Force & Pressure Summary Class 8 NCERT Summary

Introduction

• Actions like picking, opening, shutting, kicking, hitting, lifting, flicking, pushing, and pulling are often used to describe certain tasks.
• Each of these actions usually results in some kind of change in the motion of an object.

Force

• A push or pull on an object is called a force.

• Push: When an object is moving away from the applier of force.
  Example: Pushing a door to open it, Pushing a shopping cart in a grocery store.
• Pull: When an object is moving towards the applier of force.
  Example: Pulling a rope during a tug-of-war game. Pulling a chair out from under a table.
• Force is a push or a pull that changes or tends to change the state of rest or uniform motion, the direction of motion, or the shape or size of a body.
  
• Force is any action that tends to change the position, shape, or size of an object.
• The interaction of one object with another object results in a force between the two objects.
• The effect of force depends on the magnitude and direction of the force.
• The forces applied in the same direction added to one another.
• The forces applied in the opposite direction, the net force is given by the difference between the two forces.
• Force can move a body that is initially at rest and can bring a moving body to a stop.
• Force can change the direction of a moving body. Force can change the speed of a moving body.
• Force can change the shape of a body. Force can change the size of a body.

Muscular force

• It involves the action of muscles.
• Examples: Animals make use of muscular force to carry out their physical activities and other tasks. 
  Lifting weights during exercise, Kicking a football during a game.

• It is an opposing force that acts between surfaces in contact moving with respect to each other.
• The direction of the frictional force is always opposite to the direction of motion.
• Frictional force always acts between two moving objects, which are in contact with one another.
• Frictional force always acts opposite to the direction of motion.
• The frictional force depends on the nature of the surface in contact.
• Non-contact force comes into play even when the bodies are not in contact.

Magnetic force

• Force acting between two magnets or a magnet and a magnetic material.
  Examples: iron, steel, nickel, cobalt, etc.
• It can be attractive and repulsive.

Electrostatic force

• Force due to electric charges.
• It can be attractive and repulsive.
• Example: Rubbing a balloon on your hair creates a charge, and the balloon can attract small pieces of paper.

Gravitational force

• It is a kind of attractive force that comes into play because of the mass of a body.
  Example: Earth's gravitational attraction.

Pressure

• The force acting per unit area of the surface is called pressure.
• Pressure = Force/Area on which it acts.
• The unit of pressure is Newton per square meter (N/m²), which is also known as Pascal
• The smaller the area, the larger the pressure for the same force.
• Liquids exert pressure on the walls of the container.
• The pressure exerted by liquids increases with depth.
• Liquids exert equal pressure at any given depth.
• The pressure at which water comes out of the holes is directly proportional to its depth.
• Example: A knife cuts better because the sharp edge has a small area, resulting in greater pressure when force is applied.

Fluid

• The substance which can flow and has no fixed shape
• Pressure due to a liquid column of height h (p) = hrg
  where, h = Height of column
  r = Density of fluid
  g = Acceleration due to gravity
• The pressure inside a fluid increases with an increase in the depth and density of the fluid.
•  Water and gas exert pressure on the walls of their container.
• The atmosphere exerts pressure on the surface of the Earth.
  
• Atmospheric pressure = Weight of the atmosphere per unit area.
• The pressure inside our body is equal to the atmospheric pressure and cancels the pressure from outside.
• The air surrounding the Earth: atmosphere
• Air exerts pressure on its surroundings: thrust on a unit area is called atmospheric pressure.
• Example: Drinking from a straw works because the atmospheric pressure outside the straw pushes the liquid into your mouth once you create lower pressure by sucking.