25. A car is accelerating when it is (a) traveling on a straight, flat road at 50 miles per hour. (b) traveling on a straight uphill road at 30 miles per hour. (c) going around a circular track at a steady 100 miles per hour.

(c) going around a circular track at a steady 100 miles per hour.

Explanation:

The acceleration of an object is equal to the rate of change of velocity of the object:

where

is the change in velocity of the object

is the time interval

We notice that velocity is a vector, so it has both a magnitude and a direction. This means that , the change in velocity, is either caused by a change in magnitude of velocity, or by a change in the direction.

Therefore, the acceleration is non-zero if at least one of the two is verified:

- There is a change in magnitude of the velocity

- There is a change in direction

Let's now analyze the three statements:

(a) traveling on a straight, flat road at 50 miles per hour. --> here there is no change in magnitude or direction of the velocity, so there is no acceleration.

(b) traveling on a straight uphill road at 30 miles per hour. --> here there is no change in magnitude or direction of the velocity, so there is no acceleration.

(c) going around a circular track at a steady 100 miles per hour. --> here there is no change in magnitude of the velocity, but the direction is changing (circular track), therefore there is acceleration.

c) up is the direction the magnetic force is acting on the charge.