# Homework Help: Need help finding change in velocity and momentum

1. Mar 28, 2013

### Sneakatone

An automobile of mass 1500 kg traveling at 24 m/s crashes into a similar parked automobile. The two automobiles remain joined together after the collision. Suppose that the collision last for 0.020s, and pavement with a coefficient of friction uk=0.63. What change of momentum and what change of speed does the friction force produce in the joined automobiles in the interval of 0.020 s ?

Delta p=?
Delta v=?

for p I did 24m/s *1500kg=36000
3600/12m/s=3000
The Friction force = µ . m . g =0.63*9.81*3000=18541 N
The change in momentum produced by this force = F . t =18541*0.02=371 kg*m/s
delta p=36000-371=35629 kg*m/s

to get v
35629/3000=11.87m/s
12m/s-11.87m/s=0.15 m/s

2. Mar 28, 2013

### voko

What exactly do you need help with?

3. Mar 28, 2013

### Sneakatone

the numbers that I input are apparently wrong.

4. Mar 28, 2013

### voko

I am not sure what you mean by this. I have checked your computations and my results agree with yours.

5. Mar 28, 2013

### haruspex

Does the offending vehicle have its brakes on?

6. Mar 28, 2013

### Sneakatone

it does not

7. Mar 28, 2013

### haruspex

So... what's the error in the equation I quoted?

8. Mar 29, 2013

### Sneakatone

I should have used 1500kg for mass instead.

9. Mar 29, 2013

### haruspex

Exactly. Does it accept your answer now?