Term I - Revision I - Exploring forces — lesson. Physics CBSE Live product, Class 8 (2026-27).

Forces are pushes or pulls that can change the state of motion of objects. They are mainly classified into contact forces, which need physical contact, and non-contact forces, which act without touching. These forces play an important role in everyday life and natural phenomena.

Effects of applying force on objects:

The application of force on an object can produce the following effects.

1. Starting motion:

A force can make a stationary object move, such as pushing a door to open it.

2. Changing speed:

A force can increase or decrease the speed of a moving object, like applying brakes to slow down a bicycle.

3. Changing direction of motion:

A force can change the direction in which an object is moving, for example, hitting a moving ball with a bat.

4. Changing shape:

A force can change the shape of an object, such as stretching a rubber band, squeezing a lemon or rolling a chapati.

5. Producing multiple effects:

In some cases, a force may cause more than one effect at the same time.

Interaction between objects:

When you push a table, your hand applies a force on the table. In this situation, your hand and the table are two objects that interact with each other. This interaction results in the application of force.

Objects involved in applying force:

In all the actions, there are always at least two objects involved. For example, a bat and a ball, a hand and a drawer, or a foot and a football. Force is produced only when two objects interact with each other in some way.

From these examples, we can infer that a force comes into existence only when there is interaction between two objects. A single object by itself cannot produce a force.

Force:

A force is defined as a push or a pull on an object resulting from its interaction with another object.

Unit of force:

The SI unit of force is the newton, written with a small ‘n’, and its symbol is \(N\).

Forces acting on a stationary object:

If an object is at rest, it does not mean that no force is acting on it. Instead, it means that the forces acting on the object are balanced. When forces balance each other, there is no change in the state of rest or motion of the object.

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Balanced force acting on the wooden block

Forces act in pairs:

When you push a table or car with your hand, you also feel a force acting on your hand.
This force exists only as long as you are pushing the table or car.
The moment you stop pushing, the force on your hand disappears.
This shows that whenever two objects interact, each object experiences a force exerted by the other.
When the interaction between the two objects stops, the forces acting on them also cease.

Contact forces:

Contact forces are forces that act only when two objects are in physical contact with each other.

Muscular force:

Muscular force is the force generated by the contraction and relaxation of muscles in the body during physical activity.

Role of muscular force:

It helps us perform movements like walking, running, lifting, pushing, and stretching. Animals and humans also use it for carrying loads and daily activities. Inside the body, it helps in chewing food and pumping blood through the heart. It is an important example of a contact force.

A Bullock cart

Friction:

Friction is the force that comes into play when an object moves or tries to move over another surface. It always acts in a direction opposite to the motion or the attempted motion of the object.

Cause of friction:

Friction is a contact force because it occurs when two surfaces are in contact. It exists only when objects touch each other.

Friction is caused by tiny irregularities on surfaces. Even smooth surfaces have small bumps that interlock and resist motion when they slide over each other.

Friction in liquids and gases:

Friction also acts in liquids and gases, not just solids. Moving objects face resistance from air and water while passing through them. To reduce this drag, vehicles like aeroplanes, ships, boats, cars, and trains are given streamlined shapes for smoother and faster movement.

Streamlined shape of an aircraft

Non-contact forces:

Non-contact forces are forces that act on objects without any physical contact between them.

Magnetic force:

A magnet can attract objects made of magnetic materials like iron. When two magnets are brought close, like poles (North–North or South–South) push each other away, while unlike poles (North–South) pull each other towards each other.

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A horseshoe magnet attracting iron nails

Magnetic force is the force exerted by a magnet on another magnet or a magnetic material from a distance, without direct contact, and hence it is a non-contact force.

Static electric charges:

Static electric charges are formed when certain materials are rubbed together and charges build up on their surfaces. These charges stay at rest and the object becomes a charged object. A charged object can attract uncharged objects like bits of paper without touching them, so this is a non-contact force.

Types of static electric charges:

When two balloons are charged in the same way, they get similar charges and repel each other, showing that like charges repel. During rubbing, both objects get charged with opposite charges and attract each other, showing that unlike charges attract. The two types of static electric charges are positive and negative.

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Like and unlike charges

Electrostatic force:

Electrostatic force is the force exerted by a charged object on another charged or uncharged object without direct contact, and hence it is a non-contact force.

Electric current:

When electric charges start moving, they form an electric current in an electrical circuit. This current is responsible for making a lamp glow, producing heat, and creating magnetic effects.

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