Shivendra Sarairasi

 

KNOWLEDGE ACADEMY AYODHYA

Class 8 – Science

Chapter: Force and Pressure

1. Force

Definition: A force is a push or pull acting on an object.

Examples:

1. Pushing a table.
2. Kicking a football.
3. Pulling a cart.
4. Stretching a rubber band.

Effects of Force

Force can:

1. Make a stationary object move.
2. Stop a moving object.
3. Change the speed of an object.
4. Change the direction of motion.
5. Change the shape or size of an object.

SI Unit of Force

The SI unit of force is the Newton (N).

2. Types of Force

Forces are mainly classified into two types.

A. Contact Force

A force that acts when two objects are in physical contact is called a contact force.

Examples:

1. Muscular force
2. Frictional force

B. Non-contact Force

A force that acts without physical contact between two objects is called a non-contact force.

Examples:

1. Magnetic force
2. Gravitational force
3. Electrostatic force

3. Muscular Force

The force applied by the muscles of our body is called muscular force.

Examples: Lifting a bag, pushing a cart and kicking a ball.

4. Frictional Force

The force that opposes the motion of one surface over another surface is called frictional force.

Examples: Brakes stopping a bicycle and a ball slowing down on the ground.

5. What Is Pressure?

Definition: Pressure is the force acting per unit area of a surface.

Formula of Pressure

Pressure = Force ÷ Area

[ \boxed{P=\frac{F}{A}} ]

Where:

• P = Pressure
• F = Force
• A = Area

SI Unit of Pressure

The SI unit of pressure is the Pascal (Pa).

[ 1,Pa=1,N/m^2 ]

6. Relationship Between Force, Area and Pressure

1. Pressure increases when force increases, provided the area remains constant.
2. Pressure decreases when area increases, provided the force remains constant.
3. Pressure increases when area decreases, provided the force remains constant.

Examples from Daily Life

1. Sharp knife: A sharp knife has a smaller contact area and produces greater pressure, so it cuts easily.

2. Broad school bag straps: Broad straps spread the force over a larger area and reduce pressure on the shoulders.

3. Camel's feet: Broad feet distribute the camel's weight over a larger area, helping it walk on sand.

4. Pointed nails: The pointed ends have a small area, producing greater pressure and allowing them to enter wood easily.

7. Pressure Exerted by Liquids

Liquids exert pressure on the bottom and walls of their containers.

Properties of liquid pressure:

1. Liquids exert pressure in all directions.
2. Liquid pressure increases with depth.
3. Liquid pressure depends on the density of the liquid.

Example: Water pressure is greater at the bottom of a deep swimming pool than near the surface.

8. Pressure Exerted by Gases

Gases exert pressure on the walls of their containers.

Examples:

1. Air inside an inflated balloon exerts pressure on its walls.
2. Air inside a bicycle tyre exerts pressure on the tyre's inner surface.

9. Atmospheric Pressure

Definition: The pressure exerted by the air surrounding the Earth is called atmospheric pressure.

Air has weight and exerts pressure on objects in all directions.

Examples:

1. Drinking juice through a straw.
2. Using a dropper to draw liquid.
3. A suction cup sticking to a smooth surface.

10. Important Formulae

Pressure:

[ P=\frac{F}{A} ]

Force:

[ F=P\times A ]

Area:

[ A=\frac{F}{P} ]

Where:

• P = Pressure in pascals (Pa)
• F = Force in newtons (N)
• A = Area in square metres (m²)

11. Solved Numericals

Question 1

A force of 100 N acts on an area of 5 m². Calculate the pressure.

Solution:

Given:

Force = 100 N

Area = 5 m²

Formula:

[ P=\frac{F}{A} ]

[ P=\frac{100}{5} ]

Answer: 20 Pa

Question 2

A pressure of 50 Pa acts on an area of 2 m². Calculate the force.

Solution:

[ F=P\times A ]

[ F=50\times2 ]

Answer: 100 N

Question 3

A force of 500 N produces a pressure of 100 Pa. Calculate the area.

Solution:

[ A=\frac{F}{P} ]

[ A=\frac{500}{100} ]

Answer: 5 m²

12. Important Questions and Answers

Q1. What is force?

Ans. A push or pull acting on an object is called force.

Q2. What are the two main types of force?

Ans. Contact force and non-contact force.

Q3. What is the SI unit of force?

Ans. Newton (N).

Q4. Define pressure.

Ans. Pressure is the force acting per unit area of a surface.

Q5. What is the SI unit of pressure?

Ans. Pascal (Pa).

Q6. Why does a sharp knife cut better than a blunt knife?

Ans. A sharp knife has a smaller contact area, so it produces greater pressure for the same force.

Q7. Why do school bags have broad straps?

Ans. Broad straps increase the contact area and reduce pressure on the shoulders.

Q8. What is atmospheric pressure?

Ans. The pressure exerted by the air surrounding the Earth is called atmospheric pressure.

Q9. How does liquid pressure change with depth?

Ans. Liquid pressure increases with depth.

Q10. Give two examples of non-contact forces.

Ans. Magnetic force and gravitational force.

Conclusion

Force can change the motion, direction, speed or shape of an object. Pressure depends on the force applied and the area over which the force acts. Liquids and gases also exert pressure.