### Video Transcript

A cat has a mass of three
kilograms. The cat moves four meters in a
straight line in a time of two seconds. What is the momentum of the
cat?

Okay, so first of all, here is
our cat. And we can draw a diagram
showing that it moves four meters in a straight line. Now, since we haven’t been told
which direction the cat actually moves in the question, we can simply assume
that the cat is moving towards the right. This is an arbitrary
choice. But let’s just choose the right
for now.

At this point, we can recall
that momentum, which we’ll call 𝑝, is defined as the mass of an object
multiplied by the velocity of the object. Now, if we’re trying to find
the momentum of the cat, then we need to know the mass of the cat and the
velocity of the cat. Well, luckily, we already know
the mass of the cat. It’s three kilograms. However, we haven’t explicitly
been given the velocity of the cat. But, this doesn’t matter
because we can recall that the velocity of an object is defined as the
displacement of the object, which we’ll call 𝑠, divided by the time taken for
the object to be displaced that much.

Now, since we’ve been told that
the cat moves four meters in a straight line and we can recall that displacement
is the straight line distance between two positions, this means that we know the
displacement of the cat because we know that it starts here, as we’ve drawn in
the diagram, and finishes up here. The straight line distance
between these two positions is four meters. And hence, we know the
displacement of the cat.

As well as this, we know the
time taken for that displacement to occur. We’ve been told that the cat
moves four meters in a time of two seconds. This means that we can work out
the velocity of the cat. This ends up being four meters
divided by two seconds. And because both displacement
and velocity are vector quantities, we need to state their direction. And once again, because we
haven’t been given the direction in which the cat moves, we’ve chosen to say
that the cat is moving towards the right. Hence, the velocity of the cat
is four meters divided by two seconds to the right. Then, when we evaluate this
fraction on the right-hand side, we find that the velocity of the cat is two
meters per second to the right.

So at this point, we know the
mass of the cat and now we know the velocity of the cat. This means we can work out the
momentum of the cat. The momentum ends up being the
mass of the cat, which is three kilograms, multiplied by the velocity, which is
two meters per second to the right. However, at this point, we’re
going to stop using this convention to the right because we haven’t actually
been told this in the question. We just chose it randomly. So we do not want to give this
as a final answer. The only reason we even
mentioned the fact that the cat was moving to the right in the first place was
because we were dealing with vector quantities. However, realistically, if we
haven’t been given the information in the question, then when we give our final
answer for the momentum, we should only give its magnitude or size. However, in general, it’s good
practice to work with directions as well, especially, if we’ve been given that
information.

But, anyway, so the momentum of
the cat, or rather the magnitude of the momentum of the cat, is equal to three
kilograms multiplied by two meters per second. And so we find that the
momentum of the cat is six kilograms meters per second. And this is our final answer to
the question.