Question

Which of the following describes a situation requiring net force?
(A) A car starts from rest and reaches a speed of $80kmph$ after 15 seconds.
(B) A bucket is lowered from a rooftop at a constant speed of $2 m/s$.
(C) A skater glides the ice, gradually slowing from $10 m/s $ to $5m/s$.
(D) A body moving along a straight line path with constant speed.

Answer 1

Hint: When we define the physical quantity ‘Force’ it is defined in physics as the push or pull on a mass object that causes it to change velocity. A force is an external agent capable of altering a body's state of rest or motion. There is a magnitude and a direction to it. To measure the force, we use the S.I. unit Newton ($N$).

Complete step by step answer:
We can assume that if anything is subjected to force it either changes its shape or starts to be in motion. Considering the mechanics system it is considered that a force either leads a body to be in motion when it is at rest or it can stop a body if it is in motion. So, if a body is in motion under constant velocity or it is at rest then no net force acts on that body or we can assume that the net force on these bodies is zero. Here, the 1st option in which a car reaches a speed of $80kmph$ after 15 seconds has a constant or net force acting on it and the 3rd option where skater gradually slows down is also an example of net force acting on it.
Therefore, both ‘A’ and ‘C’ are correct.

Note:
We can see that a body velocity either remains constant or the body is under rest without making any kind of changes if the net force is zero on that body. The force allows the body either to increase its velocity or to change its state of rest to motion or from motion to rest, it also changes the direction of the moving body.