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: Part (a), the swimmer pushing water backward, involves two separate bodies, swimmer and water, exerting equal and opposite forces on each other at the same time, which is the defining condition of Newton's Third Law of Motion.

Step 2: The water's backward-pushed reaction force acts forward on the swimmer, propelling them through the pool, confirming the action-reaction pairing.

Step 3: Part (b) instead compares one applied force across two different masses. From Newton's Second Law, \( F = ma \), so \( a = \dfrac{F}{m} \); for the same force \( F \), a smaller mass \( m \) gives a larger acceleration \( a \).

Step 4: Since a tennis ball has a smaller mass than a cricket ball, the same applied force produces a greater acceleration in the tennis ball, governed by Newton's Second Law. \[ a_{\text{tennis}} > a_{\text{cricket}} \text{ for the same } F \]