Force And Pressure

A push or a pull on an object is called a force. It arises due to the interaction between two objects. Force has magnitude as well as direction. It may change in the state of motion of an object or it may bring about a change in the shape of an object. The unit of force is Newton.

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. Force may bring thechange in the state of motion of an object.

Forces which act only when there is physical contact between two interacting objects are known as Contact forces. Example-muscular force. Whereas Forces which can act without physical contact between objects, i.e. those that can act from a distance, are called non-contact forces or field forces. Example-magnetic force.

A goal keeper applies force for saving a goal. By his action the goalkeeper tries to apply a force on the moving ball. This leads to change in state of motion of ball.
Force is applied to the shape of a ball of dough when it is rolled to make a chapatti.

It is difficult to hold a school bag having a strap made of a thin and strong string because it apply the large pressure on the shoulders due to very small contact surface area .The pressure is inversely proportional to the surface area on which the force acts. Therefore pressure decreases if surface area increases.

Porters wear turbans when they have to carry heavy loads on their heads, to increase the area of contact. This reduces the pressure on the head.

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. 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

Lorries and trucks carrying heavy loads have 8 tyres instead of four. Also the tyres are broader because this increases the area of contact with the ground, thus reducing the pressure exerted on the ground.

When we press the sucker, most of the air between its cup and the surface escapes out. The sucker sticks to the surface because pressure of atmosphere acts on it. To pull the sucker off the surface, the applied force should be large enough to overcome the atmospheric pressure.

Force per unit area is called pressure. Pressure is inversely proportional to the surface area on which it acts. Smaller the area, larger the pressure on the surface for the same force.

When we stop pedalling the bicycle, it slows down and gradually stops because of force of friction between the surface of tyres of cycle and the ground that brings moving bicycle to rest. The direction of force of friction is always opposite to the direction of motion.

The force acting against the relative motion of surfaces in contact is called frictional force or friction. Friction is an example of contact force. Force of friction arises due to contact between surfaces.

A magnet can exert force on another magnet without being in contact with it; this force is known as magnetic force. Like poles of two magnets repel each other. Unlike poles of two magnets attract each other.

Every object exerts a force on every other object. This force is known as gravitational force. Gravity is not a property of the earth alone. Every object in the universe whether large or small exerts gravitational force on every other object.

The force acting on a rocket is:

• Frictional force due to air
• Force of gravity acting in downward direction.

Agent exerting pressure-fingers.

Object- lemon.

Effect of force is observed- lemon juice is coming out, change in shape of lemon.

Agent exerting pressure- fingers.

Object- tooth paste tube

Effect of force is observed- tooth paste is coming out, change in shape of tooth paste.

The force due to hammering changes the shape of the hot piece of copper to make tools.

Agent exerting pressure-An athlete.

Object- body

Effect of force – jumping

Atmosphere is the envelope of air surrounding our earth but the atmospheric pressure is the pressure exerted by this air.

Friction is the force responsible for changing the state of motion of objects but contact force is the forces which act only when there is physical contact between two interacting objects.