Which one of the following statements is true ?
- A.The force of gravity of the Earth on the Moon is greater than the force of gravity of the Moon on the Earth.
- B.The force of gravity of the Moon on the Earth is greater than the force of gravity of the Earth on the Moon.
- C.The force of gravity of the Earth on the Moon and of the Moon on the Earth are equal in magnitude and are in the same direction.
- D.The force of gravity of the Earth on the Moon and of the Moon on the Earth are equal in magnitude but are in opposite directions.
Correct answer
Explanation
The correct answer is D: the two forces are equal in magnitude but opposite in direction. This follows from Newton's third law and his law of gravitation.
Newton's law of gravitation gives the force between two bodies as F = Gm₁m₂/r². The same expression applies to both bodies, so the pull of the Earth on the Moon and the pull of the Moon on the Earth have exactly the same size. Each force acts along the line joining their centres and points towards the other body, so they are opposite in direction. They form an action-reaction pair. The Earth moves far less in response only because its mass is about 81 times that of the Moon, so its acceleration is much smaller.
A is wrong: the Earth's larger mass does not make its pull larger, since both masses appear in the formula. B is wrong for the same reason. C is wrong: the forces point towards each other, not the same way.
Exam tip: gravitational forces between two bodies are always equal and opposite; only the accelerations differ, inversely with mass.
Practice Questions
View allA negative work is done when an applied force F and the corresponding displacement S are
- A.perpendicular to each other.
- B.parallel to each other.
- C.anti-parallel to each other.
- D.equal in magnitude.
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Work is negative when the force acts opposite to the displacement, because the angle between them is then 180°. Work done is W = F S cos θ, where θ is the angle between the force and the displacement.
Read the full explanation →Which one of the following statements about a satellite orbiting around the Earth is correct ?
- A.Satellite is kept in orbit by remote control from ground station.
- B.Satellite is kept in orbit by retro-rocket and solar energy keeps it moving around the Earth.
- C.Satellite requires energy from solar panels and solid fuels for orbiting.
- D.Satellite does not require any energy for orbiting.
Show answer
Once a satellite is placed in orbit with the right speed, the Earth's gravity keeps it going round without any engine.
Read the full explanation →Which one of the following is not a conservative force ?
- A.Frictional force
- B.Electric force
- C.Gravitational force
- D.Spring force
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Friction is a non-conservative force because the work it does depends on the path taken and the energy it removes is lost as heat.
Read the full explanation →A microphone converts
- A.electrical signals to sound waves
- B.sound waves to electrical signals
- C.microwaves to sound waves
- D.sound waves to microwaves
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A microphone is a transducer that changes the pressure variations of sound into a matching electrical signal. In a common dynamic microphone, sound waves make a thin diaphragm vibrate.
Read the full explanation →When a light ray passes through from air to water with a non-zero angle the ray will be
- A.bending towards the normal
- B.bending away from the normal
- C.propagating in straight line
- D.reflected towards the opposite direction
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Light slows down when it enters water from air, so a ray that strikes the surface at an angle bends towards the normal.
Read the full explanation →