A 1000 kg Toyota collides into the rear end of a 2200 kg Cadillac stopped at...

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...a red light.? The bumpers lock, the brakes are locked, and the two cars skid forward 2.9 m before stopping. The police officer, knowing that the coefficient of kinetic friction between tires and road is 0.45, calculates the speed of the Toyota at impact. What was that speed?
 
This sounds like a homework question, so I will just help you out a little.

The energy lost due to friction:

Efr = (coefficienct of friction) * Mass * g (gravity) * distance

= 0.45 * (1000 kg + 2200 kg) * 9.8m/s^2 * 2.9 m

The energy lost is equal to the combined kinetic energy of the vehicles at the time of the collision:

Ekinetic = (1/2) * m * v^2 = 0.5 * 1000kg * v^2

So, put these equations together (Efr = Ekinetic) and you can calculate the velocity (speed) of the vehicle.
 
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