Class 8 Force and Pressure Notes

Force and Pressure Notes

Force :-

A push or pull upon an object ; it results from its interaction with another object . { The S.I. unit of Force is Newton }

Effect of force :-

• Change in the state of motion
• Change in size/shape of an object
• Change in direction of an object

Some examples:
(i) A boy want to move a tyre faster it has to be pushed repeatedly.

(ii) Change in the direction of moving ball after it strikes the ruler placed in its path .

Do you know ?

An object continues to be in motion , with the same speed and in the same direction , unless acted upon by an unbalanced force . It is a natural tendency of objects to resist any change in their state of rest or motion . This is known as the Law of Inertia .

Direction and Magnitude of Force:

(i) The measurement of strength and amount of force is called magnitude of force.
(ii) Two or more forces on the same object can be applied in the same direction and opposite direction.

1. When two or more forces are applied over an object in the same direction:
When two or more forces are applied in the same direction, then the total or net force is the addition of magnitude of both the forces.
Example: When two persons push the box in the same direction with the magnitude of forces of each 2 N, then resultant force will be-
Net force:-  2N + 2N= 4N (In the direction of applied force)

2. When two forces are applied over an object in the opposite direction:
When two forces are applied over an object in the opposite directions, the total effective or net force is the difference of magnitude of two forces.
Example:
(i) When two persons push the box in the opposite direction with the magnitude of forces of each 2 N, then resultant force will be-
Net force:-  2N – 2N= 0N (Box will not move)
(ii) If one person is applying a force of 6 unit in one direction and another person is applying a force of 8 unit in opposite direction,
Then the resultant force: 8N – 6N= 2N (In the direction of higher magnitude force)
In this case, force will act in the direction of higher magnitude of force.

Types of forces

1 . CONTACT FORCES

2. NON-CONTACT FORCES

CONTACT FORCES :-

The force which acts only when objects are in physical contact with each other .

(a) Muscular force:
(i) Force due to the action of muscles is called muscular force. We can say force resulting because of action of muscle is called muscular force.
(ii) Muscular force is applied only after interaction with the object.
Examples:
(a) When we push the object like school bag or lift the bucket of water we use the muscular force.
(b) Animals also use of muscular force to carry out their task.

(b) Friction Force:
(i) The forces of friction arises between the surfaces of two objects
(ii) The force of friction always acts on all moving object and its direction is always opposite to the  direction of motion.
(iii)Force of friction comes into action only after interaction between two objects, thus, it is a type of  contact force.Examples:
(a) Due to force of friction between the surface of the ball and the ground that brings the moving ball to rest.
(b) When we stop pedalling a bicycle, it gradually slow down and finally comes to s stop due to force of friction.

NON – CONTACT FORCES :-

The force of interaction which acts on an object without its having any direct physical contact with another body .

(a) Magnetic Force:
(i) A magnet can exert a force on another magnet without being in contact with it.
(ii) The force exerted by a magnet is an example of a non-contact force.
Example: Opposite pole of two magnets attract each other and same pole repel each other.

(b) Electrostatic Force:
(i)When a charged body exert a force on another charged or uncharged body is called electrostatic force.
A charged body attracts an uncharged body.
(ii) A positively charged body attracts a negatively charged body and repels a positively charged body without coming in contact, thus it is a non-contact force.

(c) Gravitational Force:
(i) Gravitation force is exerted by earth, moon, sun and other planets to others.
(ii) Earth and other planets attract all objects towards them. Since, earth attracts all objects even without coming in contact, thus gravitational force is a non-contact force.
Example: When we release an object from a height, it falls over the ground because of gravitational  attraction of earth.

PRESSURE

PRESSURE — the perpendicular force ( thrust ) acting per unit area of the surface . When the contact area is less , its pressure , on the surface in contact , becomes more .

THRUST — the force acting on a surface in a direction perpendicular to the surface .

The formulae of PRESSURE —

PRESSURE = force / area acts on it

The S.I. unit of pressure = pascal .

Liquid pressure

The pressure exerted by a liquid known as liquid pressure . It is also known as hydrostatic pressure .

PROPERTIES OF LIQUID PRESSURE :-

• Liquid exert pressure on the wall of the container .
• pressure exerted by liquid increase with depth .
• liquids exert equal pressure at the same depth .
• The pressure at which water comes out of the holes is directly proportional to its depth

Atmospheric pressure

Atmospheric pressure — it is defined as the force per unit area , exerted on a given surface , by the weight of the air above the surface .

(i) Our atmosphere is made of different types of gases. Since gases exert pressure, thus Pressure exerted by air (present in atmosphere) is called atmospheric pressure.
(ii) The weight of air in a column of the height of the atmosphere and area 10 cm × 10 cm is as large as 1000 kg. The reason we are not crushed under this weight is that the pressure inside our bodies is also equal to the atmospheric
(iii) A pressed rubber sucker on a plane surface does not come off because of atmospheric pressure .