Newton's First Law And Inertia

Question

A passenger standing in a bus suddenly lurches forward when the moving bus comes to an abrupt halt at a signal. Which principle best explains this sudden forward motion of the passenger?

RankGuru · Accepted Answer

As stated in NCERT Class 11, Chapter 5 (Laws of Motion), Newton's First Law tells us that a body continues in its state of uniform motion unless acted upon by an external force; this tendency is called inertia. When the bus is moving, the passenger's whole body shares the bus's forward velocity. When brakes are applied, the floor and the feet of the passenger decelerate quickly due to friction, but the upper body, having no immediate force acting on it, tends to preserve its forward velocity. This unbalanced situation throws the upper body forward, which is inertia of motion. The option inertia of rest is wrong because that describes the backward jerk felt when a stationary bus suddenly starts. Conservation of momentum is incorrect because the bus and passenger are not an isolated system here; the road exerts an external braking force. Action-reaction pairing is irrelevant because the two forces in a Newton pair act on different bodies and never explain a single body's motion. Plausibility check: the effect is forward on stopping and backward on starting, exactly matching everyday experience, confirming inertia of motion as the cause.

Newton's First Law And Inertia

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