Force and Pressure Class 8 Notes Science Chapter 8

• Force: A push or a pull on an object is called a force. Force may impart motion to an object.
  Force is a Push or a Pull
• Forces applied on an object in the same direction add to one another.
• When two forces act in the opposite direction on an object, the net force acting on the object is the difference between the two forces.
• Magnitude: The strength of a force is usually expressed by its magnitude.
• While talking about a force we have to specify the direction in which a force is acting and also its magnitude.

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

• State of Motion: The state of motion of an object is described by its speed and the direction of motion. At rest the state of the object is considered the state of zero speed.
• Muscular Force: The force resulting due to the action of muscles is known as the muscular force.
• Contact Force: Force that can be applied only when it is in contact with an object is called a contact force. For example, muscular force can be applied only when it is in contact with an object. Friction is also an example of contact force.

The force of friction always acts on all the moving objects and its direction is always opposite to the direction of the motion.

• Non-Contact Force: The force exerted on an object without touching it is known as non-contact force. For example, the force exerted by a magnet on a piece of iron.

Another example is Electrostatic Force. It is the force exerted by a charged body on another charged or uncharged body.

• Gravitational Force: The attractive force of the earth which acts upon all the objects is known as the force of gravity or just gravity. Every object in the universe, whether small or large, exerts a force on every other object. This force is called as the gravitational force.
• Pressure: The force acting on a unit area of a surface is called pressure. It is especially true for those forces which act perpendicular to the surface on which the pressure is to be computed.

Lesser the area, greater is the effect of force.
A porter places a round folded long piece of cloth on his head while carrying heavy load. By doing this he increases the area of contact of the load with his head. So, the pressure on his head at a particular place of head is distributed to adjoining areas. As a result, he finds easier to carry the load (Fig. 11.1).
The base of the dam is made quite wide. This reduces pressure exerted by the enormous amount of stored water (Fig. 11.2).
For the similar reason, it is easier to push pointed nail into a wooden plank than the blunt edged nail. The pointed nail reduces the surface for the force acted upon it.
For the same reason the foundation of the walls of a building is made wider than the wall to be erected on them (Fig. 11.3).

• The liquid exerts pressure on the walls of a container.
• Gases also exert pressure on the walls of their containers.
• Atmosphere: The envelope of air all around us is known as atmosphere. The atmospheric air extends up to many kilometres above the surface of the earth.
• Atmospheric Pressure: The pressure exerted by atmospheric air is termed as atmospheric pressure. This is the weight of air column over a unit area (1 cm²). For example, the area of the head of a boy is 10 cm × 10 cm. Then the weight of air column of the height of the atmosphere and area 10 cm × 10 cm is as large as 1000 kg. This heavy weight does not crush us because the pressure inside our bodies is also equal to the atmospheric pressure as it cancels the pressure outside.