Question

When a speeding bus stop suddenly, passengers are thrown forward from their seats because:
A. The back of the seat suddenly pushes the passengers forward.
B. Inertia of rest stops the bus and takes the body forward.
C. upper part of the body contains to be in the state of motion whereas the lower part of the body in contact with the seat remains at rest.
D. upper part of the body comes to rest whereas the lower part of the body in contact with the seat begins to move.

Answer 1

Hint: This question is based on the first law of Newton. Here we will first see the statement for Newton’s first law and then see the definition of inertia. After this we see different types of inertia. The answer for the question will be obtained when talking about different types of inertia.

Complete answer:
Newton’s laws are part of classical mechanics to understand the motion of objects. Newton presented his three laws of motion in 1687 in his ‘philosophiae Naturalis principia Mathematica’, usually called the Principia.
Let us see about the Newton’s first law:
According to Newton’s first law:
If net force on a body is zero, if the body is at rest , it will remain at rest and if the body is moving with constant velocity, it will continue to do so.
The first law of Newton gives the definition of term inertia.
Inertia:- It is property of the body to resist the change in its state unless an external force is applied on it. There are three types of inertia:
(a) Inertia of rest- Here if the body is in rest, it will remain in rest unless an external force acts on it.
(b) Inertia of Motion- If the body is moving with a constant velocity then will remain moving with that velocity unless an external force cats on it.
(c) Inertia of direction- A body moving in particular direction will move in that direction unless a force acts on it.
Now coming to the question, here the bus is in the motion with constant velocity. Passenger seating on his seat in the bus will have inertia of motion as no net force acts on the passenger. But as soon as the bus stops the lower part of the body comes to rest instantaneously but the upper part of the body is still in motion due to inertia of motion. Therefore, the passenger will fall ahead.
So, option C is correct.

Note: For Newton's first law to be valid, the net force on the object should be zero i.e. either no force acts on the object or if forces act on the object then their resultant must be zero. Newton’s first law only applied in inertial frames and this law helps to identify inertial frames.