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## Homework Statement

A car of mass 10 kg hits another car at 10 m/s and they both come to rest at the same time. In the process, the car that was hit moved back 10 meters. What is the average force exerted on it?

## Homework Equations

Average force = change in momentum divided by time

## The Attempt at a Solution

I'm taking it so everything is going to the right, so positive rightwards.

Final momentum is 0. Initial is 10kg * 10m/s so: Average force = -100kg*m/s (because final - initial) divided by the time which is 1 second I think. 10m = 10m/s * t = 1 second.

So -100kg*m/s divided by 1 second = -100N.

But this is wrong. I can't find my error but I'm told I calculated the time incorrectly. How do I get the correct time?