Satellites and moons
3 marks · EasyA small stone is tied to a string and whirled in a horizontal circle above your head.
The Moon orbits Earth in a similar way, but with no string.
Explain what provides the force that acts like the string for the Moon's orbit.
State what happens to the Moon if the Earth's gravity suddenly disappeared.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Gravity from the Earth acts as the force that pulls the Moon towards the Earth, just like the string pulls the stone.
(b) It would move off in a straight line (at a tangent to its orbit).
Common mistakes: magnetism; air resistance
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Identifies gravity as the force replacing the string for the Moon.
M2 (a) · 1 mark · States gravity pulls the Moon towards the Earth (providing centripetal force).
B1 (b) · 1 mark · States the Moon moves off in a straight line or tangent if gravity disappears.
Worked solution
a) Gravity from the Earth acts as the force that pulls the Moon towards the Earth, holding it in orbit. This force is what keeps the Moon moving in a circle, just as the string does for the stone.
b) If Earth's gravity disappeared, the Moon would move in a straight line at a tangent to its orbit.