Explain the following terms.

(a) Force

(b) Unbalanced force

(c) Frictional force

(d) Gravitational force

(e) Static friction

(a) Force: Force is defined as a pull or push acting on a body which tends to change its state of rest or motion, or the speed or direction of motion, or its shape or size. For example, kicking a football applies force that makes it move

(b) Unbalanced force: When two or more forces act on a body and one force is greater than the other, the body changes its state of rest or motion. Such forces are called unbalanced forces. For example, if two boys push a box from opposite sides but one pushes harder, the box moves in the direction of the stronger force.

(c) Frictional force: The force that opposes the relative motion of two bodies is called frictional force. It slows down things by acting in the direction opposite to the motion of the moving body. It is a contact force as it arises due to contact between surfaces. For example, when we slide a book on a table, it gradually slows down and stops because of friction.

(d) Gravitational force: Gravitational force is the force by which the Earth pulls all objects towards itself. It is a non-contact force that acts at a distance. For example, when we throw a ball upward, it falls back due to Earth’s gravity. This is also the reason why we stay firmly on the ground.

(e) Static friction: Static friction is the frictional force that prevents an object from moving when a small force is applied and it has to be overcome to make the object start moving. For example, it is hard to push a heavy box at first because static friction resists its motion. Once the box starts sliding, kinetic friction takes over, which is smaller.