[STPM] Physics

[shawblade]

Hi! Anyone can solve the below question for me?
Lower six physics, chapter 2 kinematics and dynamics

~~ The coefficient of kinetic friction between the tyres of a car and the road is 0.40. In an accident, the length of the skid mark of a vehicle is 52m. What was the speed of the vehicle before in skidded?

ANS: 20m/s

Given the coefficient of kinetic friction, you can find the acceleration which is -3.924. Negative because the car is being decelerated before it comes to a halt. Then use v^2=u^2+2as, sub a=-3.924, s=52 and final v=0.

[jypang]

Actually I don't really understand the questions (lol), but anyway, I'm going to give it a shot.

v^2 = u^2 - 2as

When the car came to a full stop, v = 0, s = 52m

k (coefficient of friction) = friction/weight of car
= ma/mg

Hence a = kg = 0.4g

Substitiute all values into the first equation to find u, the initial speed.

Hmm, i thought answer was already given in the book. 0.o ?

Well, the answer from the book is like this~~
http://img146.imageshack.us/img146/8193/unledjd.jpg (http://imageshack.us/photo/my-images/146/unledjd.jpg/)

Uploaded with ImageShack.us (http://imageshack.us)

What's mean by " retarding force" in the ans?

But anyway, I am appreciate the explanation from Cactus although i can't understand well.:)

Given the coefficient of kinetic friction, you can find the acceleration which is -3.924. Negative because the car is being decelerated before it comes to a halt. Then use v^2=u^2+2as, sub a=-3.924, s=52 and final v=0.

Hi, how can i obtain a=-3.942?
Thanks for your explanation

[Cactus]

Well, the answer from the book is like this~~
http://img146.imageshack.us/img146/8193/unledjd.jpg (http://imageshack.us/photo/my-images/146/unledjd.jpg/)

Uploaded with ImageShack.us (http://imageshack.us)

What's mean by " retarding force" in the ans?

But anyway, I am appreciate the explanation from Cactus although i can't understand well.:)



Hi, how can i obtain a=-3.942?
Thanks for your explanation

Retarding force means the force that causes the car to decelerate.
Skidding occurs when the tyres stop rotating but the car still moves, hence leaving a skidding mark/trail of 52m in the scenario of the question.

When the tyres do not rotate, we may consider the car as a rigid body undergoing ONLY linear motion, and apply the equations of kinematics to deduce its speed RIGHT AFTER the tyres stop rotating completely (or RIGHT BEFORE the skidding mark begins).

Would appreciate if any Engineering students were to give a more realistic/less presumptuous explanation concerning skidding/rotational motion etc.

[jypang]

If a graph plotted F against t, area under the graph represent momentum?

[Cactus]

If a graph plotted F against t, area under the graph represent momentum?

change in momentum from t1 to t2.

[markwongsk]

change in momentum from t1 to t2.

Or also the total impulse in the given period. But change in momentum from t1 to t2 is more consistent with force :)

[jypang]

hey guys! do your teacher assist you when doing experiments ???

[markwongsk]

hey guys! do your teacher assist you when doing experiments ???

When I took it no.

[erikanyam]

When I took it no.

wah so fast do experiment.My teacher says only in the upper 6 just do the experiments:nuts

[jypang]

wah so fast do experiment.My teacher says only in the upper 6 just do the experiments:nuts

I am lower six, I had took 2 experiments in 2 months!:P

[ruffyloohy]

Hi! anyone know where to get the book Fundamentals of Physics (Halliday, Resnick and Walker)? How much would it cost? :D

[sugan]

I dont' think you REALLY need it, unless u are preparing for IPHO or leisure or for futher clarification .


i use ace ahead use most of the time.

but , still it will be useful later in university if u are still in a field that requires physics.

[jypang]

dose anyone could help in this question?

A ball of mass M released from a height h0 above the floor rebounces to a height h1, after one bounce. The graph shows the variation of h1 with h0.
http://i688.photobucket.com/albums/vv241/Jianye0304_2009/Untitled-11.jpg

1) If the ball was dropped from a height h, what is the kinetic energy of the ball immediately after the third impact?
2)After how many impacts would kinetic energy of the ball becomes half of its initial potential energy?

[jypang]

Anyone help me to solve this question???
http://i688.photobucket.com/albums/vv241/Jianye0304_2009/phy.jpg