# 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

**Explanation:**

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 *mv*^{2} = *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 *mv*^{2} = μN*s*, which leads to 1/2 *mv*^{2} = *mgs*. At this point the mass cancels out on both sides, which leaves 1/2 v^{2} = μ*gs*. The working equation becomes . The solution to the problem is