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CBSE NOTES ⇒ Class 8th ⇒ Science ⇒ Chapter 11. Force and Pressure

## Chapter 11. Force and Pressure

### Force And Pressure:

Force: A push or a pull on an object is called a force.

Strength Of Force: A force could be larger or smaller than the other. The
strength of a force is usually expressed by its magnitude.

Force is a vector quantity which has both magnitude and direction

If the direction or the magnitude of the applied force changes, its effect also changes.

Net force: If the two forces act in the opposite directions on an object, the net force acting on it is the difference between the two forces.

Features of Force:

(i) A force can change the state of motion: A change in either the speed of an
object, or its direction of motion, or both, is described as a change in its state of
motion. Thus, a force may bring a change in the state of motion of an object.

There are two conditions:

(a) A force applied on an object may change its speed. If the force applied on the object is in the direction of its motion, the speed of the object increases.

(b) If the force is applied in the direction opposite to the direction of motion, then it results in a decrease in the speed of the object.

(ii) A Force can Change the Shape of an Object: A ball of dough when it is rolled to make a chapati, This happens due to applying force on dough and it changes its shape. In this example we see that the application of force on an object may change its shape.

Kind of Force:

There are two kinds of force.

1. Contact force

2. Non-contact force

1. Contact Force: Muscular force can be applied only when it is in contact with an object, it is also called a contact force.

Type of Contact Force:

There are two types of contact force:

(i) Muscular force – The force resulting due to the action of muscles is known as the muscular force

Examples of Muscular Force:

(a) we push an object like a school bag or lift a bucket of water.

(b) Animals like bullocks, horses, donkeys and camels are used to perform various tasks for us like pulling carts.

(ii) Frictional force  -  The force acts opposite to the direction of motion and this is responsible for changing the state of motion of objects. It arises due to contact between the two surfaces.

Example of Frictional Force:

(i) Rubbing of shoes during walking.

(ii) The force between the surface of the ball and the ground that brings the
moving ball to rest.

(iii) Friction between water and the boat brings it to a stop once you stop rowing.

Features of Frictional Force:

(a) Frictional force acts always against/opposite the motion of an object.

(b) It always works between two suraces or two bodies. '

(c) It changes the state of motion.

2. Non-contatct Force: The force which is felt by the objects without any contact is known as non-contact force.

Types of Non-contact Force:

There are two types of Non-contact Force.

(i) Magnetic Force - A magnet can exert a force on another magnet without being in contact with it. The force exerted by a magnet is an example of a non-contact force.

Magnet applies two types of forces:

(a) Attacting force (b) Repulsing force

(ii) Electrostatic force – The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.

Examples:

(i) A straw rubbed with paper attracts another straw.

(iii) Gravitational force – Every object attracts each other and pulls toward itself. Such force which applied on objects is known as Gravitational force and gravitation.

Examples:

(i) The Sun attracts the Earth and other planets.

(ii) The Earth attracts the moon.

Gravity: Objects or things fall towards the earth because it pulls them. This force
is called the force of gravity, or just gravity. This is an attractive force. The force of gravity acts on all objects.

In other words, The force applying by the earth on every object on the earth is called garvity

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