Physics quiz 2009

[VivienChen]

My school physics teacher is damn good in physics and subject he is teaching(not that im boosting around) Mr Tan Yong Kun...bet many students around kerian know him very well.

He had trained students to go for the quiz and 1 of his fav student was NO.9 in whole Malaysia in physics quiz that year but this few years he had stop training students to go for the physics quiz cz he had been applying for retirement(he is about 45-50) cz he wanted to go for a long vacation :((sort of) and this year his apply has been proven so he kind of not so 'in' to wth these quiz

So i was wondering can any1 tells me whats all bout the quiz thx alot!!
and can send me the past year quiz paper ?? thx a million!!

[Eurytos]

http://pkukmweb.ukm.my/~ipho/npc2008.htm

Above is the link to last year's competition questions complete with solutions. Normally the competition is dominated by students by Chung Ling High School & Malacca High School as they have seniors and ipho trainers teaching them. If you want to qualify for the International Physics Olympiad team, you need to finish within the top 5 in the competition. You can start revising by reading university 1st year physics books written by Giancoli or Halliday & Resnick. Being able to solve problems using calculus would give u a distinct advantage over the rest of field too.

The competition will roughly be held in August. With the top 5 students being announced as winners. After that, the rest of the field will be awarded Gold, Silver and Bronze Medals according to marks obtained. Oh yeah, if you happen to be interested, do start early ! Because the competition covers everything in STPM and beyond !!!! : )

[VivienChen]

thx alot for the website but i forgot to mention that im sitting for spm this year
so im in the secondary level ^_^

[lowxuan]

physics quiz is tomoro~~~ any1 taking here???? i'm studying now..do u all spot any chapt?

[unsolvedcryptex]

physics quiz is tomoro~~~ any1 taking here???? i'm studying now..do u all spot any chapt?

yeah im joining too 2mr..haha

haiz NPC is damn hard!!haiz

[frankchong]

This is not exactly training for Math and Physics Olympiad, but if you are looking for good instructions in those areas, you can register in these courses and take them from home. The instructors/tutors know their subjects very well and if you want to be trained for competitions, some of the instructions are directly involved in training for competitions too.

http://epgy.stanford.edu/overview/us.html

You can take up courses up to university levels. It may look expensive, but if you can afford it, it is worth it. These courses started as research project in computerized instruction at Stanford University and they have been used successfully to trained talented youths for many years.
The text books they used are well written, I have gone through some of the materials in Physics and Maths when members in our family were taking them.

--------------------------------------------------------------------------
Since you are into Physics, you may be interested in these lectures as well:


http://research.microsoft.com/apps/tools/tuva/index.html

Bill Gates has made them available for free on the web.


Please make sure you are taking you math seriously as well when you are doing your physics, otherwise you will be held up by you ability in math at some point.

regards,
Frank

[Restl3ss]

How was the NPC today?

For me, the NPC questions today regarding Chap 2:Forces and Motion were exceptionally hard. And sitting in the hall cracking my head for two and a half hour continuously was not as easy as I thought. I hope I did OK. My teacher said the second individual winner last year got 46 questions correct. So if you can score more than 30 questions, it may not be a problem to get a merit. That is if you can score. >.<

[unsolvedcryptex]

How was the NPC today?

For me, the NPC questions today regarding Chap 2:Forces and Motion were exceptionally hard. And sitting in the hall cracking my head for two and a half hour continuously was not as easy as I thought. I hope I did OK. My teacher said the second individual winner last year got 46 questions correct. So if you can score more than 30 questions, it may not be a problem to get a merit. That is if you can score. >.<

speaking from that,restless,u must b from kwang hwa rite?haha.

[Restl3ss]

Gee, and what makes you say that? :wussI thought we were supposed to discuss some physic questions? xD:offtopic
Let's see....


5. A student tosses a ball directly upward. Its total time in the air is t. Its maximum height is h.What is its height after it has been in the air at time 1/4 t? Neglect air resistance.

A. 1/4 h
B. 1/3 h
C. 1/2 h
D. 2/3 h
E. 3/4 h

[unsolvedcryptex]

Gee, and what makes you say that? :wussI thought we were supposed to discuss some physic questions? xD:offtopic
Let's see....


5. A student tosses a ball directly upward. Its total time in the air is t. Its maximum height is h.What is its height after it has been in the air at time 1/4 t? Neglect air resistance.

A. 1/4 h
B. 1/3 h
C. 1/2 h
D. 2/3 h
E. 3/4 h


the max. height is h.and the time taken is t/2 since its total time in the air is t.that means to reach the max height,it take t/2
now they r asking,at the instant of t/4,what is the height?therefore it shud be 1/2h.

like i said,at t/2,they reached max.height.at t/4 means half of the maximum height.so answer is 1/2h.btw,i got this question wrong.coz i thot they r saying if the total time in the air is t/4.
sighz.then onli i realise.but too late d.

i think ure from kwang hwa coz as i know,in klang kwang hwa and acs got AMC.the rest of the skul dunno.shud b dun hv.

[gman]

well, my answer is different form the one suggested by unsolvedcryptex. i am not sure whether my working is flawed or not.

at T= 0.5t (at max height)

v= u+aT
0= u+(-10)(0.5t)
u= 5t

v^2= u^2+ 2as
0= 25t^2+ 2(-10)h
h= (25t^2)/20

at T=0.25t,

v= u+aT
v= 5t - 10(0.25t)
v=2.5t

v^2=u^2+ 2as
6.25t^2= 25t^2-20s
20s= 18.75t^2
s= (18.75t^2)/20

when compared, the value of s to h is 3/4. hence, the answer is 3/4h.

[unsolvedcryptex]

well, my answer is different form the one suggested by unsolvedcryptex. i am not sure whether my working is flawed or not.

at T= 0.5t (at max height)

v= u+aT
0= u+(-10)(0.5t)
u= 5t

v^2= u^2+ 2as
0= 25t^2+ 2(-10)h
h= (25t^2)/20

at T=0.25t,

v= u+aT
v= 5t - 10(0.25t)
v=2.5t

v^2=u^2+ 2as
6.25t^2= 25t^2-20s
20s= 18.75t^2
s= (18.75t^2)/20

when compared, the value of s to h is 3/4. hence, the answer is 3/4h.
:cry:cry:cry:cry..ur answer sounds so...............correct..didnt know it was that complicated.

[Restl3ss]

Gosh! I forgot acceleration should be negative. >.<
I was thinking about using equations like that but I can't relate it with the info given, you're really good gmam, thanks. :)
However, I submitted the answer unsolvedcryptex initially suggested at last. ; ) I used the same concept as what he thought too. x)

Then what about this?

11. Which of the following is NOT an example of accelerated motion?
A. Vertical component of projectile motion
B. Circular motion at constant speed.
C. A swinging pendulum
D. Earth's motion about sun
E. Horizontal component of projectile motion

[unsolvedcryptex]

Gosh! I forgot acceleration should be negative. >.<
I was thinking about using equations like that but I can't relate it with the info given, you're really good gmam, thanks. :)
However, I submitted the answer unsolvedcryptex initially suggested at last. ; ) I used the same concept as what he thought too. x)

Then what about this?

11. Which of the following is NOT an example of accelerated motion?
A. Vertical component of projectile motion
B. Circular motion at constant speed.
C. A swinging pendulum
D. Earth's motion about sun
E. Horizontal component of projectile motion

this stupid question?haiz...i dun say its stupid.but i also was very much confused.i chose B.from wat i think,its either B or D.A is out.bcz if vertical b deceleration since gravity is pulling down.and its also an example since deceleration is also acceleration.but somewhat negative and bla bla.same goes for pendulum bcz its speed is decreasing due to damping.horizontal can b either accelerating,decelerating or constant.quite doubtful.but D,im not sure bout the earth's motion.can i say that sometimes the earth move fast and its not moving at constant speed?bcz i felt that earth's orbit is oval.mayb when its near to the sun,its speed increase slightly.and when far,decrease slightly.so i got no choice but to choose B.but dunno y i chose that.cant justify it

ok sry if i have wrong infos.hahaha..im not good in physics at all:P

[jiacheng91]

i think its E
A) gravitational acceleration
B) changing direction of velocity means thrs acceleration as rate of chg of velocity exist
C) changing direction of velocity
D) also circular motion
E) we treat horizontal and vertical component separately and horizontally assuming no air drag, experience 0 net force and hence 0 acceleration

[unsolvedcryptex]

i think its E
A) gravitational acceleration
B) changing direction of velocity means thrs acceleration as rate of chg of velocity exist
C) changing direction of velocity
D) also circular motion
E) we treat horizontal and vertical component separately and horizontally assuming no air drag, experience 0 net force and hence 0 acceleration

swt..rly ar?no idea.

[withoutwax]

The answer is E.
Circular motion involves change in direction of object even though the speed is constant. That means there is acceleration during circulation of an object and also orbiting of Earth around the Sun.
Horizontal component of projection is constant in velocity, ie no force is acting in this horizontal direction.
________
Sunshine_fire cam (http://camslivesexy.com/cam/sunshine_fire)

[gman]

i agree with both jiacheng91 and withoutwax that the answer is E. my rationale is similar to those offered by the 2 of them. by the way, are these questions from the form 5 or form 6 quiz?

[jiacheng91]

i think the questions are for form 5 students
they are structured in a way that form 6 students can answer them rather smoothly but form 5 students would have to ponder abit longer on them.

[unsolvedcryptex]

i think the questions are for form 5 students
they are structured in a way that form 6 students can answer them rather smoothly but form 5 students would have to ponder abit longer on them.

yeah form 5.see me so noob in answering edi know its form 5 loo..haahahaha

try to look on my signature.kinda speaks for myself

[chiunlin]

My school physics teacher is damn good in physics and subject he is teaching(not that im boosting around) Mr Tan Yong Kun...bet many students around kerian know him very well.

He had trained students to go for the quiz and 1 of his fav student was NO.9 in whole Malaysia in physics quiz that year but this few years he had stop training students to go for the physics quiz cz he had been applying for retirement(he is about 45-50) cz he wanted to go for a long vacation :((sort of) and this year his apply has been proven so he kind of not so 'in' to wth these quiz

So i was wondering can any1 tells me whats all bout the quiz thx alot!!
and can send me the past year quiz paper ?? thx a million!!
Well, my secondary school physics teacher has produced two NPQ winners, one winning team, one 2nd-placed team, a few top-10s and many gold medals over the years. :D

If you're serious about NPQ, you should prepare early. Over the years I have seen threads created in Recom asking for tips for NPQ a few weeks before it. That's way too late to start. Start preparing at least half a year ahead. There used to be a workbook containing all the past year questions(when Physics was taught in Malay, no idea whether it was translated). So yea, for those who are in form 4, and aspires to do well in NPQ, start preparing now!

5. A student tosses a ball directly upward. Its total time in the air is t. Its maximum height is h.What is its height after it has been in the air at time 1/4 t? Neglect air resistance.

A. 1/4 h
B. 1/3 h
C. 1/2 h
D. 2/3 h
E. 3/4 h
Gman's answer is correct but if you're aiming for a top-10 finish, doing calculations like that will cost you precious time.

How I did it in less than 10 seconds - After reading the question, I pictured the projectile motion in my head and it's an inverted parabola. From the fact that it's an inverted parabola and since it takes t/2 to reach h, at t/4, the ball is at 3h/4.

[Restl3ss]

I have a problem here, can anyone please help me to solve this mathematics question? :))

!. The smallest possible value that √(x? + y?) + √((x-1)? + y?) + √(x? + (y-1)?) + √((x-3)? + (y-4)?)
can have is

A. 5
B. 4+√3
C. 6
D. 5+√2
E. 7

2. How many numbers from 1, 2, 3, 4, ....., 2008 have a cubic number other than 1 as a factor?

A. 346
B. 336
C. 347
D. 254
E. 393


P.s. I'm impressed with chiunlin totally! :wink Cause, some of my teachers can't even solve the problem yet he can make it look so simple. Thanks for the guide! =)

[Smilehoe]

For the first problem, plot all 4 circles with r=1. Since the value of the equation is the distance of the center of the circles to the x,y value you set. The minimum could then be obtained by choosing x,y value which are straight lines from the center of all 4 circles.

which will give y = -x + 1 and y = 4x/3, => x = 3/7, y = 4/7. Substituting into the equation gives 6 + sqrt(2). Verify with wolfram (http://www03.wolframalpha.com/input/?i=minimize+%E2%88%9A(x%C2%B2+%2B+y%C2%B2)+%2B+%E2%88%9A((x-1)%C2%B2+%2B+y%C2%B2)+%2B+%E2%88%9A(x%C2%B2+%2B+(y-1)%C2%B2)+%2B+%E2%88%9A((x-3)%C2%B2+%2B+(y-4)%C2%B2))

I don't know the second problem. Will try later. Btw, those questions feel more like maths quiz than physics.

[youngyew]

Haha Restl3ss (http://www.recom.org/forum/member.php?u=9572) you posted in the wrong thread la. Post maths questions in the maths thread instead.

There are only 12 cubic numbers from 1 to 2008: 1 to 12^3. In this question you don't have to count the compound number's cubes like 4^3, 6^3, 8^3, 9^3, 10^3 and 12^3. The reason is simple - once you have counted the numbers divisible by 2^3, that count would have already included all those numbers divisible by 4^3, 6^3, 8^3, 10^3 and 12^3. And the 9^3 would have been counted when you were counting numbers divisible by 3^3.

So in the end just find out how many numbers are divisible by 2^3, 3^3, 5^3, 7^3, and 11^3, AND subtract the numbers that you have double-counted in the process namely numbers divisible by both 2^3 and 3^3 and numbers divisible by both 2^3 and 5^3.

If you want a nice summary for the calculations needed, here it is:

[2008/2^3] + [2008/3^3] + [2008/5^3] + [2008/7^3] + [2008/11^3] - [2008/(6^3)] - [2008/10^3]

(the square bracket above is the floor function, which means the integer part of the answer. E.g. [100/7] = 14)

And HERE (http://www04.wolframalpha.com/input/?i=floor%5B2008%2F2%5E3%5D+%2B+floor%5B2008%2F3%5E3%5D+%2B+floor%5B2008%2F5%5E3%5D+%2B+floor%5B2008%2F7%5E3%5D+%2B+floor%5B2008%2F11%5E3%5D+-+floor%5B2008%2F%286%5E3%29%5D+-+floor%5B2008%2F10%5E3%5D&asynchronous=false&equal=Submit) is the full calculation from WolframAlpha.

[henry_yew]

11. Which of the following is NOT an example of accelerated motion?
A. Vertical component of projectile motion
B. Circular motion at constant speed.
C. A swinging pendulum
D. Earth's motion about sun
E. Horizontal component of projectile motion

(Direction going up the y-axis is taken as positive vertical motion; direction to the right of the y-axis is taken as positive horizontal motion)

A is wrong because in a projectile motion, the object travels in a vertical motion (in addition to the horizontal one as well). You will notice that as the object travels vertically, it's positive vertical velocity decreases, stops at one point of time, and increases in the negative direction. This can only be as a result of acceleration - more precisely, gravitational acceleration.

B is wrong because an object in a circular motion always experiences centripetal force, which is actually a product of mass and acceleration. Actually the force is measured as F=(mv^2)/r, where m is mass, v is velocity and r is the radius of the circle. A practical way of explaining this situation is that an object which is travelling in a circular motion is constantly changing its velocity (magnitude of speed may be the same but the direction is changing constantly) - change of velocity is affected by acceleration. You can conduct an experiment where you tie a ball to a string and swing it with your hand to make the ball travel in a circular motion. You will notice that the inertia of the ball tends to make the ball travel in a straight line but you are forcing it to travel in a circular path, thus overcoming the inertia. That requires force, which is indirectly related to acceleration.

C is wrong because a swinging pendulum, like the projectile motion, as a vertical component as well as a horizontal component. The ball travels upwards, stops at a point, then comes down again and swings to the other horizontal direction, where the same scenario happens. Gravitational acceleration is playing a role here.

D is wrong because, like the scenario where the ball tied to the string is swung with your hand (the ball represents the Earth and your hand represents the Sun), the Earth changes velocity constantly. According to the law of inertia, the Earth tends to travel in a straight line and would eventually fly off its orbit had it not for the gravitational pull of the Sun. The Sun's gravitational pull imposes a centripetal force which acts towards the direction of the Sun and keeps the position of the Earth in place. Whenever centripetal force is in existence, acceleration must be involved. No exception.

E is the correct answer because the horizontal component of a projectile motion is not affected by the gravitational pull. Assuming that there is no air resistance, no friction and no other external forces, the horizontal component of the projectile motion would remain unchanged, thus proving the fact that there is not acceleration involved.

[youngyew]

The other not-so-technical way of explaining the answer is this:

A, B, C and D are all under the influence of gravitational force, and we all know that F=ma. E is perpendicular to the gravitational force and is hence independent of it.

[henry_yew]

The other not-so-technical way of explaining the answer is this:

A, B, C and D are all under the influence of gravitational force, and we all know that F=ma. E is perpendicular to the gravitational force and is hence independent of it.

LOL. Yeah.. but it's useful to explain how the influence works. Besides F=ma, F=(mv^2/r) for centripetal force. Therefore, if you are asked the centripetal acceleration, a(centripetal) = v^2/r.

Centripetal acceleration cannot be found using linear kinematics (like a=(v+u)/t). This is an important point. It is more closely related to rotational kinematics.

P/S: At least these are topics I can help out with and still related to my engineering discipline. :amuse

[Johnivan]

I wonder did anyone here took the Physics quiz for the Pre-U level... It was super duper hard... For an A student, i don't think I could even answer a single section of the subjective questions....:cry

[lonewolf5999]

C is wrong because a swinging pendulum, like the projectile motion, as a vertical component as well as a horizontal component. The ball travels upwards, stops at a point, then comes down again and swings to the other horizontal direction, where the same scenario happens. Gravitational acceleration is playing a role here.If you look carefully at the swinging pendulum, you notice that when resolving the gravitational force along and perpendicular to the direction of motion, the perpendicular component of the gravitational force is directed radially outwards from the string, and therefore it is not the force providing the centripetal acceleration, which must act radially inwards along the string. That force would be the tension in the string.

The other not-so-technical way of explaining the answer is this:

A, B, C and D are all under the influence of gravitational force, and we all know that F=ma.

Well, no mention of gravitational force was made for the "circular motion at constant speed" option, so we can't say that the acceleration in B was caused by gravity.

Also, I'd like to say that chiunlin's solution to the projectile motion problem was incredibly elegant and insightful!

I wonder did anyone here took the Physics quiz for the Pre-U level... It was super duper hard... For an A student, i don't think I could even answer a single section of the subjective questionsI never took the quiz, but past year papers are easily available from the NPC website. It seems that some problems required the use of calculus, including integration. I don't think STPM teaches the use of calculus in problem solving.

[unsolvedcryptex]

hi everyone.

as some of u edi know or dunno
the NPC result is out.check it out.

http://pkukmweb.ukm.my/~ipho/SecondaryIndividual2009.htm (http://pkukmweb.ukm.my/%7Eipho/SecondaryIndividual2009.htm)

as for myself,i din get it la of coz.onli one person from my town got it.hes my fren.not so close.acquaintance.ok la at least he knows me also.haha.

check if ure the winner.

[Restl3ss]

Gah! I hope everyone did great, cause i was lousy. x]
When i check through the answers online, I got a merely 50%... Sad...T.T

[unsolvedcryptex]

Gah! I hope everyone did great, cause i was lousy. x]
When i check through the answers online, I got a merely 50%... Sad...T.T

eler..50% of top score.hahahah
i din check..scared to check

[cfliang]

Anyone has any idea whether the certificate for pre-u gold category is useful in application for Singapore or US universities?

[unsolvedcryptex]

Anyone has any idea whether the certificate for pre-u gold category is useful in application for Singapore or US universities?

wow ..u got ur result d for pre-u?

ok personally,(seriously personal view,might not work for others and might b misinterpreted)i think its useful

Useful to the extend that it brush ur acdemic achievements.

Sry,wasnt able to get more info bout u.wat i meant?

U c,if ur whole life is dedicated to academic and u get achievements onli from academic activites,the addition is not really important

For most of the us unis or other scholarships bla bla,u had to b more than a bookworm la.obvious.
so thats y i need to know if ure a real bkworm or not.coz this question is just too subjective.it varies from everyone

for eg,someone is able to get like gold in the competition also,hes also the bla bla bla bla(co-curricular),volunteered,....etc.then it rly shows he is able to balance everything and that gold means something.

if ure a real bkworm ,i would say gold is really good for me.but somehow when it comes to application mayb they expect more from a bookworm.

Thats wat i think.
Coz i got the feeling ure asking if the gold really boosts u into universities.from wat i think,there could be a lot of things that can boost u instead of this competition(not looking down at the gold,i would say congrats coz i dun think i can get it).

so if u onli have such achievements,i cant answer u.im not the adcom,but if ure real all-rounded and can do more than studying,i wud say brilliant.(unlike me)

[lowxuan]

i thought they will be bronze , gold, silver medal? y it doens't show out?

i mean the web page

[unsolvedcryptex]

i thought they will be bronze , gold, silver medal? y it doens't show out?

i mean the web page

if thats the case,then the page is gonna b loaded.so onli top 10 will b displayed.
besides,where do u get the info that those 3 medals winner will b displayed in the website?
i dun think all along they show u know

[lowxuan]

if thats the case,then the page is gonna b loaded.so onli top 10 will b displayed.
besides,where do u get the info that those 3 medals winner will b displayed in the website?
i dun think all along they show u know

xD... i dunno bout that...ha...really...coz i heard my teacher say that there will be bronze, silver, gold medal for student who score 30%,50%, 75% in whole malaysia...(some sort of that)

[unsolvedcryptex]

xD... i dunno bout that...ha...really...coz i heard my teacher say that there will be bronze, silver, gold medal for student who score 30%,50%, 75% in whole malaysia...(some sort of that)


there will b four awards for those who din get top 10

a) Gold - > highest score of 90%
b) Silver - > 78% but < 90%
c) Bronze - > 65% but < 78%
d) Distinction - > 50% but < 65%

not 30% la..too low d.

[frankchong]

It is a myth to think that top universities prefer all rounder over specialists. Place like Harvard look for all rounded student body, so in a student body, there will be all rounders and there will also be a lot of specialists. In places like Caltech, it is a science/engineering school, they have to make sure you like and will do well in science/engineering, otherwise, you would not fit in.
The fact that you get a gold in pre-U physics quiz and if you take a leadership role or volunteer in a science club, do a science project, it shows that you are into science. Do not try too hard to do something totally not related, it is better to concentrate in a few and do well.
Do not over worry, if you are not the gold medallist in the International Physics Olympiad, it helps, but not a necessary condition, an important criteria in top US university admission process is to see whether you make use of the opportunities available to you. If you did, the hope is that you would make use of the opportunities make available to you once you are in the U.
There is nothing wrong in studying hard, and by being good in an area does not mean you can not acquire skills in other areas. Some of the best managers I have seen in MNCs are engineers who have never gone to business schools. People used to make jokes about Bill Gates (in the 80s), describing him as someone as sexy as a floppy disc, but you know what he did and what he is doing now.
Once you move down from the few top US Us, to universities like UC Berkeley etc, most of UC Berkeley's admits depends on school and exem results (SAT, SAT2 etc). They have (or used to) a formula to compute a candidate's academic index, and from the academic index one can tell roughly the chance of a student getting admitted. I do not know how is it applied to international students.
In the case of Singapore, they realized they need to nurture special talent, hece they have established NUS High School of Science and Math to cater to thes students. In NTU, they have the C.N. Yang scholar program, modelled after the Caltech undergraduate program.
So, for those of you who have passion in science, go as far as you can. There will be a lot of all rounders, but very few who are at the forefront of their profession, who can bring the profession to new heights, create new products, companies and extend the boundary of human knowledge.

regards,
Frank Chong

[youngyew]

Well, no mention of gravitational force was made for the "circular motion at constant speed" option, so we can't say that the acceleration in B was caused by gravity.
You are right. I kind of translated it to orbital motion unconsciously.

[frankchong]

Originally Posted by Restl3ss

11. Which of the following is NOT an example of accelerated motion?
A. Vertical component of projectile motion
B. Circular motion at constant speed.
C. A swinging pendulum
D. Earth's motion about sun
E. Horizontal component of projectile motion

Let me answe these questions in a different way:
By definition: Acceleration = rate of change of velocity, it is a vector quantity. Hence for acceleration to be zero, velocity has to be constant.
A. vertical component of velocity is changing(magnitude), so acceleration is not zero
B. Direction of velocity is changing, so acceleration is not zero
C. Velocity is changing in both magnitude and direction, so acceleration is not zero
D. Same as B
C. Horizontal component of velocity is constant, hence horizontal acceleration is zero.

The art in understanding physics is to develop the ability to decompose complex problems into simple components and once you have done that, there is very little you have to memorize in physics. You can go very far with a few definitions and physical laws.

regards,
Frank

[Restl3ss]

The way you explained the question is really clever and simple! =)
Thanks for the tips and explanation.

[unsolvedcryptex]

I just received my results.Bad.din get wat i wanted.u shud b able to get back ur results now.my teacher announced that she received it last friday.

[Restl3ss]

Since we were in this together, really hoped that you can get some good results. But never mind, cheer up! =) and thanks for giving me that info. Though I can't give myself high hopes.. x/ :puke

[unsolvedcryptex]

Since we were in this together, really hoped that you can get some good results. But never mind, cheer up! =) and thanks for giving me that info. Though I can't give myself high hopes.. x/ :puke

thx..edi got it tho..u haven get?

[henry_yew]

Bumping this thread with some new Physics questions.

1. Sound can travel in different media, be it solid, liquid or gas. Arrange the media in the order of decreasing speed that the sound can travel in each of the respective medium.

A. Gas, liquid, solid
B. Solid, liquid, gas
C. Liquid, gas, solid
D. Gas, solid, liquid
E. Solid, gas, liquid

2. Given that a cube of ice is floating in a cup of warm water. After the ice melts, will the volume of water in the cup increase or decrease?

A. The volume should increase
B. The volume should decrease
C. The volume should not change

3. An oscilloscope is wired to a piezometer which can produce sounds of up till 30 kHz. The oscilloscope is first set to 7500 Hz and the sound pressure level is measured to be 85 dB at a distance of 0.3m, using a decimeter that is able to measure sound pressure levels at any given frequency. At 7500 Hz, the human ear is able to pick up the sound clearly. The oscilloscope is then tuned to a higher frequency, and as the frequency gradually approaches the ultrasonic frequency, it becomes gradually softer until the ear can no longer detect any sound (the oscilloscope has been tuned into the ultrasonic range). Assuming that the energy loss is negligible and that the sound pressure level of the surrounding is 45 dB (when the oscilloscope is not in use), what is the sound pressure level that will be detected by the decimeter at 25 kHz?

A. 45 dB
B. 85 dB
C. 60 dB
D. 95 dB

[Restl3ss]

My little attempt. xD

For question 1, is it B? This is because sounds travel through vibrations of particles, and solid had the most compact arrangement, following by liquid and gas?

As for question 2, the volume does not change? Cause when the ice cubes are added in, the volumes has already increased?

[henry_yew]

Both answers are correct. For question 2, the reasoning has to do with the fact that the volume displaced by the ice is the volume of the ice that is under the water. When it melts, the extra volume (from the ice) will fill in the displaced volume of water, thus there will be no increase in the water level after the ice melts.

But what about question 3?

[eve88]

just a nitpick : note the difference between volume of water and water level. Volume of water does increase (you add 5ml more water to the cup, because before that it wasn't water it was ice) but water level stays the same.

=p

other than that, carry on....

[henry_yew]

just a nitpick : note the difference between volume of water and water level. Volume of water does increase (you add 5ml more water to the cup, because before that it wasn't water it was ice) but water level stays the same.

=p

other than that, carry on....

Right. My bad on this one. But if we were to conduct this experiment using a measuring cylinder instead of a cup, the volume of water (with the ice in it) would remain the same after the ice melts. That means the volume of water that is recorded before the ice melts is actually the volume of water plus the volume of ice that is under the water surface. Thus, in this sense, the volume does not change after the ice melts.

But if you want to go right down to the very bottom of things and say that water in the form of liquid and water in the form of solid are separate entities, then yes, I agree with you whole-heartedly that the volume of water (in liquid form) increases after the ice melts.

It's just a matter of how we word the question. My bad, my bad.

So, question 3?

[frankchong]

Question 2:
To the first order, the water level in the cup/container would stay constant as the ice melts. So, if you fefer to the water level, you should be able to get out of the question of whether water includes ice or not. However, if you really want to be exact, if you consider the temperature of ice and water initially, and of the total water afterwards, calculate the volume factoring the initial and final temperature, volume change of ice to water, water due to temperature change and the container due to temperature change, it is unlikely the level would stay exactly the same.
This brings out the point that one needs to know the problem at hand and make the right simplifying assumptions. If you read too much into the problem, you could be in trouble too.

Question 3:
"oscillescope" should read as "oscillator" and "piezometer" should be "piezo electric transducer". An oscillescope is a waveform display instrument, an oscillator is a signal generator that can be used to drive a transducer. A transducer is a device that converts one form of energy to another form of energy.

regards,
Frank

[chongkeat]

Questions like Question 2 have been confusing me for quite a while now.

I mean, the increase in the level of water when the ice is placed into the water is only equal to the amount of water displaced by the submerged part of the ice, right?

When the ice melts, won't it have an effect equivalent to me pushing the ice and submerging it completely, i.e. raising the water level?

But then again, that effect could perhaps be canceled out by the reduction in volume as ice turns into water.....

I dunno, enlighten me here, please?

[youngyew]

Question 2 is the direct corollary of the Archimedes principle. When the solid ice is floating happily on the water, the volume of the water it displaces is the same as its weight (which is conveniently the same value as its volume using our SI units). Later when the solid ice melts, it turns into water which has the same volume as its weight, and this new water replaces the "hole" where the original submerged ice was. Since the original displaced volume is the same as its weight (= Archimedes principle) which in turn is the volume, the water level will not change.

Or to make it more concrete:

Let's assume that in the beginning, x ml of ice is submerged in the water and y ml of ice is above the surface. How much is the weight of the ice? It's x grams (due to Archimedes' principle). When it melts later, there's a "hole" where that x ml of ice used to be. How much water will we get from the molten ice? It's obviously x ml (as the ice's weight is x grams). So you have a "hole" of x ml but it's replaced by x ml of freshly molten water, therefore you get an unchanged water level.

I have not considered the temperature change that Frank has pointed out with much insight. :)

[eve88]

gosh, if you're going into temperature changes im going to include effects of evaporation as well....

okay, since henry keeps asking for ppl to attempt qn 3:

(disclaimer : im using MCQ-guess reasoning instead of physics as i have *no clue* what hes talking about)

working in done in order of likelyhood that the assumption is correct.

1) Background rates, thus answer must always be different. (A is out)
2) assumption 2 : something changes. thus, something else must change. (B is out)

C vs D guess. I guess D (flipped a coin)

[Weng]

this Physics quiz 2009 tat u all are discussing, is it the "THE NATIONAL PHYSICS COMPETITION 2009 (NPC 2009)"...??

[yanno_yamster]

this Physics quiz 2009 tat u all are discussing, is it the "THE NATIONAL PHYSICS COMPETITION 2009 (NPC 2009)"...??

Yes, they are.

[Weng]

Yes, they are.

secondary level..? are the results out yet..?

[unsolvedcryptex]

yes the results r out.click the link here.i received my results as well d.so wait till holiday is over.but u cant check wat u get thru the website.onli top 10 r listed


here (http://pkukmweb.ukm.my/%7Eipho/SecondaryIndividual2009.htm)

[Weng]

yes the results r out.click the link here.i received my results as well d.so wait till holiday is over.but u cant check wat u get thru the website.onli top 10 r listed


here (http://pkukmweb.ukm.my/%7Eipho/SecondaryIndividual2009.htm)


u got top 10...?? how did u receive ur results..??

[unsolvedcryptex]

u got top 10...?? how did u receive ur results..??
eh no la.dun praise me.i din .restless skulmate got.i got very bad results.thats all .so malu to tell my results.

one monday,actually last monday,my teacher sud went up stage and announced it suddenly.luckily i came and din ponteng.so i received my results on stage.so memalukan when ur results sucked.lolz.jk.at least i can stand with other pro in my skul.

[Weng]

eh no la.dun praise me.i din .restless skulmate got.i got very bad results.thats all .so malu to tell my results.

one monday,actually last monday,my teacher sud went up stage and announced it suddenly.luckily i came and din ponteng.so i received my results on stage.so memalukan when ur results sucked.lolz.jk.at least i can stand with other pro in my skul.


what is restless real name...??

[unsolvedcryptex]

what is restless real name...??

restless real name is restless.come on la.dun use death note here.hahahahah:pwned

not restless get.his skulmate!

[Restl3ss]

hey guys, and when did this became a discussion of my real name?:roll: Make sure you don't use the death note on me. :P

It's true that someone from my school was listed as the top ten. But i believe there are many more talented people out there. So let's just not get out of the topic and concentrate on the physic questions asked here for now?

But by the way, anyone from recom got good results from this quiz? I still don't know my results yet though, sad. :'(

[unsolvedcryptex]

hey guys, and when did this became a discussion of my real name?:roll: Make sure you don't use the death note on me. :P

It's true that someone from my school was listed as the top ten. But i believe there are many more talented people out there. So let's just not get out of the topic and concentrate on the physic questions asked here for now?

But by the way, anyone from recom got good results from this quiz? I still don't know my results yet though, sad. :'(

wait la.ur skul haven gif ma.holiday wat.my skul doesnt has holidays for pmr.thats y

[Weng]

i'm form Chan Wa seremban... i still don noe my result yet....

[unsolvedcryptex]

i'm form Chan Wa seremban... i still don noe my result yet....
slowly wait lo.

[frankchong]

"gosh, if you're going into temperature changes im going to include effects of evaporation as well...."

Actually, it would be a good exercise to estimate what is the impact of evaporation too. However, fingering how to calculate evaporation rate is a lot more difficult. However, if you manage to do it, you would learn more physics than all the quizs people talk about here. The ability to analyse problems from first principles would make you highly valuable in years to come.
An experimental method would be easier for most people here. The easiest way out would be just cover the container.

A big part of learning physics is acquiring the ability to do order of magnitude estimation, in this care if you cover the container, water can stay in the container for a very very long time!

Please take note, all the problem sets in physics books and quizes have been solved before, the fact that you solve it another time adds no intrinsic value, unless you learn something (hopely physics) from the process.

Frank

[Tysle]

Yes, the results are out already now. My school has already received the results and award certificates. :D

May I know where I can find the answers to this year's NPC?

Never mind. I have already found the answers :)

[henry_yew]

Question 3:
"oscillescope" should read as "oscillator" and "piezometer" should be "piezo electric transducer". An oscillescope is a waveform display instrument, an oscillator is a signal generator that can be used to drive a transducer. A transducer is a device that converts one form of energy to another form of energy.


Right, "oscillator" makes more sense. Gosh, my group and I had been using the term "oscilloscope" without realising the mistake, and neither did my lecturer correct us. (Possibly because the oscillator itself is also an oscilloscope which can generate the signal and display the waveform.) As for the "piezometer", I accept that "piezo electric transducer" is a better term although it's been stated as "piezometer" in our standard drawings and dealings with the manufacturers (surprisingly, they understood what we wanted).

Thanks anyway.


okay, since henry keeps asking for ppl to attempt qn 3:

(disclaimer : im using MCQ-guess reasoning instead of physics as i have *no clue* what hes talking about)

working in done in order of likelyhood that the assumption is correct.

1) Background rates, thus answer must always be different. (A is out)
2) assumption 2 : something changes. thus, something else must change. (B is out)

C vs D guess. I guess D (flipped a coin)

Aww... OK, I'm sorry if you had no clue what I was talking about. Perhaps the "oscilloscope" and "piezometer" confused you.

I forgot to add a particular word to the question, but it doesn't matter anymore. Although you cannot possibly calculate the answer out if you don't have the proper formulae given, a good guess is actually C.

(Sound pressure level, in not so technical terms, refers to the sound volume, or sound intensity.)

As the frequency of the sound wave increases from 7500 Hz to 25 kHz, the energy loss per unit length in the sound wave becomes greater. An acoustic sound is able to propagate at a longer distance (greater than 1.0 m), but an ultrasound would have lost up to 60% of its energy after travelling a distance of 0.3 m to 0.9 m (1 ft to 3 ft).

(What I'm trying to say is that while your scream can be heard perhaps 20 metres away, an ultrasound with the same initial energy will not be able to travel that distance of 20 m.)

Now, sound pressure level is measured according to the energy of the sound wave. If the energy level decreases, then the sound pressure level decreases, too. Therefore, the sound pressure level of the ultrasound (at 25 kHz) should be less than 85 dB.

Recognising that ultrasound has a sound pressure level is important because while we cannot hear ultrasound, that doesn't mean that it cannot cause damage to our ears. Studies (Heinrich Kutruff, 1991) and some standards have advised that the maximum sound pressure level for any device that produces ultrasound for whatsoever function should be set at a maximum of 100 dB, because at sound pressure levels higher than that, it can cause damage to the ears and may also cause local heating.

For your info, sound wave energy is directly proportional to the square of sound pressure. And sound pressure level, SPL = 10 log (P/Pref)^2, where P = sound pressure from the source, Pref = reference pressure = 2 x 10^-5 N/m^2.

I'm no expert in sound engineering, but it's part of my curriculum and the topic of ultrasound was used in my team project last semester, which proved one thing (at least for me and my team): ultrasound is not good to get rid of pestiferous birds (such as swallows and swifts) which are staying happily 30 metres above you!