Class 8 Science – Force and Pressure

Class 8 Science – Force and Pressure (One-Line Notes)

1. Basics of Force

• Force: A push or pull on an object that can change its state of motion.
• SI unit of force: Newton (N).
• Effects of force: Can change the shape, speed, or direction of an object.
• Balanced force: Forces acting on an object that do not change its motion.
• Unbalanced force: Forces that change the motion of an object.

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2. Types of Force

Type of Force	Description	Example
Gravitational force	Force of attraction between two masses	Earth pulling objects down
Muscular force	Force applied by muscles	Pushing a box
Frictional force	Force opposing motion between surfaces	Sliding a book on table
Magnetic force	Force of attraction or repulsion between magnets	Magnet attracting iron
Electrostatic force	Force between charged objects	Rubbing balloon on hair

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3. Pressure

• Pressure: Force applied per unit area on a surface.
• Formula: $P = \frac{F}{A}$
• Unit of pressure: Pascal (Pa) = $1 \, N/m^2$
• High pressure: Small area with large force.
• Low pressure: Large area with same force.

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4. Examples of Pressure in Daily Life

• Sharp knife: Small area → high pressure → cuts easily.
• Snow shoes: Large area → low pressure → prevents sinking.
• Balloon: Force spread over area → low pressure → doesn’t burst easily.

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5. Fluid Pressure

• Fluid pressure: Pressure exerted by liquids and gases.
• Pascal’s law: Pressure applied to a confined fluid is transmitted equally in all directions.
• Applications: Hydraulic lifts, syringes, and water supply systems.

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6. Atmospheric Pressure

• Atmospheric pressure: Pressure exerted by air on objects.
• Measurement: Using a barometer.
• Effect: High altitudes → low atmospheric pressure; sea level → high pressure.

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7. Summary Table: Force vs Pressure

Quantity	Definition	Formula	Unit
Force	Push or pull on an object	–	Newton (N)
Pressure	Force per unit area	$P = F/A$	Pascal (Pa)

MCQ

1. Which of the following best describes a force?
   A. Something that changes motion or shape of an object
   B. The weight of an object
   C. The speed at which an object moves
   D. The distance an object travels
   Answer: A
2. A push or pull on an object can:
   A. Change its shape
   B. Make it start moving
   C. Change its direction
   D. All of the above
   Answer: D
3. Which force keeps the planets revolving around the Sun?
   A. Magnetic force
   B. Friction
   C. Gravity
   D. Elastic force
   Answer: C
4. When you lift a book from a table, the type of force you use is:
   A. Non-contact force
   B. Contact force
   C. Gravitational force
   D. Magnetic force
   Answer: B
5. Which of these does not require contact to act?
   A. Pulling a rope
   B. Friction
   C. Magnet attracting a pin
   D. Pushing a chair
   Answer: C
6. What is the standard unit of force?
   A. Pascal
   B. Joule
   C. Newton
   D. Watt
   Answer: C

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2. Pressure Basics

7. Pressure is defined as:
   A. Force multiplied by area
   B. Force divided by area
   C. Area divided by force
   D. Force multiplied by distance
   Answer: B
8. Which unit measures pressure in the SI system?
   A. Newton
   B. Joule
   C. Pascal
   D. Bar
   Answer: C
9. Pressure increases when:
   A. Force increases (area constant)
   B. Force decreases
   C. Area increases
   D. Area stays the same
   Answer: A
10. Why does a needle pierce easily through cloth compared to a blunt stick?
    A. Needle is heavier
    B. Needle applies force on a smaller area
    C. Needle is sharper in color
    D. Needle applies less force
    Answer: B

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3. Pressure in Liquids and Gases

11. Fluids (liquids and gases) exert pressure:
    A. Only downward
    B. Only upward
    C. In all directions
    D. Only sideways
    Answer: C
12. The weight of air above us creates:
    A. Muscle pressure
    B. Atmospheric pressure
    C. Water pressure
    D. Contact pressure
    Answer: B
13. Camels can walk on sand without sinking because:
    A. They are light
    B. Their feet are wide, spreading their weight
    C. They move slowly
    D. Sand is very hard
    Answer: B

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4. Applications

14. A simple way to demonstrate atmospheric pressure is:
    A. Using a rubber suction cup
    B. Using a spring balance
    C. Using a pendulum
    D. Using a watch glass
    Answer: A
15. Pressure is:
    A. Work done per unit time
    B. Force applied per unit area
    C. Energy stored in a body
    D. Mass multiplied by speed
    Answer: B
16. Which device is used to measure liquid pressure?
    A. Thermometer
    B. Voltmeter
    C. Manometer
    D. Barometer
    Answer: C

Short Questions & Answers

1. Force

Q1. What is a force?
A: A force is a push or pull that can change the motion or shape of an object.

Q2. Name two effects of a force.
A: A force can change the speed or direction of an object and can also change its shape.

Q3. Give an example of a contact force.
A: Pushing a chair or lifting a book.

Q4. Give an example of a non-contact force.
A: Gravity or magnet attracting a pin.

Q5. What is the SI unit of force?
A: Newton (N).

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2. Pressure

Q6. Define pressure.
A: Pressure is the force applied per unit area.

Q7. Write the formula for pressure.
A: Pressure = Force ÷ Area

Q8. What is the SI unit of pressure?
A: Pascal (Pa)

Q9. Why does a needle pierce easily compared to a blunt object?
A: Because it applies force over a smaller area, producing higher pressure.

Q10. How can pressure be increased on a surface?
A: By increasing the force or decreasing the area.

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3. Pressure in Fluids

Q11. In which directions do fluids exert pressure?
A: In all directions (upwards, sideways, and downwards).

Q12. What is atmospheric pressure?
A: The pressure exerted by the weight of the air above us.

Q13. Why don’t camels sink in sand?
A: Because their wide feet spread the weight, reducing pressure on sand.

Q14. Name a device used to measure pressure in liquids.
A: Manometer

Q15. Give an example to demonstrate atmospheric pressure.
A: Pressing a rubber suction cup on a wall.

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4. Miscellaneous

Q16. What happens when a force is applied on an object at rest?
A: The object may start moving.

Q17. What happens when a force is applied on a moving object?
A: Its speed or direction may change.

Q18. Can force change the shape of an object? Give an example.
A: Yes, e.g., pressing a sponge.

Q19. Why do sharp knives cut better than blunt ones?
A: Sharp knives have a smaller contact area, producing higher pressure.

Q20. What is the relationship between force, area, and pressure?
A: Pressure increases with force and decreases with area (P = F ÷ A).

Important One-Word / One-Line Answers for Exams

1. A push or a pull – Force
2. Force applied by muscles – Muscular
3. Force applied without contact – Non-contact
4. Force applied with contact – Contact
5. Force due to gravity – Gravitational
6. Force exerted by magnet – Magnetic
7. Force exerted by charged object – Electrostatic
8. Force that changes shape – Deforming
9. Force that changes speed – Force
10. Force that stops a moving object – Opposing
11. Force exerted by air – Air pressure
12. Force exerted by water – Water pressure
13. SI unit of force – Newton
14. Device used to measure force – Spring balance
15. Force applied by stretched spring – Elastic
16. Force acting on a body due to earth – Gravity
17. Direction of gravitational force – Downward
18. Force responsible for falling objects – Gravity
19. Force exerted per unit area – Pressure
20. SI unit of pressure – Pascal
21. Pressure increases when area is – Decreased
22. Pressure decreases when area is – Increased
23. Pressure exerted by gases – Air pressure
24. Instrument used to measure pressure – Barometer
25. Pressure exerted by liquids increases with – Depth
26. Force that can change direction of motion – Force
27. Effect of force on object’s shape – Deformation
28. Force exerted by muscles of animals – Muscular
29. Force applied by earth on objects – Gravitational
30. Force exerted by magnets without contact – Magnetic

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