A parachutist descends toward the Earth at a constant speed. The reaction force to the force of gravity acting on the parachutist is:

Question

BlackTom AI · Accepted Answer

Newton's third law states that for every force there is an equal and opposite force. The weight (gravitational force) on the parachutist is the Earth pulling the parachutist downward; its reaction is the parachutist pulling the Earth upward with the same magnitude.

1. Relevant knowledge points and concepts
- Newton's third law: forces come in action–reaction pairs, equal in magnitude and opposite in direction, and each acts on a different body.
- Weight: gravitational force from Earth on a mass m is F = G M_earth m / r^2 (approximately mg near Earth's surface).
- Other forces (air resistance, tension) are different interaction pairs and have their own reaction partners.

2. Step-by-step reasoning
- Identify the action force: Earth exerts gravitational force on the parachutist (the parachutist’s weight).
- By Newton’s third law the reaction must be a gravitational force of the parachutist on the Earth, equal in magnitude and opposite in direction.
- The fact the parachutist descends at constant speed only tells us the net force on the parachutist is zero (weight balanced by upward air resistance/tension), but it does not change the identity of the reaction to gravity.

3. Intermediate expressions
- Weight on parachutist: F_E_on_p = G M_earth m / r^2 ≈ m g downward.
- Reaction force: F_p_on_E = −F_E_on_p = −m g (upward on Earth). Magnitudes equal: |F_E_on_p| = |F_p_on_E|.

4. Why the correct answer is correct
- The reaction to the gravitational pull by Earth on the parachutist is the gravitational pull by the parachutist on Earth; this is exactly option b.

5. Why other options are incorrect
- a: "parachutist pulling down on the parachute" is a different interaction (parachutist–parachute), not the gravitational action–reaction pair.
- c: "air resistance pushing upward" is a separate force (air on parachute), not the reaction to gravity.
- d: "tension in cords pulling upward" is an internal contact force in the parachute–parachutist system, not the gravitational reaction.

6. Conclusion
- Therefore the reaction force to gravity acting on the parachutist is the gravitational attraction of the parachutist pulling the entire Earth toward the parachutist (option b), which matches the provided answer.

A parachutist descends toward the Earth at a constant speed. The reaction force to the force of gravity acting on the parachutist is:单项选择题

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