CLASS: VIII NCERT (CBSE)
Physics
Force And Pressure
SUMMARY
Force
1) Force is a kind of push or a pull on an object. The interaction of one object with another results in a force between the two objects. If a force acts on an object in the direction of its motion, then the object moves faster. If a force acts on an object opposite to the direction of its motion, then the objects slows down.
2) Force may also change the direction of motion of an object. The effect of force changes with the direction of its application. Forces acting on an object in the same direction add to each other, while forces acting opposite to each other results in the difference of the forces.
3) The strength of force is called its magnitude. The international unit used to measure force is newton.
4) A force can change the shape and size of an object. There are many types of forces.
5) The force resulting from the action of muscles is called muscular force. We make use of muscular force of animals like bullocks, horses and camels get our activities done.
6) The point where force is applied on an object is called the point of contact.
7) Friction is another type of contact force that always acts opposite to the direction of motion of the objects. Magnetic force andelectro-static force are non-contact forces as they act on an object from a distance. The force of gravity is also a non-contact and an attractive force exerted by the earth on objects, due to which objects fall to the ground.Gravitational force is an attractive force between any two objects in the universe, and is a non-contact force.
Pressure
The thrust acting per unit surface area is pressure. The force acting on an object perpendicular to its surface is thrust. Pressure is measured in newton/metre2, which is equal to 1 pascal (Pa). Liquids and gases exert pressure on the walls of the vessel in which they are carried. Air exerts pressure on all objects. The pressure exerted by air is called atmospheric pressure. A rubber sucker sticks to the surface of an object as atmospheric pressure acts on it. In order to fix nails to walls, sharp nails are used, as the area of contact of the nail with the wall is small and more pressure acts on it for a given force.
A suction pump sucks out all the air from a closed container, creating a vacuum in it.
