Question

In the examples given below, which of Newton's Law of Motion will apply? Justify your answer.
(a) When a Swimmer pushes the water backwards.
(b) When a cricket and tennis ball are thrown with the same force, the tennis ball moves with greater acceleration.

Hint:

Always remember: Third Law is about pairs of forces, while the Second Law is about the result of force on a specific mass.

Answer 1

Step 1: Understanding the Concept:
Newton’s Laws explain the relationship between a body and the forces acting upon it. We must identify which specific principle (Inertia, Acceleration, or Action-Reaction) governs the movement.
Step 2: Detailed Explanation:

1. (a) Swimmer pushing water backwards: This applies Newton's Third Law of Motion (Law of Action and Reaction). - Justification: The swimmer exerts a force on the water (Action), and the water exerts an equal and opposite force on the swimmer (Reaction), pushing them forward.
2. (b) Cricket vs. Tennis ball: This applies Newton's Second Law of Motion (Law of Acceleration). - Justification: The law states $F = ma$ (Force = mass $\times$ acceleration). If Force ($F$) is constant, acceleration is inversely proportional to mass ($a \propto 1/m$). Since a tennis ball has less mass than a cricket ball, it achieves greater acceleration.
Step 3: Final Answer:
(a) Third Law (Action-Reaction). (b) Second Law (Force and Acceleration).