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FORCE AND PRESSURE A push or a pull on an object is called Force. Force imparts motion to an object. FORCE IS DUE TO AN INTERACTION  Force between two objects is result of interaction of these two objects. At least two objects must interact for a force to come into play.  Force applied on an object in the same direction adds to one another.  Net force of two forces acting in the opposite directions on an object is the difference between the two forces.  The strength of a force is usually expressed by its magnitude. A FORCE CAN CHANGE THE STATE OF MOTION  State of motion of an object is described by the speed and the direction of motion. The state of rest is considered to be the state of zero speed. State of motion may be rest or in motion.  Force induces motion to an object and changes its speed or its direction of motion or both. Thus, a force changes the state of motion of an object. FORCE CAN CHANGE THE SHAPE OF AN OBJECT  Applying force on an object bring change in the shape based on texture of the respective object. Example: Applying force on a lump of dough changes its shape. 

CONTACT FORCES  Contact force can be applied only when it is in contact with an object. For an example, Muscular force and frictional force are contact forces i.e. can be applied only when it is in contact with an object.  Force resulting due to action of muscles is termed muscular Force. Many activities like bending, movement, breathing, digestion of food, etc involve muscular force.  Opposing force which acts against all moving object in the direction opposite to the moving object is called friction. It changes state of any moving object.  Non-contact force can be exerted on an object without touching it. Magnetic force, electrostatic force and gravitational force are non-contact forces  Force exerted by magnet on a piece of iron or magnet is called magnetic force.  Force exerted by a charged body on another charged or uncharged body is known as electrostatic force. Example, a plastic ruler rubbed with hair attracts small pieces of paper.  The force of attraction between all masses in the universe is called gravitational force/gravity. The attraction of the earth's mass for bodies near its surface is known as gravitational force of earth.

PRESSURE   

Force acting on a unit area of a surface is called pressure. Mathematically, Pressure= Force/Area on which it acts o i.e pressure is more if strong force applied on smaller area of surface. Liquid and gases exert pressure on the walls of container.

PRESSURE EXERTED BY LIQUID

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Envelope of air surrounding earth is called atmosphere. Pressure exerted by atmosphere is called atmospheric pressure. The weight of air in a column of height of atmosphere and area of 10 cm2 is as large as 1000 kg.


Cbse class viii science force and pressure by lhakpa wangyal