Van and car collision: why is my momentum answer wrong?

[cybernerd]

Homework Statement

A 2100kg van collides with a 1200kg car that is at rest. They lock together and move together at a speed of 4.5 m/s. What is the initial speed of the van?

Homework Equations

Eki = Ekf
1/2mv^2

The Attempt at a Solution

Okay, so I know that the energy before the collision must equal the energy after the collision. I know everything I need to figure out the energy after the collision:

E = 1/2mv^2
(1/2)(2100kg + 1200kg)(4.5m/s)^2
=33412.5 J

So now I need to figure out the other side of the equation using that number. So:

33412.5 = 1/2mv^2
v^2 = 2(33412.5J)/m
=2(33412.5J)/2100kg
Square Root of the result gives me:

5.6 m/s

That looks right to me, but my worksheet says the answer should be 7.07 m/s. What am I doing wrong?

 

[rock.freak667]

Use the law of conservation of linear momentum.

If ever you have two objects colliding and they stick together, kinetic energy is not conserved. This type of collision is called an inelastic collision. Only in an elastic collision is kinetic energy conserved.

 

[cybernerd]

Oh yeah...that's my problem.

I've got it now, thank you so much.