# Need help with an inelastic collision problem (1 Viewer)

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#### formulajoe

A car weighing 900 kg is waiting at a stop sign. A car weighing 1200 kg hits the oter car. The cars move .76 m after the collision. The coefficient of friction between the sliding tires and the pavement is .92. I need to find the speed of the car right before the collision.
I dont know what to use. Conservation of momentum doesnt work because I need at least one of the velocities.

#### arcnets

formulajoe,
I think conservation of momentum is the right idea. I think you got to use it.
You got to find the final momentum. The idea is that the cars stick together after collision. So the final momentum is $(m_1 + m_2)v_{final}$. You got to find $v_{final}$.
You can do that. The cars are decelerated by a constant force of friction. This force is $F = .92(m_1 + m_2)g$. From $F$ and $d = .76m$ you can calculate $v_{final}$.

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