Video Transcript
A satellite orbits Earth at a
constant speed, traveling in a circle around Earth. Which of the following diagrams
correctly shows the forces acting on the satellite, where the forces are shown by
black arrows? The satellite is completely outside
Earth’s atmosphere and has no engine. Is it option (A), (B), (C), (D), or
(E)?
To answer this question, we need to
identify the diagram that correctly shows the forces acting on a satellite which
orbits Earth.
Let’s start by unpicking some of
the statements in the question. We are told that the satellite is
completely outside Earth’s atmosphere. This means that the satellite is
not in contact with anything that could exert a frictional force on it like the
atmosphere. A frictional force always opposes
the motion of the object. So in this case, it would act in
the opposite direction to the satellite’s velocity like this. We are also told that the satellite
has no engine. An engine would exert a forward
force on the satellite in the direction of its velocity like this. Since there’s no friction and no
engine, we know that the correct diagram must show neither of these forces. So, straight away we can rule out
options (C) and (E).
But there must be some kind of
force acting on the satellite, or else it wouldn’t travel in a circle around the
Earth. To understand why this is the case,
we can think about Newton’s first law of motion, which tells us that the motion, and
thus velocity, of an object only changes if there is a net force acting on the
object. Recall that velocity refers to both
the speed and direction of the object. If there was no net force acting on
the satellite, it would move with a constant speed and in a constant direction. This would mean that the satellite
moved on a completely straight line off into space.
But this does not happen. The direction of the satellite’s
motion is constantly changing, which keeps it on a circular path around the
Earth. Since the satellite’s direction is
changing, its velocity must be changing even though its speed is constant. Since the satellite’s velocity is
changing, there must be a net force acting on the satellite. This net force is provided by the
satellite’s weight. Weight is a force which due to
gravity pulls objects towards the center of the Earth. We can represent weight with an
arrow like this. There are no other forces acting on
the satellite. We can see that this matches option
(B), which must be the correct answer to this question.
It is worth noting that although we
got to the right answer by thinking about which forces do and do not act on the
satellite, we could actually have used Newton’s first law to work this out straight
away. We’ve already said that Newton’s
first law tells us that there must be a net force acting on the satellite. Well, (B) is the only diagram that
shows this. In options (A), (C), (D), and (E),
there is no net force acting on the satellite. In option (A), there are no forces
acting on the satellite at all. And in (C), (D), and (E), the
forces that do act are balanced, so the net force is always zero. Even if we knew nothing else about
the situation or what kind of forces act on the satellite, we could have used this
simple rule to work out that option (B) is the correct answer.
