SAT Subject Physics Practice Question 667: Answer and Explanation

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Question: 667

3. A car skids a distance of 54 m on a dry road. What was the velocity of the car when the skid began if the coefficient of friction between the tires and the road is .46?

A. 16 m/s
B. 22 m/s
C. 28 m/s
D. 34 m/s
E. Velocity cannot be determined with the information given.

Correct Answer: B


The correct answer is (B). The problem is a work energy conversion. Set the kinetic energy of the car equal to the frictional work to be done by the tires. 1/2 mv2 = Fs. The mass of the car is not given, but if we expand the equation a little more we find it is not necessary. 1/2 mv2 = μNs, which leads to 1/2 mv2 = mgs. At this point the mass cancels out on both sides, which leaves 1/2 v2 = μgs. The working equation becomes . The solution to the problem is

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