# Initial velocity (and momentum)

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

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

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

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

Doc Al
Mentor
Exactly.

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?

Doc Al
Mentor
That's correct.

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

Doc Al
Mentor
is that mean the final momentum is also 6200kg*m/s?
Sure. Momentum is conserved.

thanks Doc Al