Chapter 11: Force and pressure

These NCERT Class 8 Science Chapter 11 notes explain the concepts of force and pressure in a simple and structured way. The chapter focuses on how forces act on objects and bring changes in motion and shape.

It discusses types of forces such as contact and non-contact forces, gravitational force, magnetic force and electrostatic force. It also explains balanced and unbalanced forces and introduces the concept of pressure.

Chapter 11 – Force and Pressure is important for understanding motion, mechanics and everyday physical phenomena. It is highly relevant for school examinations and foundation level competitive preparation.

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1. Introduction: what is force?

• Force means a push or a pull acting on an object.
• A force can change the shape of an object.
• It can change the speed of a moving object.
• It can change the direction of motion.
• A force can start or stop the motion of an object.

2. Effects of force

• Force can change the shape of objects like stretching a rubber band.
• Force can increase or decrease the speed of an object.
• Force can change the direction of a moving object.
• Force can change the state of motion from rest to motion or vice versa.
• The overall effect depends on how the force is applied.

3. Contact forces

• Contact force is a force applied by touching an object.
• Muscular force is applied by muscles during lifting or pushing.
• Carrying a bag or pushing a cart involves muscular force.
• Frictional force opposes motion between two surfaces in contact.
• Friction slows down a bicycle when pedalling stops.

4. Non-contact forces

• Non-contact force acts without physical contact.
• Gravitational force pulls objects toward the Earth.
• Falling of an apple is due to gravitational force.
• Magnetic force can attract or repel magnetic materials.
• Electrostatic force acts between electrically charged objects.

5. Net force and its effect

• When more than one force acts on an object, the total effect is called net force.
• Forces acting in the same direction add up.
• Forces acting in opposite directions subtract.
• Net force determines the final motion of the object.
• If two people push a box in opposite directions equally, it does not move.

6. Balanced and unbalanced forces

• Balanced forces are equal and opposite, resulting in zero net force.
• Balanced forces do not change the state of motion.
• Tug of war with equal strength is an example of balanced force.
• Unbalanced forces are unequal and result in non-zero net force.
• Unbalanced forces cause motion or change in motion.

7. Pressure and its meaning

• Pressure is defined as force applied per unit area.
• Smaller area results in more pressure.
• Larger area results in less pressure.
• High-heel shoes exert more pressure than flat shoes.
• Pressure depends on both force and area.

8. Pressure exerted by liquids

• Liquids exert pressure on the bottom and sides of containers.
• Liquid pressure increases with increasing depth.
• Dam walls are made thicker at the bottom to withstand more pressure.
• Pressure acts in all directions in a liquid.
• Greater depth means greater pressure.

9. Atmospheric pressure and daily life

• Air exerts pressure called atmospheric pressure.
• Drinking through a straw works due to atmospheric pressure.
• Rubber suction cups stick to walls because of air pressure.
• A sharp knife cuts easily due to high pressure on small area.
• Broad camel feet reduce pressure on sand.

10. Key exam-oriented keywords explained

• Force: A push or pull that can change motion or shape.
• Push: Force applied to move something away.
• Pull: Force applied to bring something closer.
• Contact force: Force that acts through physical contact.
• Muscular force: Force applied by muscles of the body.
• Friction: Force that opposes motion between two surfaces.
• Non-contact force: Force that acts without touching.
• Gravitational force: Force of attraction between Earth and objects.
• Magnetic force: Force exerted by magnets.
• Electrostatic force: Force between electrically charged objects.
• Net force: Total force acting on an object.
• Balanced force: Equal and opposite forces resulting in no change in motion.
• Unbalanced force: Unequal forces causing motion or change.
• Pressure: Force applied per unit area.
• Atmospheric pressure: Pressure exerted by air.

11. One-page summary for quick revision

• Force is a push or pull that affects motion and shape.
• Forces can be contact or non-contact.
• Balanced forces do not change motion, while unbalanced forces do.
• Pressure depends on force and area.
• Liquids and air exert pressure in all directions.

Understanding NCERT Class 8 Science Chapter 11 is essential to learn how forces affect objects and how pressure works in daily life. The chapter clearly explains net force, atmospheric pressure and liquid pressure.

The concepts help students build a strong foundation in physics and problem-solving. It is highly useful for school exams and foundation competitive preparation.

Continue reading NCERT Class 8 Science Chapter 12 – Friction to understand how friction affects motion and machines.

FAQs

Q1. What is force?
Force is a push or a pull that can change the shape, speed or direction of an object.

Q2. What are contact and non-contact forces?
Contact forces act by touching an object, while non-contact forces act without physical contact.

Q3. What is pressure?
Pressure is the force applied per unit area.

Q4. What is atmospheric pressure?
Atmospheric pressure is the pressure exerted by air around us.

Q5. Why is this chapter important for exams?
It explains force, pressure and motion concepts, which are important for school exams and foundation preparation.

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