Video Transcript
The following images show the
positions of objects at times one second apart. The objects start from rest. Which image shows an object that
has uniform acceleration. Is it (A), (B), or (C)?
The question asks about the motion
of some box-shaped objects. The objects all accelerate from
rest. An object that accelerates
uniformly increases its speed by equal amounts in equal time intervals. The images do not directly show the
speeds of the objects in equal time intervals. However, the images show the
positions of the objects at one-second time intervals.
It is possible to compare the
average speeds of an object in the time intervals between two different positions of
the object. The greater the distance between
the position of the object at the start of the time interval and at the end of the
time interval, the greater the average speed of the object in that time
interval. For example, we can see here that
the average speed of this object is greater between one second and two seconds than
it is between zero seconds and one second.
Let us now look at the options in
the question.
First, option (A), we can see that
the speed of the object increases in each of the time intervals except for the last
time interval. In the last time interval, the
speed of the object decreases. As the speed of the object changes
from increasing to decreasing, the object cannot have been uniformly
accelerating. We reject option (A).
Let us now look at option (B). We can see that the speed does not
decrease in any of the time intervals. Let us look closely at two of these
time intervals between one and two seconds and between two and three seconds. We see that the distance between
the positions of the object at the start and end of each interval are equal. In these intervals, the object had
zero acceleration. In other time intervals though, the
speed of the object did increase. And so the acceleration of the
object was nonzero. The object cannot then have
accelerated uniformly. We reject option (B).
Finally, consider option (C). Here, the distances traveled in
successive time intervals always increases. We can see then that option (C)
shows a uniformly accelerating object. Option (C) is correct.
