NCERT Solutions Class 8th Science Chapter – 8 Force and Pressure
Chapter – 8
Force and Pressure
Notes
Force arises due to the interaction between at least two objects
Force has both Magnitude and Direction
Force applied on an object in the same direction add to one another.
If the two forces act in the opposite direction on an object, the net force acting on it is the difference between the two forces.
In general, more than one force may be acting on an object at any given point. However, a force acting on an object is always the mean net force acting on it.
• Two friends pushing a heavy load (a) in the same direction and (b) in the opposite direction
A Force can Change the State of Motion
A change in the speed of an object or the direction of its motion or both implies a change in its state of motion.
Force acting on an object may bring a change in its state of motion or a change in its shape.
• To move a tyre faster it has to be pushed repeatedly
• A magnet can exert a force without being in contact with it. The force exerted by a magnet is an example of a non-contact force.
Thrust – The force that acts on a surface in a direction perpendicular to it, is known as thrust.
Pressure – Thrust per unit area is called pressure. So, pressure = force/area on which it acts.
• Liquids exert pressure on the walls of the container in which they are kept.
• Gases exert pressure in all directions.
Electrostatic force – A force exerted by a charged body on another charged or uncharged body is known as electrostatic force.
Atmospheric pressure is defined as the pressure exerted on a surface by the weight of air above that surface.
Atmospheric Pressure – The pressure exerted by atmospheric air around us is known as atmospheric pressure.
Contact Force – A force that can be applied only when it is in contact with an object is called a contact force. For example, hammering a nail.
Electrostatic Force – When a charged body, either having a (+) or (-) charge, exerts force on another charged or uncharged body, that force is known as electrostatic force.
Force – Simply a push or pull exerted by an object on another is a force.
Friction – The force of friction always acts on all the moving objects and its direction is always opposite to the direction of motion.
Gravitational Force – The force exerted by the earth to pull the objects towards itself is called the force of gravity.
Gravity – Every object in the universe, whether small or large, exerts a force on every other object. This force is called the force of gravity or just gravity.
Magnetic Force – The force exerted by a magnet to pull/push a metallic object is called magnetic force.
Muscular Force – The force resulting due to the action of muscles is known as muscular force. For example, writing, cutting vegetables.
Non-Contact Forces – A force that can be applied without any contact between two objects is called non-contact force. For example, the magnet pulls the iron-pieces.
Pressure – The force acting on per unit area, applied to an object in the direction perpendicular to the surface is called pressure.
Nuclear forces
The nuclear force acts between all the particles in the nucleus. i.e., between two neutrons, between two protons and between a neutron and a proton.
It is an attractive force in all cases.
It is the force that keeps the nucleus intact by overcoming the enormous repulsive force between positive protons.
Distribution of pressure
Force acting on a smaller area applies more pressure than the same force acting on a larger area.
Examples – Porters place a round cloth on their heads to increase surface area and reduce pressure.
A sharp knife cuts better as more pressure is exerted over a smaller area.
