<p>Question:</p>
<p>Which of the following statements are true for a satellite in outer space orbiting the Earth?</p>
<p>I. There are no forces acting on the satellite.
II. The force of gravity is the only force on the satellite.
III. The force of gravity is balanced by the outward force of the object.</p>
<p>A) I only
B) II only
C) III only
D) either I or III may be correct
E) either II or III may be correct</p>
<p>The answer is B, but I don't understand why. Isn't there also a centripetal force acting on the satellite? Or is the centripetal acceleration caused by the force of gravity?</p>
<p>Whenever something moves in circular motion, there has to be a centripetal force to keep it there. But it’s not a new force, ‘centripetal force’ is merely a name given to a force when it keeps something moving in a circle. So in this case, gravity is the centripetal force.</p>
<p>Let’s take this one option at a time:
I. There are no forces acting on the satellite.
This is obviously false due to Newton’s First Law. If there were no forces acting on the satellite, then it would go off in a straight line.</p>
<p>II. The force of gravity is the only force on the satellite.
By drawing a free-body diagram of the satellite, we see that the only force acting on the satellite is the gravitational force from Earth. This is the force that provides the centripetal force needed to keep the satellite moving in a circular motion. So this can be a valid option.</p>
<p>III. The force of gravity is balanced by the outward force of the object.
First of all, there is no outward force acting on the satellite. Second, if there were, then the object would just stay in place. So this is a false option.</p>
<p>Therefore only Option II is the valid one, and the correct answer is B.</p>