A couple is traveling by car down a straight highway at 45 km/h . They see an ac

Question

A couple is traveling by car down a straight highway at 45 km/h . They see an accident in the distance, so the driver applies the brakes, and in 7.0 s the car uniformly slows down to rest.

The direction of the acceleration vector is

A. opposite to the velocity vector.
B. in the same direction as the velocity vector.
C. at 90 relative to the velocity vector.

By how much must the velocity change each second from the start of braking to the car’s complete stop? Find the magnitude of the acceleration.

Express your answer using two significant figures.

Explanation / Answer

The direction of the acceleration vector is (A) opposite to the velocity vector. That's why the car's speed decreases to zero.

The velocity must change by (-45 km/h)/(7.0 s) = -6.4 km/h/s.

Normally acceleration is stated in m/s2, and if that is what you want, you must convert km/h to m/s.

45 km/h = 45000 m/h = 45000/3600 m/s = 12.5 m/s

And in that case, the magnitude of the acceleration is (12.5 m/s) / 7.0 s = 1.8 m/s2.

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