# Friction: decelerating vehicle on dry and wet concrete

1. Oct 11, 2012

### tratata

1. The problem statement, all variables and given/known data

A pick up truck is travelling at 58 km/h on a dry horizontal concrete surface when the driver applies the brakes locking the wheels. (a) How far does the car travel before coming to rest? (b) How long does it take the car to stop after the driver applies the brakes? (c) How far does the car travel before coming to rest if the concrete surface is wet?

2. Relevant equations

3. The attempt at a solution

I can't even understand where to start! Please offer any suggestions. I can't find any equation where just the velocity would be sufficient. I have V1 and V0 but absolutely nothing else - well also μk and μs. ok wait if I have μs that should somehow help me....but how? Any help greatly appreciated! Kind regards,
Kate

2. Oct 11, 2012

### tiny-tim

hi tratata!

from µ, you can find the acceleration

does that help?

3. Oct 11, 2012

### Simon Bridge

Don't try to find an equation to plug numbers into to get the answer. Instead, use your understanding of physics to write an equation.

Can you sketch a v-t diagram of the motion?
Can you draw a free-body diagram for the forces on the car?
Do you know the coefficient of kinetic friction for the surfaces involved?

4. Oct 11, 2012

### tratata

Tiny-tim, but how? ;)) a little more help please or direction to go! Ive been struggling with this one half the night

5. Oct 11, 2012

### Simon Bridge

What is the relationship between friction force, $\mu_k$ and acceleration?
Hint: Newton's laws.

This is why you want the fbd.

6. Oct 11, 2012

### tiny-tim

exam questions usually tell you everything you need to know

if you can't see what the answer is, go back to fundamentals and ask yourself what everything in the question means

in this case, what is meant by the coefficient of friction?

it's the … ?

7. Oct 11, 2012

### tratata

Simon: a=μk*g? ;)

8. Oct 11, 2012

### Simon Bridge

acceleration is a vector :)
if the pickup starts out moving in the +x direction, then $a=-\mu_k g$.
but you should not just present someone with your end result - show your reasoning.

Would the deceleration be constant or would it change?
Can you sketch a velocity-time graph of the motion?