Question

Give reason:
(a) While riding a bicycle, if we stop pedaling the bicycle begins to slow down
(b) Athletes in pole jump events fall on cushioned surfaces and not on the floor.

Answer 1

Hint:In the first part, we will be discussing the frictional forces which lead to slowing down the bicycle. While in the second part, we will learn about the impulsive force which helps the athletes from not getting injured.

Complete answer:
(a) Firstly, we will revise our concepts for frictional force. Frictional force is generated when two surfaces are either contacting or sliding against each other. Frictional force depends upon the force impelling them together and to the surface texture. And in our question peddler is also feeling frictional force with the road. When we stop pedaling, the bicycle begins to slow down. The reason is friction starts acting opposite to direction of motion which leads to slowing down the bicycle.

(b) In the second part, athletes in pole jump events fall on cushioned surfaces and not on the floor. The reason is to increase the time of fall of the athlete and to lower the momentum of the fall of the athlete.Let us assume the athlete is jumping on the ground, in this case he will make a short contact with the ground and in this short contact time momentum will come to zero.This short momentum will give high impulsive force to ground and will hurt him. While using a cushioned surface, impulsive force being passed to the cushion reflects negligible reactive force and he will get hurt.Hence, cushioned surfaces are used by the athletes in the pole event.

Note:An impulsive force is generated in the collision that results in change in velocity or momentum on the objects experiencing the collision. The force that acts for a very short interval of time on an object.Impulse is defined as the product of net external force and time. $\Delta p = {F_{net}}\Delta t$