Question

According to Newton's Third Law of Motion (Law of Action and Reaction), forces always occur in equal and opposite pairs. Which of the following sports movements directly demonstrates this law during execution?

• A. \( \text{A swimmer pushing backward against the water to drive their body forward through the pool.} \)
• B. \( \text{An athlete choosing a heavier shotput ball to slow down its acceleration rate.} \)
• C. \( \text{A sprinter continuing to slide along the track after crossing the finish line.} \)
• D. \( \text{A passenger leaning sideways as a bobsled rounds a sharp corner.} \)

Hint:

To identify a Third Law example on the exam, look for movements that involve pushing against a surface or medium (like jumping off the ground, rowing a boat, or swimming) to generate a forward reaction force.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Isaac Newton’s Third Law of Motion states: "For every action, there is an equal and opposite reaction."
Crucially, these forces act on {different} bodies. If Object A exerts a force on Object B, Object B exerts a force of equal magnitude but opposite direction back on Object A.
In sports, this law is fundamental to locomotion. We move forward by pushing {against} something else (the ground, water, or a starting block).
Step 2: Detailed Explanation:
1. Swimming Mechanics (Newton's 3rd Law):
When a swimmer performs a stroke, they apply a force by pushing the water backward with their palms or feet. This is the Action.
According to the 3rd Law, the water exerts an equal and opposite force on the swimmer's body, pushing them forward. This is the Reaction.
Without this reaction force from the water, the swimmer would remain stationary.
2. Evaluating other options:
- Option (B): Relating mass to acceleration ($F=ma$) is a direct application of Newton's Second Law.
- Option (C): A sprinter sliding past the finish line is demonstrating Inertia—the tendency of an object in motion to stay in motion. This is Newton's First Law.
- Option (D): Centrifugal force and the tendency to lean relate to inertia and circular motion, falling primarily under the First and Second laws.
Step 3: Final Answer:
The swimming example perfectly illustrates the duality of action and reaction forces between two mediums (the athlete and the water).