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

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

Option a: The tension force in the cords pulling the parachutist upwards.
- This is not the reaction to gravity on the parachutist. The tension in the cords acts on the parachutist as an upward force, balancing some of the weight, but the question asks for the reaction to gravity acting on the parachutist, not the internal force within the parachute system. The cord tension is an effect within the parachute-parachutist system, not the external reaction force to the gravitational pull on the parachutist.

Option b: The force of air resistance pushing upward on the parachute.
- Air resistance or drag is another external force acting upward on the parachutist/parachute, but it is not the reaction to gravity. It is an independent force that balances part of the weight to yield constant speed, yet it is not the reaction force that Newton's third law assigns to gravity.

Option c: The gravitational attraction force of the parachutist pulling the entire planet Earth towards the parachutist.
- According to Newton's third law, every action has an equal and opposite reaction. The gravitational force that Earth exerts on the parachutist is an attraction, and the reaction to that force is the parachutist exerting an equal and opposite gravitational attraction on Earth. This reaction acts on Earth, not on the parachutist, and is the counterpart to the parachutist-Earth gravitational interaction. Although extremely small for Earth, it is conceptually the reaction force described by the action–reaction pair.

Option d: The force of the parachutist pulling down on the parachute.
- This force is part of the internal interaction within the parachute-parachutist system (the parachutist pulling on the parachute). While it contributes to the internal forces and how the parachute behaves, it is not the reaction to the gravitational force acting on the parachutist. The reaction to gravity is the Earth experiencing an equal and opposite pull from the parachutist, not the parachutist pulling the parachute.

In summary, to identify the reaction force to gravity on the parachutist, one must invoke Newton's third law: the Earth and the parachutist exert equal and opposite gravitational forces on each other. The force exerted by the parachutist on the Earth is the reaction to the gravitational force that Earth exerts on the parachutist. The other options describe forces that are either internal to the system or external but not the precise action–reaction pair to Earth's gravitational pull on the parachutist.

MUF0121 Physics Unit 1 - Semester 2, 2025

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

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