Question

When a fast moving vehicle is stopped suddenly the person sitting in it tend to fall forward. This is due to _________.

• A. inertia of rest.
• B. inertia of motion
• C. inertia of direction.
• D. All the above.

Answer 1

The correct option is B

inertia of motion

Explanation of correct option:

Option (B).

When a fast moving vehicle is stopped suddenly the person sitting in it tend to fall forward. This is due to inertia of motion.

Inertia of motion:

1. If a body is in a state of uniform motion, it continues to remain in the same state unless an external force act on it.

Example:

1. When a boy who is running is tripped by a stone, he falls, because his feet come to rest whereas the upper portion of his body continues to move forward due to inertia of motion.
2. A passenger standing in a moving bus tends to fall forward when the driver of the bus suddenly applies the brakes. This is because the feet of the passenger (which are in contact with the bus) come to rest along with the bus but the upper portion of the body continues to move forward due to inertia of motion.

Explanation of incorrect options:

Option (A). Inertia of rest:

1. lt is the resistance of the body to change its state of rest or of motion.

Example:

1. When books are piled one above the other and the lowest book is suddenly pulled out, the rest of the books remain intact. This is due to inertia of rest.
2. When the bus starts suddenly, the standing passengers in the bus tend to fall backward. When the bus is at rest, the entire body of a passenger is at rest but when the bus starts suddenly, the lower portion of the body of passenger which is in contact with the bus moves along with the bus while the upper part remains at rest due to inertia of rest. Hence, the standing passengers tend to fall backwards.

Option (C). Inertia of direction:

1. It is the inability of a body to change its direction of motion on its own.

Example:

1. When a car takes a sudden turn towards the left, the person seated inside it is pushed towards the right. This is because when the car takes a sharp turn it changes its direction whereas the person seated inside tends to move in the original direction due to inertia. Hence, he is pushed towards the right.

Thus, if a fast moving vehicle is stopped suddenly the person sitting in it tend to fall forward, is due to inertia of motion.

Hence, the option B is the right answer.