A car travels a certain distance along a straight road and on the way must stop

Question

A car travels a certain distance along a straight road and on the way must stop at traffic lights and obey the local speed limits. The Figure shows the distance the car travels as function of time. Choose all the correct answers (i.e., B, AC, BCD . . .) which apply to the speed of the car.

Refer to the Figure and choose ALL the correct answers which apply to the acceleration of the car.

     A) The acceleration is non-zero at 2.3 min.
     B) The acceleration is not zero at 3.3 min.
     C) The acceleration is non-zero at 6.4 min.
     D) The acceleration is never negative.

E) The acceleration is zero at 0.6 min and 3.7 min.

Explanation / Answer

A. Speed is given by the slope of distance time graph. Since slope of distance time graph is constant from 2 min to 3 min, speed is constant. Hence acceleration is zero at 2.3 min. A is incorrect.

B. We can see from the plot that the car is decelerating at 3.3 min. Hence B is correct.

C. Car is moving at constant spped at 6.4 min. Hence acceleration is zero. C is incorrect.

D. After 3 min the car decelerates until it speed becomes zero. Hence the acceleration is negative. D is incorrect.

E. At 0.6 min and 3.7 min car is not moving hence both speed and acceleration are zero. E is correct.

B and E are the correct options.

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