Question

Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?

Answer 1

Hint: Inertia is the property of any body by virtue of which it tends to remain in its state. For example, if it is at rest, it tends to remain at rest and if it is in motion, it will tend to remain in motion.

Complete Step by Step Solution:
Inertia plays an important role in such problems.

Inertia is the property of any body by virtue of which it tends to remain in its state until an external force is applied. For example, if it is at rest, it tends to remain at rest and if it is in motion, it will tend to remain in motion.

When a bus is moving, the people sitting inside it will also be moving with the same speed. So, the bus and the people inside, both are in motion. When brakes are applied, the bus starts stopping. But, the people sitting/standing inside the bus tend to remain in motion due to the property of inertia. Since the lower part of their bodies is in contact with the bus, it stops first and the upper part if their body stops after that. The upper part resists the change of state of motion due to inertia. Therefore, they tend to fall in the forward direction.

When a bus is at rest, the people sitting inside will also be at rest. The part of their bodies is in contact with the bus. Due to inertia of rest, these bodies tend to remain at rest. When the bus accelerates, the lower part of their bodies accelerates first. But, the upper part of their bodies tends to remain at rest state due to inertia. So, they tend to fall backwards. Further, the upper part of their bodies also starts moving along with the bus.

Additional Data:
There are three types of inertia- Inertia of rest, inertia of motion and inertia of direction.

Note: The students may commit mistakes if they ignore the fact that the lower parts of the bodies of people sitting/standing in a bus are in direct contact with the bus. Inertia holds their upper part of the body at the same state for a short time interval.
The concept of falling comes because the person feels a torque (a turning force) due to the combined effect of the change in state of the bus and the inertia of his/her upper body.