### Video Transcript

Fill in the blank. A velocity is a speed in a
particular direction. Velocity is the change of blank
with time. (A) Distance, (B) displacement, (C)
speed.

This question is asking us to
recall the definition of velocity. Let’s look at each of the options
given to us.

First, we can rule out option
(C). It’s incorrect to say velocity is a
change in speed with time. A change in speed with time is
actually equal to the quantity of acceleration. So this is not the answer.

Next, let’s consider option (A),
the change of distance with time. Recall that this is actually equal
to the definition of speed. The question actually tells us that
a velocity is a speed in a particular direction. This reminds us that, unlike speed,
velocity is a vector quantity. It has to have both magnitude and
direction. For example, if we wanted to
describe the motion of this person, we could say their speed is three miles per
hour. But to describe their velocity,
we’d have to say three miles per hour east. Velocity and speed are similar, but
they are not the same. So option (A) cannot be the
answer.

Next, let’s consider option (B),
the change in displacement with time. Displacement is a similar quantity
to distance. But it is important to be aware
that displacement and distance are not interchangeable. The distance traveled by an object
is equal to the length of the path that it took. The displacement of the object is
the distance from an object’s starting point to its endpoint in a particular
direction. For example, if a person is walking
and they changed direction during their journey, their displacement is different to
the distance they traveled.

Let’s think about this person who
walked from point 𝐴 to point 𝐵 and then changed direction and went to point
𝐶. In this case, the distance traveled
is the length of this path. But the displacement is measured
between the starting point and the endpoint of the journey. So the distance traveled by the
person is greater than the person’s displacement.

It is also important to remember
that displacement is a vector. As well as measuring the magnitude
of the displacement, we must also specify that the displacement is in this
direction. If we were to measure the change in
displacement with time for this person, we would find their speed in this one
particular direction. This is consistent with the
definition of velocity we’ve been given.

So option (B) is the correct
answer. Velocity is the change of
displacement with time.