# Physics

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A 920 kg sports car collides into the rear end of a 2300 kg SUV stopped at a red light. The bumpers lock, the brakes are locked, and the two cars skid forward 2.5 m before stopping. The police officer, knowing that the coefficient of kinetic friction between tires and road is 0.42, calculates the speed of the sports car at impact. What was that speed?

• Physics - ,

first, find from momentum the speed of the two cars just after impact.

920V=(920+2300)Vi
or Vi= 920/(3220) V

Now,
where a= F/m= mu*(3220)g/3220
Vi^2=.42g*2.5

Find Vi, then put it in

V= Vi(3220/920)

• Physics - ,

I got that vi=10.29 then when I sub in I get that v=11.23 but this is the wrong answer. Am I doing something wrong?

• Physics - ,

You forgot to put the 2 back in when finding Vi.

It says Vi^2= 0.42*g*2.5

It should be Vi^2= 2*0.42*g*2.5

That should take care of the wrong answers you come up with. (I know this is kind of late to answer your question but it might help someone else later on.)

• Physics - ,

@Ryan, is the "g" just gravity? Why do we need gravity in this equation? I keep doing this problem and getting the initial speed to 3.6 m/s which is not right.