# Initial velocity (and momentum)

1. Nov 28, 2007

### shiri

A 2020kg car moving at 20.0m/s collides and locks together with a 1400kg car at rest at a stop sign. Show that momentum is conserved in a reference frame moving at 10.0m/s in the direction of the moving car.

initial momentum

final momentum

I don't understand why that the answer for initial momentum is not 20200 kg*m/s due to a reference frame moving at 10.0m/s in the direction of the moving car. Can anyone help me out here, that will be awesome

Last edited by a moderator: Jan 7, 2014
2. Nov 28, 2007

### Staff: Mentor

In that new reference frame, what's the initial velocity (and momentum) of the 1400 kg car?

Last edited by a moderator: Jan 7, 2014
3. Nov 28, 2007

### shiri

was it 10.0m/s and 14000kg*m/s in the opposite direction?

4. Nov 28, 2007

### Staff: Mentor

Exactly.

5. Nov 28, 2007

### shiri

then do I have to sum it together (20200+ (-14000))kg*m/s? and the answer should be 6200kg*m/s for the initial momentum?

6. Nov 28, 2007

### Staff: Mentor

That's correct.

7. Nov 28, 2007

### shiri

is that mean the final momentum is also 6200kg*m/s?

8. Nov 28, 2007

### Staff: Mentor

Sure. Momentum is conserved.

9. Nov 28, 2007

### shiri

thanks Doc Al