Question

Raj was riding his bicycle. After some time, he got tired and stopped paddling the cycle. The cycle stopped after traveling some distance. Which laws of motion help us to explain this phenomenon?
(A) First law of motion
(B) Second law of motion
(C) Third law of motion
(D) Law of conservation of energy

Answer 1

Hint When raj stops paddling, the bicycle continues to move with its previous velocity. But it encounters the drag force and the friction force which gradually slow down the motion of the bicycle and stop it.

Complete Step by step answer
Whenever raj applies a force on the paddles of the bicycle to rotate them, the bicycle moves forward. When he stops paddling, the cycle still continues to travel (it does not stop abruptly). This demonstrates the first law of motion, the law of inertia.
The first law of motion dictates that an object will continue to be at rest or keep moving with the same velocity until a force is applied to it.
The cycle when not paddled tries to move with the same velocity, but it encounters two other forces, which cause it to slow down. These forces are the Drag force by air and the friction force by the surface. Both of these act opposite to the direction of motion of the bicycle. And they arise due to the motion of the bicycle itself. This demonstrates Newton’s second and third laws of motion.
The value of friction force depends on the weight (the normal force) of the bicycle. Which in turn depends on its mass. The second law of motion states that acceleration of an object depends on its mass.
Whereas the drag depends on the velocity of the bicycle. The third law of motion states that for every action force, there is a reaction force.
Since there is a loss of energy due to the frictional and drag forces, the law of conservation of energy doesn’t apply here.

Therefore options A, B, and C are correct.

Note If the whole universe is taken as a system, then no energy loss occurs due to this event. The drag force, friction force, and heat loss will still remain in the system, thus the energy will be considered as such. But here only the bicycle is considered a system, therefore it does not follow the conservation of energy.