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 — D, the forces are equal in magnitude but opposite in direction. By Newton's third law, the gravitational pull of the Earth on the Moon and of the Moon on the Earth form an action–reaction pair: equal in magnitude and opposite in direction, regardless of the huge difference in their masses.
- (a)Earth's pull on Moon is greater — Wrong — the two gravitational forces are equal in magnitude; the Earth being more massive does not make its pull on the Moon larger than the Moon's pull on it.
- (b)Moon's pull on Earth is greater — Wrong for the same reason — the action–reaction forces are equal in magnitude, not larger for the smaller body.
- (c)equal and in the same direction — Wrong on direction — action and reaction act on different bodies and point in opposite directions, not the same direction.
Newton's law of universal gravitation gives the same F = G m1 m2 / r^2 for the force each body exerts on the other, and Newton's third law makes them an action–reaction pair. So Earth↔Moon gravitational forces are equal in magnitude and opposite in direction. (The Moon accelerates more only because it has far less mass, by F = ma.)
Don't let the mass difference mislead you: gravitational action–reaction forces are always equal and opposite. The unequal accelerations come from unequal masses, not unequal forces.
- Newton's third law: forces come in equal, opposite action–reaction pairs.
- Gravitation: each body pulls the other with F = G m1 m2 / r^2 — the same magnitude.
- The Moon accelerates more than the Earth only because it has less mass.
- Action and reaction act on different bodies, so they never cancel.
- Thinking the more massive body exerts the larger force.
- Confusing equal forces with equal accelerations.
Asked as comparing the Earth–Moon gravitational forces, testing Newton's third law.
No directly related past PYQ was found.
- practice — not a real PYQ
Two objects of different mass attract each other gravitationally. The forces they exert on each other are
- (a)larger for the heavier one
- (b)larger for the lighter one
- (c)equal in magnitude, opposite in direction
- (d)zero
Answer(c) equal in magnitude, opposite in direction.
- practice — not a real PYQ
If Earth's pull on the Moon and the Moon's pull on Earth are equal, why does the Moon orbit the Earth?
- (a)the forces are unequal
- (b)the Moon has much less mass, so it accelerates more
- (c)gravity acts only on the Moon
- (d)the Earth is fixed in space
Answer(b) the Moon has much less mass, so it accelerates more.