# Force

Force can be defined as the physical cause that tends to change the resting position or uniform motion or to change the shape and size of the body.

## Classification of Forces

Forces are classified as contact and non – contact forces.
• Forces can be applied on a body by making physical contact or otherwise.
• Forces which act on a body through physical contact are contact forces.
• Forces which act on a body without making any physical contact with the body are known as non – contact forces.

Based on their nature, forces can also be classified as:

1. Mechanical force
2. Magnetic force
3. Gravitational force
4. Electric force
5. Nuclear force

### Mechanical force

To hit a nail into a wall, we use a hammer.  The force exerted by the hammer on the nail rives into the wall.  Thus, in this case, the force makes a physical contact and displaces the mail Such a force is called contact force or in this case, it can also be called mechanical force.

### Magnetic Force

Place a small iron piece near a magnet.  The iron piece moves wards the magnet.  This force applied by the magnet is called magnetic force.
Magnetic force is a non – contact force, since the iron piece experiences a force though there is no physical contact.

### Gravitational Force

A ball thrown vertically upwards returns to the ground after some time . This is on account of the attractive force the Earth exerts on the ball . Similarly , the Earth also exerts an attractive force on celestial bodies such as the moon , the sun , the planets , etc. This force of attraction between any two masses is known as gravitational force.

### Electric Force

Glass rod attracting a paper When a glass rod is rubbed with wool , the glass rod acquires electric charge . Now , when the glass rod is brought near some pieces of paper , they cling to the glass rod . This is due to the electric force of the glass rod.

### Nuclear Force

A nuclear force is a force that exists only if the interacting particles are protons or neutrons or both . It is different from gravitational and electromagnetic forces.

## Units of Force

Absolute unit of force:
If m = 1 unit, a = 1 unit, then the value of F = 1 unit
C.G.S.  unit of force is ‘dyne‘ and S.I.  unit of force is ‘newton‘ (N).
Dyne: It is the force which when acting on a body of mass one gram produces in it an acceleration of 1 cm s-2 in its direction.
Newton: It is the force which when acting on a body of mass 1 kg produces in it an acceleration of 1 m s-2 in its direction.
Therefore
1 newton (N) = 1 kg × 1 m s-2
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