1. The problem statement, all variables and given/known data 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? 2. Relevant equations Average force = change in momentum divided by time 3. 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?