# Homework Help: Impulse and Momentum

1. Nov 7, 2006

### rmarkatos

A 1550kg car, traveling with a velocity of 12m/s plows into a 1220kg stationary car. During the collision the two cars lock bumpers and then move together as a unit. (a) what is tehir common velocity just after the impact?
(b) What fraction of the initial kinetic energy remains after the collision.

(a) Pf=Pi

(m1+m2)vf=m1vi1
(1550kg+1220kg)vf=1550kg(12m/s)
2770kgvf=18600kg m/s

vf=6.71 m/s
I know my answer for part A is correct but part B i really have no idea how to approach it.

2. Nov 7, 2006

### PhanthomJay

What is the initial kinetic energy of the system just before impact? What is the kinetic energy of the system immediately after impact? What is the ratio of the 2 values?

3. Nov 7, 2006

### rmarkatos

would the kinetic energy of the system just be the kinetic energy of the object the first car since the second one was stationary.

By the way the answer to the question is .559J from the odd selected answer section but i am not sure how to get the answer

ke=1/2mv^2
(1/2)(1550kg)(12m/s)^2
ke=111,600J

would the kinetic energy after be the sum of masses of the 2 cars with the shared final velocity?

ke=1/2mv^2
=(1/2)(2770kg)(6.71m/s)^2
=62358J the difference is not equal to .559J

4. Nov 7, 2006

### PhanthomJay

Your calcs are correct, but they did not ask for the difference, they asked for the fraction of initial energy left. KE_f/KE_i = ??

5. Nov 7, 2006

### rmarkatos

ohhhhh your right thanks for clearing that up for me that makes a big difference