Question

Each of the strobe photographs (a), (b), and (c) in Figure OQ2.18 was taken of a single disk moving toward the right, which we take as the positive direction. Within each photograph, the time interval between images is constant.

(i) Which photograph shows motion with zero acceleration?

(ii) Which photograph shows motion with positive acceleration?

(iii) Which photograph shows motion with negative acceleration?

Short answer

(i) Figure (b) shows motion with zero acceleration.

(ii) Figure (c) shows motion with positive acceleration.

(iii) Figure (a) shows motion with negative acceleration.

Step-by-step solution

Defining constant non zero velocity and acceleration

There should be no net external force exerted on an object that is travelling at a constant non-zero velocity. Otherwise, the object's motion (velocity) would have been adjusted.

In terms of both speed and direction, the rate at which velocity varies over time. If a point or an object moves in a straight line faster or slower, it is said to be accelerated.

Determining motion with zero acceleration

(i) The figure (b) shows the discs have equal spacing.

This means it has constant nonzero velocity and constant zero acceleration.

Thus figure (b) shows motion with zero acceleration.

Determining motion with positive acceleration

(ii) By observing all three figures, it is clear that figure (c) shows positive acceleration throughout the duration of time.

Thus figure (c) shows motion with positive acceleration.

Determining motion with negative acceleration

(iii) Figure (a) shows negative (leftward) acceleration in the first four images.

So, (a) is the correct option.