Newton's third law: action and reaction

Complete the statement of Newton's third law: "If a body A exerts a force on a body B, then B exerts on A another force with the same $\dots$, the same $\dots$, and $\dots$ direction". Then give an everyday example.

When walking, your foot pushes the ground backwards, yet you move forwards. Identify the action and reaction forces: explain who exerts each one and which body it acts upon.

If you punch a wall, you hurt your hand, even though the wall did not move. Explain, using Newton's third law, where the force that causes the pain comes from.

A swimmer moves forward because her arms push the water backwards. Explain, using Newton's third law, why pushing the water backwards propels her forwards.

For a rowing boat to move forward, the rower pushes the water backwards with the blades of the oars. Explain what force the water exerts on the oars and why the boat moves forward.

Two ice skaters, stationary and facing each other, push off each other with their hands, as shown in the figure. Both slide away in opposite directions.

1. a) Identify the action and the reaction.
2. b) If one has a mass of $40\ \text{kg}$ and the other $80\ \text{kg}$, which one moves faster? Justify your answer by combining the third and second laws.

You inflate a balloon and release it without tying it: it shoots off while deflating. What force propels it? Identify the action and reaction in this case, indicating which body each force acts upon.

A rocket ascends because it expels gases downwards at high speed, as shown in the figure.

Identify the action-reaction pair and explain why the rocket accelerates upwards.

A helicopter stays in the air because its blades push the air downwards. Explain the action-reaction pair and why the helicopter does not need to 'push off' the ground to climb.

A student claims: "If every action has an equal and opposite reaction, the two forces cancel each other out and nothing could ever move." Explain where their mistake lies, reasoning about which body each force acts upon. Think about the case of a person walking.

In a fable, a horse refuses to pull a cart, arguing: 'If I pull the cart forward with a force, the third law says the cart pulls back on me with an equal force; so we can never move.' Gently explain to the horse where its reasoning is wrong, indicating which body each force acts upon and what other forces (those of the ground on its hooves) make the movement possible.

A boy is inside a train carriage and pushes the back wall with all his strength: the carriage does not move. He steps outside onto the platform, pushes the same wall from there, and the carriage begins to roll slowly.

1. a) Explain using Newton's third law why he cannot move it from inside: think about the forces exerted between the boy and the carriage (including at his feet).
2. b) Why can he move it from the outside? Which body receives the reaction force now?
3. c) Relate this to the impossibility of 'lifting oneself up by pulling one's own hair'.