Question

When a car accelerates rapidly forward, the heads of the people in the car seem to jerk backward. This is best explained by which of the following:
A. The action is the car accelerating forward; the reaction is the heads accelerating backward.
B. Large forward velocities cause large backward acceleration.
C. The inertia of people’s heads is not as big as the inertia of the car.
D. The people’s heads initially have no force to accelerate them forward. The heads momentarily remain at rest until the necks and/or heads rests apply forward forces to the heads to accelerate them forward.
E. The forward momentum of the car results in an equal backward momentum of the people’s heads to conserve momentum.

Answer 1

Hint:We know Newton’s first law of motion tells us about the concept of inertia. In which it defines that, a body on its own can’t set back to motion and the same a body cannot stop itself from motion on its own. For all the motions the body requires an external force. The capacity of the body to withstand the same state of motion is termed as inertia. The more the mass a body possesses, the more inertia it has.

Complete answer:
When a car accelerates forward, the people sitting in the car experience a jerk backwards. This happens because, as initially the car plus the person system was at rest.But when the car accelerates the feet of the person are only in contact with the car and not the whole body. Due to inertia the body wants to remain at rest but as the feet accelerate with the car.

The car moves forward and the neck remains backward as the feet are in contact they make the neck also to travel forward. This process takes some time and results in a backward jerk. The neck wants to be in rest but the motion moves it forward.

Hence, the correct answer is option D.

Note:Inertia is also useful to us in our daily lives. As when we dust carpet the dust particle remains at rest just the carpet moves which results in cleaning of carpet. And the same case happens when a moving car stops suddenly the passengers experiences a forward jerk as their body is moving forward and the car stops which makes the feet stop but the body is still in motion forward.