Video Transcript
Which of the following
distance–time graphs shows an object initially moving with constant speed that then
stops moving and then starts moving again with a greater constant speed?
Let’s start by recalling that a
distance–time graph shows us how the total distance traveled by an object varies
over time. As we can see from the answer
options provided, distance–time graphs show distance on the vertical axis and time
on the horizontal axis. We need to figure out which of
these graphs shows an object which moves in the manner described in the
question. So we want to choose the graph that
shows us an object which moves at a constant speed, then stops, then moves again at
a greater constant speed.
There are a couple of rules we can
use to help us do this. Firstly, a horizontal line on a
distance–time graph represents a stationary object. And secondly, a straight sloped
line represents an object moving at a constant speed. Looking at the two graphs provided,
we can see that they both share some characteristics. Firstly, they both consist of three
straight-line segments. And these straight-line segments
follow a similar pattern on each graph. Let’s look at these segments in
time order, that is, moving from left to right on each graph.
We can see that the first line
segment on each graph is a straight sloped line. We know that this represents an
object moving at a constant speed. The next segment on each graph is
horizontal, which we know represents a stationary object. And the third segment on each graph
is again a straight sloped line representing constant speed. So we can see that both of these
graphs show us an object which moves at a constant speed, then stops moving, and
then moves at a constant speed again. The difference between these graphs
relates to how steep these sloped sections are. In order to interpret these
differences, we need to remember another rule. The slope of the line is equal to
the speed of the object. This means that steeper sections of
the graph, like this and this, correspond to faster motion than less steep sections
of the graph, like this and this.
Once we’ve spotted this, we can see
that the graph given in option (A) shows an object which moves relatively quickly,
which then stops moving for a bit and then moves relatively slowly, whereas the
graph given in option (B) shows us an object which moves relatively slowly, then
stops moving, and then moves again relatively quickly. So we can see that the correct
answer is option (B). This graph shows us an object
initially moving with a constant speed that then stops moving and then starts moving
again with a greater constant speed.
