Short Notes and Formulas of class 8 science chapter-Force

May 13, 2022, 16:45 IST

Class-8 science chapter-Force

Force is an external effort in the form of a push or pull acting on the body. Forces are used in our everyday actions like pushing, pulling, lifting, stretching, twisting and pressing. A force cannot be seen, tasted or felt. However, we can see or feel the effect of a force. A force can produce the following effects :

• A force can move a stationary body. For e.g. a football at rest moves when it is kicked.
• A force can stop a moving body. For e.g. a moving car stops when the force of brakes is applied.
• A force can change the direction of a moving body. For e.g. when a moving cricket ball is hit by a bat, the ball moves in a different direction due to the force applied by the bat.
• A force can change the speed of a moving body. For e.g. the speed of a falling ball increases because the earth applies a pulling force on it (force of gravity). Whereas, if ball is thrown vertically upwards, the pull of the earth causes the speed of the ball to decrease.
• A force can change the size and shape of a body. 

Types of Force

The forces that we come across in our daily life, are divided into two types:

1. Balanced Forces
2. Unbalanced Forces

Balanced Forces: If the resultant of all the forces acting on a body is zero, the forces are called balanced forces.

Whew balanced forces act on a body (here a heavy box), they do not produce any motion in it.

Unbalanced Forces: If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced forces.

• The heavy box shown in figure can be moved with a very strong force. This is because in that case the force of push will become greater than the opposing force of friction. An unbalanced force will then act on the heavy box and make it move.
• A ball rolling on the ground stops after some time because an unbalanced force of friction acts on it.

Different Types of Forces:

• Force arises from interaction of bodies either due to contact or from a distance. So it can be classified as contact force and non-contact force.

Contact Force is such type of force, which arises when a body is in physical contact with another.

Example: String tension, spring force, reaction force and friction force etc.

Non-Contact Force is such type of force, which does not involve physical contact between two bodies, but act through empty space. It is also known as field force.

• The force of gravitational attraction between two objects is an example of this type of force. Magnet and electric charges can also interact in vacuum.
• Force can also be classified as conservative or non-conservative. If under the action of a force work done in a round trip is zero or work is path independent, the force is said to be conservative otherwise not. Gravitational, electric and elastic forces are conservative while frictional and viscous forces are non-conservative. In presence of Non conservative force there is loss of mechanical energy, which is usually converted into heat.
• Force can also be classified as internal and external. Internal forces are those, which arise from the interactions with other particles that are parts of the system while external forces are those which originate beyond the system under consideration. Same force can be external or internal depending on the system. If we consider a body as the system the force becomes internal. Total internal force acting on a system is always zero as these are parts of action-reaction pairs and cancel.

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