A boxer punches an opponent on her nose and knocks her over. The force of the fist acting on the nose is:

Question

BlackTom AI · Accepted Answer

Newton's third law states that when two bodies interact they exert forces on each other that are equal in magnitude and opposite in direction.

1) Relevant concepts
- Newton's third law (action = -reaction): F₁ on 2 = -F₂ on 1.
- Newton's second law relates force to acceleration: F = ma. Different masses or durations give different accelerations even with the same force.
- Impulse (force × time) changes momentum; equal and opposite forces produce equal and opposite impulses.

2) Step-by-step reasoning
- When the boxer’s fist contacts the opponent’s nose, the fist applies a force on the nose (call it Ff→n).
- Simultaneously, the nose applies a force on the fist (Fn→f). By Newton’s third law, Fn→f = −Ff→n, so the magnitudes are equal: |Fn→f| = |Ff→n|.

3) Illustrative calculation (intermediate steps)
- Assume the fist exerts a force of F = 200 N on the nose (example number).
- By Newton’s third law the nose exerts −200 N on the fist; magnitude 200 N. (No further calculation is needed to show equality.)
- If the nose (or head) effective mass is 5 kg, acceleration of the head = a = F/m = 200/5 = 40 m/s². If the boxer’s body mass is 70 kg, acceleration of the boxer’s fist/body = 200/70 ≈ 2.86 m/s². Different accelerations occur because masses differ, not because forces differ.

4) Why the correct answer is correct
- The forces are an action–reaction pair: same magnitude, opposite direction. The fact that the opponent is knocked over comes from the nose/head having a different mass and receiving a different net effect (and possibly balance loss), not from unequal contact forces.

5) Why the other options are incorrect
- Option (a) “less than” contradicts Newton’s third law which requires equality of magnitudes.
- Option (b) “greater than” likewise contradicts Newton’s third law.

6) Conclusion
- Therefore the force of the fist acting on the nose is the same as the force of the nose on the fist. This confirms answer c.

A boxer punches an opponent on her nose and knocks her over. The force of the fist acting on the nose is:单项选择题

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