NEET Physics Question - Laws of Motion

A man of mass \(80 \text{ kg}\) pushes a box of mass \(20 \text{ kg}\) horizontally. The man moves the box with a constant acceleration of \(2 \text{ m/s}^2\) but his foot does not slip on the ground. There is no friction between the box and the ground, whereas there is sufficient friction between the man's foot and the ground to prevent him from slipping.

Assertion (A): The force applied by the man on the box is equal and opposite to the force applied by the box on the man.

Reason (R): Friction force applied by the ground on the man is \(200 \text{ N}\).

(1) Both (A) & (R) are true and the (R) is the correct explanation of the (A)

(2) Both (A) & (R) are true but the (R) is not the correct explanation of the (A)

(3) (A) is true but (R) is false

(4) Both (A) and (R) are false

Solution:

Assertion (A) is Newton's Third Law, which is true. For the box: \(F_{\text{man-box}} = m_{\text{box}}a = 20 \text{ kg} \times 2 \text{ m/s}^2 = 40 \text{ N}\). By action-reaction, \(F_{\text{box-on-man}} = 40 \text{ N}\). For the man: \(F_{\text{friction}} - F_{\text{box-on-man}} = m_{\text{man}}a). So, \(F_{\text{friction}} - 40 \text{ N} = 80 \text{ kg} \times 2 \text{ m/s}^2 = 160 \text{ N}\). Thus, \(F_{\text{friction}} = 200 \text{ N}\). So (R) is true. But (R) does not explain (A).

Rankers Physics

Man Pushing a Box

Solution:

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