NCERT Revision Notes Chapter 11 Force and Pressure Class 8 Science
CBSE NCERT Revision Notes1
Answer
• Introduction
• Force
→ Push
→ Pull
• Muscular Force
• Friction
• Magnetic Force
• Electrostatic Force
• Gravitational Force
• Pressure
• Fluid
• Atmospheric Pressure
2
Answer
→ Actions like picking, opening, shutting, kicking, hitting, lifting, flicking, pushing, 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.
3
Answer
→ A push or pull on an object is called a force.
• Push: When an object is moving away from the applier of force.
• Pull: When an object is moving towards the applier of force.
→ Force is a push or a pull which changes or tends to change the state of rest or of uniform motion, or direction of motion or the shape or size of a body.
→ Force is any action that has the tendency to change the position, shape, or size of an object.
→ 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.
→ Force applied in the same direction added to one another.
→ Force applied in the opposite direction, the net force is given by the difference of two forces.
→ Force can move a body initially at rest.
→ Force can bring a moving body to rest.
→ 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.
4
Answer
→ It involves the action of muscles.
→ Animals make use of muscular force to carry out their physical activities and other tasks.
5
Answer
→ It is an opposing force that acts between surfaces in contact moving with respect to each other.
→ The direction of force of friction 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.
→ Frictional force depends on the nature of the surface in contact.
→ Non-contact force come into play even when the bodies are not in contact.
6
Answer
→ Force acting between two magnets or a magnet and a magnetic material.
Example: iron, steel, nickel, cobalt etc.).
→ It can be attractive and repulsive.
7
Answer
→ Force due to electric charges.
→ It can be attractive and repulsive.
8
Answer
→ It is a kind of attractive force that comes into play because of the mass of a body.
Example: earth's gravitational attraction.
9
Answer
→ The force acting per unit area of 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.
→ Smaller the area larger the pressure for the same force.
→ Liquids exerts pressure on the walls of the container.
→ Pressure exerted by liquids increases with depth.
→ Liquids exert equal pressure at the same depth.
→ The pressure at which water comes out of the holes is directly proportional to its depth.
10
Answer
→ 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
→ Pressure inside a fluid increases with increase in depth and density of the fluid.
→ Water and gas exert pressure on the walls of their container.
→ Atmosphere exerts pressure on the surface of the Earth.
11
Answer
Pressure inside our body is equal to the atmospheric pressure and cancels the pressure from out side.
Air surrounding the Earth: atmosphere
Air exerts pressure on its surroundings: thrust on unit area is called atmospheric pressure