When a swimmer pushes backward against the water with their arms and legs, they move forward through the pool. Which law of motion explains why pushing on the water propels the swimmer in the opposite direction?
- Newton's First Law -- an object stays at rest or keeps moving at a constant speed unless a force acts on it
- Newton's Third Law -- every force one object exerts on a second object is matched by an equal and opposite force from the second object back on the first
- The law of conservation of energy -- energy is transferred from the swimmer's muscles into the water without ever being created or destroyed
- Newton's Second Law -- the acceleration produced depends on the size of the force and the mass being pushed
Why B? And why not the others?
Correct answer: B. Newton's Third Law -- every force one object exerts on a second object is matched by an equal and opposite force from the second object back on the first
A swimmer moving forward relies on Newton's Third Law: pushing backward on the water with hands and feet is an action force, and the water pushes back on the swimmer with an equal and opposite reaction force, propelling them forward through the pool. The option describing the law of inertia is wrong because that law only describes how an object behaves when no net force acts on it, and says nothing about paired forces between two interacting objects. The option describing conservation of energy is wrong because it addresses how energy moves and changes form, not why two objects experience opposite forces of the same size. The option describing the relationship between force, mass and acceleration is wrong because that law explains how a single object accelerates under a given force, not why the water exerts a force back on the swimmer in the first place.
Source: BBC Bitesize GCSE Physics: Newton's Laws of Motion