Force and Pressure — Class 8 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 8 Science chapter "Force and Pressure" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 8 Science chapter "Force and Pressure" — 8 important questions with detailed answers for CBSE board exam p…
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NCERT Solutions for Class 8 Science — Force and Pressure. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Force and Pressure — All 8 Questions Solved
Q1. Define force. State its SI unit.
Force is a push or pull that changes or tends to change the state of rest or motion of an object. SI unit: Newton (N). 1 N = 1 kg·m/s².
Q2. A force of 100 N acts on an area of 5 m². Calculate the pressure.
Step 1: Use formula P = F/A
Step 2: P = 100/5 = 20 Pa
Pressure = 20 Pascal
Q3. Why does a sharp knife cut better than a blunt one?
A sharp knife has a smaller contact area. Since pressure P = F/A, smaller area produces greater pressure for the same force. Higher pressure cuts through objects more easily than a blunt knife.
Q4. Calculate the force exerted by a pressure of 50 Pa on an area of 2 m².
Step 1: Use P = F/A → F = P × A
Step 2: F = 50 × 2 = 100 N
Force = 100 Newton
Q5. What are contact forces? Give two examples.
Contact forces are forces that act when two objects are in direct contact. Examples: (1) Normal force when an object rests on a surface, (2) Friction between two sliding surfaces, (3) Tension in a rope, (4) Applied force when pushing an object.
Q6. State the difference between balanced and unbalanced forces.
Balanced forces: Equal in magnitude, opposite in direction; produce no net change in motion. Unbalanced forces: Unequal magnitudes or directions; cause acceleration or change in motion. Balanced forces keep objects at rest or moving with constant velocity.
Q7. A pressure of 10 Pa is applied over 4 m². Find the total force.
Step 1: F = P × A
Step 2: F = 10 × 4 = 40 N
Total force = 40 Newton
Q8. Why do we wear heeled shoes instead of flat shoes in winter on icy roads?
Heeled shoes have a smaller contact area. Smaller area means greater pressure on ice. However, in reality, both heels and flats are risky on ice. Better answer: flat shoes distribute pressure evenly, providing more friction. Heels concentrate pressure and reduce friction on ice, increasing slipping.
Force and Pressure — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. What is a push or a pull acting on an object called?
- A. Force
- B. Pressure
- C. Energy
- D. Work
Answer: A. Force — A force is defined as a push or a pull acting on an object that can change its state of motion or shape.
MCQ 2. Which of the following can a force NOT do to an object?
- A. Change its speed
- B. Change its direction
- C. Change its mass
- D. Change its shape
Answer: C. Change its mass — Force can change the speed, direction, or shape of an object, but it cannot change an object's mass.
MCQ 3. What is the SI unit of force?
- A. Pascal
- B. Joule
- C. Newton
- D. Watt
Answer: C. Newton — The SI unit of force is Newton (N). Pascal is for pressure, Joule for energy, and Watt for power.
MCQ 4. Which of these is an example of a contact force?
- A. Magnetic force
- B. Electrostatic force
- C. Frictional force
- D. Gravitational force
Answer: C. Frictional force — Frictional force acts when two surfaces are in direct physical contact. Magnetic, electrostatic, and gravitational forces are non-contact forces.
MCQ 5. A boy kicking a football is an example of which type of force?
- A. Gravitational force
- B. Muscular force
- C. Magnetic force
- D. Electrostatic force
Answer: B. Muscular force — Muscular force is a contact force exerted by the muscles of an animal or human body.
MCQ 6. If two forces of 10 N and 5 N act on an object in the same direction, what is the net force acting on the object?
- A. 5 N
- B. 10 N
- C. 15 N
- D. 2 N
Answer: C. 15 N — When forces act in the same direction, the net force is the sum of the individual forces. So, 10 N + 5 N = 15 N.
MCQ 7. Two forces, 20 N acting to the right and 12 N acting to the left, are applied to a box. What is the net force on the box?
- A. 8 N to the right
- B. 8 N to the left
- C. 32 N to the right
- D. 32 N to the left
Answer: A. 8 N to the right — When forces act in opposite directions, the net force is the difference between them, and its direction is that of the larger force. 20 N - 12 N = 8 N, directed to the right.
MCQ 8. Which force is responsible for an apple falling from a tree?
- A. Muscular force
- B. Frictional force
- C. Electrostatic force
- D. Gravitational force
Answer: D. Gravitational force — Gravitational force is a non-contact force that pulls objects towards the Earth's center.
MCQ 9. When you rub a plastic comb through dry hair, it can attract small pieces of paper. This is due to:
- A. Magnetic force
- B. Gravitational force
- C. Electrostatic force
- D. Frictional force
Answer: C. Electrostatic force — Rubbing the comb generates static electricity, and the force of attraction between the charged comb and uncharged paper is an electrostatic force.
MCQ 10. Which force causes a compass needle to align with the Earth's magnetic field?
- A. Muscular force
- B. Magnetic force
- C. Gravitational force
- D. Frictional force
Answer: B. Magnetic force — A compass needle is a small magnet, and it aligns itself due to the Earth's magnetic force.
MCQ 11. What is defined as the force acting per unit area?
- A. Energy
- B. Pressure
- C. Power
- D. Work
Answer: B. Pressure — Pressure is defined as the force acting perpendicularly on a unit surface area (P = F/A).
MCQ 12. The SI unit of pressure is:
- A. Newton
- B. Joule
- C. Pascal
- D. Watt
Answer: C. Pascal — The SI unit of pressure is Pascal (Pa), which is equivalent to Newtons per square meter (N/m²).
MCQ 13. If a force of 100 N acts on an area of 10 m², what is the pressure exerted?
- A. 10 Pa
- B. 100 Pa
- C. 1000 Pa
- D. 1 Pa
Answer: A. 10 Pa — Pressure = Force / Area. So, Pressure = 100 N / 10 m² = 10 Pa.
MCQ 14. Why do porters use a large piece of cloth on their heads when carrying heavy loads?
- A. To increase the force on their head
- B. To decrease the force on their head
- C. To increase the area of contact and reduce pressure
- D. To decrease the area of contact and reduce pressure
Answer: C. To increase the area of contact and reduce pressure — By increasing the contact area, the pressure exerted by the load on their head is reduced (P=F/A), making it less painful to carry.
MCQ 15. A sharp knife cuts vegetables more easily than a blunt knife because:
- A. It applies more force
- B. It applies less force
- C. It exerts more pressure due to smaller area
- D. It exerts less pressure due to larger area
Answer: C. It exerts more pressure due to smaller area — A sharp knife has a very small cutting edge (area). For the same applied force, a smaller area results in higher pressure (P=F/A), making it easier to cut.
MCQ 16. The pressure exerted by a liquid at a certain depth:
- A. Increases with depth
- B. Decreases with depth
- C. Remains constant with depth
- D. Is only dependent on the container's shape
Answer: A. Increases with depth — The pressure exerted by a liquid increases with increasing depth because the column of liquid above that point becomes taller, increasing the weight per unit area.
MCQ 17. Liquids exert pressure on:
- A. Only the bottom of the container
- B. Only the walls of the container
- C. Only the top surface of the liquid
- D. The bottom and walls of the container
Answer: D. The bottom and walls of the container — Liquids exert pressure in all directions (downwards, upwards, and sideways) on the bottom and walls of the container.
MCQ 18. The pressure exerted by the air around us is known as:
- A. Liquid pressure
- B. Gas pressure
- C. Atmospheric pressure
- D. Vacuum pressure
Answer: C. Atmospheric pressure — The immense weight of the air column above us exerts pressure, which is called atmospheric pressure.
MCQ 19. We usually do not feel the atmospheric pressure because:
- A. It is very small
- B. Our blood pressure is much higher than atmospheric pressure
- C. Our body exerts an outward pressure that balances the atmospheric pressure
- D. We are accustomed to it
Answer: C. Our body exerts an outward pressure that balances the atmospheric pressure — The fluids inside our body exert an outward pressure that is equal and opposite to the atmospheric pressure, thus balancing it.
MCQ 20. When an object is at rest on a surface, the force of gravity pulling it down is balanced by which force from the surface?
- A. Frictional force
- B. Magnetic force
- C. Normal force
- D. Muscular force
Answer: C. Normal force — The normal force is a contact force exerted by a surface on an object in contact with it, acting perpendicular to the surface. It balances the gravitational force for an object at rest on a horizontal surface.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.