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# What is the force which creates the jerk on our body when brakes are applied?

“What is the force which creates the jerk on our body when brakes are applied?” (Ab Waheed Ashraff asked)

When we are traveling in a vehicle and when brakes are applied suddenly, we feel a jerk. This is caused by inertia of motion of our body.

Inertia is the inherent property of every body by virtue of which resists any change in its state of rest or the state of uniform motion in a stratight line.

When we are on a moving bus, our body is moving with the same speed as the bus. When brakes are applied suddenly, our feet will come to rest along with the bus, but the upper body will tend to continue with the same speed as before due to its inertia of motion.. Thus we feel the jerk.

Fictitious force (Pseudo Force)

The case can also be explained on the basis of pseudoforce. When brakes are applied, there is a sudden deceleration. Now the frame of reference is not an inertial frame of reference; it is now a non-inertial frame of reference.

When a body is freely placed in such a system, though no net forces are directly acting on the body, it experiences and behaves as if a force is acting on it. The magnitude of this force is equal to the mass of the body multiplied by the acceleration of the frame of reference in the reverse direction.

Fictitious Force = – (mass of body) (acceleration of system)

The direction of pseudoforce will be opposite to the direction of acceleration of the frame of reference. In case of deceleration, the pseudo force will be in the direction of motion. So we feel a jerk in the direction of motion itself when brakes are applied.

PLEASE WATCH THIS LESSON FROM H C VERMA (The author of Concepts of Physics, the book referred by most of the students for preparation for IIT JEE)

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