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Exploring Forces

NCERT Class 8 · Science Based on NCERT Class 8 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter introduces the concept of force as a push or pull resulting from the interaction between two objects. It explains the various effects of force, such as changing speed, direction, or shape of an object, and classifies forces into contact forces (muscular force, friction) and non-contact forces (magnetic, electrostatic, gravitational). The big idea a student must walk away with is that force is everywhere — it is the cause of any change in motion or shape, and it always involves an interaction between at least two objects.
Key Concepts (12)
Force
A push or pull applied on an object resulting from the object's interaction with another object.
e.g. Pushing a heavy cardboard box across the floor or pulling a drawer open.
Contact force
A force that acts only when there is physical contact between the objects applying and receiving the force.
e.g. Pushing a table with your hand — your hand must touch the table.
Muscular force
The force resulting due to the action of muscles in our body, occurring when muscles contract and elongate.
e.g. Walking, running, lifting a school bag, or chewing food.
Friction
The force that comes into play when an object moves or tries to move over another surface; it always acts in a direction opposite to the direction of motion.
e.g. A ball rolling on a flat ground stops on its own after some time due to friction between the ball and the ground.
Non-contact force
A force whose effect can be experienced even if the objects are not in contact with each other.
e.g. A magnet attracting an iron nail from a distance without touching it.
Magnetic force
The force exerted by a magnet on another magnet or a magnetic material.
e.g. Two ring magnets with like poles facing each other repel, causing one to float above the other.
Electrostatic force
The force exerted by a charged body on another charged body or an uncharged body.
e.g. A plastic scale rubbed with polythene attracts small pieces of paper without touching them.
Gravitational force
The force with which the Earth attracts objects towards itself; it is always an attractive force.
e.g. A ball thrown vertically upwards falls back down to the ground due to the Earth's pull.
Weight
The force with which the Earth pulls an object towards itself; measured in newton (N).
e.g. A pencil box hung from a spring stretches the spring; the amount of stretch indicates its weight.
Mass
The amount of matter in an object; measured in grams (g) or kilograms (kg); its value remains the same everywhere.
e.g. A 1 kg object on Earth has a mass of 1 kg even on the Moon.
Buoyant force (upthrust)
The force applied by a liquid on an object in the upward direction.
e.g. When you push an empty bottle into water, you feel an upward push, and the bottle bounces back to the surface.
Spring balance
A simple device used to measure weight (force), consisting of a spring fixed at one end with a hook at the other end.
e.g. Hanging a partially filled water bottle from a spring balance shows its weight on the scale in newton.
How It Works
How friction opposes motion
  1. Step 1: Two surfaces come in contact; even smooth surfaces have minute irregularities.
  2. Step 2: The irregularities of the two surfaces lock into each other.
  3. Step 3: When one surface tries to move over the other, the locked irregularities oppose the motion.
  4. Step 4: This opposing force is friction, which acts in the direction opposite to the motion.
💡 Friction always opposes motion — it never helps motion. Students often think friction helps in walking, but the chapter defines it as opposing motion or attempted motion.
How electrostatic force arises from rubbing
  1. Step 1: Two objects of certain materials (e.g., plastic scale and polythene) are rubbed together vigorously.
  2. Step 2: Electrical charges build up on their surfaces — these are static charges.
  3. Step 3: The object that acquires static charges is called a charged object.
  4. Step 4: The charged object exerts a force (attraction or repulsion) on another charged or uncharged object without contact.
  5. Step 5: Like charges repel; unlike charges attract.
💡 Both the rubbing object and the rubbed object get charged, but they acquire opposite kinds of charges. Students often think only one object gets charged.
How weight is measured using a spring balance
  1. Step 1: Hang the object from the hook of the spring balance.
  2. Step 2: The Earth's gravitational force pulls the object downward, stretching the spring.
  3. Step 3: The amount of stretching corresponds to the weight of the object.
  4. Step 4: Read the scale on the balance — the scale is marked in newton (N) to show weight.
  5. Step 5: Ensure the object is not heavier than the maximum range of the balance to avoid damage.
💡 The spring balance measures weight (force), not mass directly. The mass scale on it is marked assuming Earth's gravity.
How objects float or sink in a liquid
  1. Step 1: When an object is placed in a liquid, two forces act on it: gravitational force (downward) and buoyant force (upward).
  2. Step 2: If the gravitational force is more than the buoyant force, the object sinks.
  3. Step 3: If the gravitational force is equal to the buoyant force, the object floats.
  4. Step 4: The buoyant force depends on the density of the liquid (and the volume of liquid displaced).
💡 Floating or sinking depends on the balance of forces, not just on the weight of the object. A heavy ship floats because its shape displaces enough water to create an equal buoyant force.
Exam Traps — Don't Make These Mistakes
⚠ Confusing mass and weight
✗ Mass and weight are the same thing, just measured in different units.
→ Mass is the amount of matter (constant everywhere), while weight is the gravitational force on that mass (varies with location). They are fundamentally different physical quantities.
✓ Mass is the amount of matter in an object, measured in kg or g, and remains the same everywhere. Weight is the force with which the Earth pulls the object, measured in newton (N), and can change with location (e.g., on the Moon, weight becomes one-sixth).
⚠ Thinking friction only acts on moving objects
✗ Friction acts only when an object is moving.
→ Friction also acts when an object tries to move (static friction) — the chapter says 'when an object moves or tries to move over another surface'.
✓ Friction comes into play when an object moves or tries to move over another surface. It opposes the motion or attempted motion.
⚠ Assuming force always requires contact
✗ Force can only be applied when two objects are touching each other.
→ There are non-contact forces (magnetic, electrostatic, gravitational) that act without physical contact.
✓ Forces can be contact forces (requiring physical contact) or non-contact forces (acting from a distance, e.g., gravitational, magnetic, electrostatic).
⚠ Thinking weight is measured in kilograms
✗ The weight of this bag is 10 kg.
→ Kilogram is the unit of mass, not weight. Weight is a force and its SI unit is newton (N). In everyday language we misuse the term, but in science it is incorrect.
✓ The weight of this bag is 98 N (approximately). The mass of the bag is 10 kg.
⚠ Confusing direction of friction
✗ Friction acts in the same direction as the motion of the object.
→ Friction always acts in the direction opposite to the direction in which the object is moving or trying to move.
✓ Friction always acts opposite to the direction of motion (or attempted motion) of the object.
Compare & Contrast
Definition and constancy
Mass
Weight
Mass is the amount of matter in an object and remains the same everywhere; weight is the gravitational force on the object and varies with location (e.g., on the Moon, weight is one-sixth of that on Earth).
Need for physical contact
Contact force
Non-contact force
Contact forces (e.g., muscular force, friction) require physical contact between objects to act; non-contact forces (e.g., gravitational, magnetic, electrostatic) act even when objects are not in contact.
Nature of force (attractive/repulsive)
Gravitational force
Magnetic force
Gravitational force is always attractive (pulls objects towards Earth); magnetic force can be either attractive (unlike poles) or repulsive (like poles).
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 SA-I (2 marks), 1 SA-II (3 marks), 1 LA (5 marks) — total approximately 12 marks
Most asked: Definition and effects of force · Difference between mass and weight · Types of forces (contact vs non-contact) with examples · Friction — direction and dependence on surface roughness · Gravitational force and weight measurement
Don't skip: Differentiate between mass and weight with examples and units.; Explain the concept of friction and why a ball rolling on a rough surface stops sooner than on a smooth surface.; Classify the following forces into contact and non-contact: muscular force, friction, magnetic force, electrostatic force, gravitational force.; Describe an activity to show that electrostatic force is a non-contact force.
One-Line Revision (read aloud before the exam)
  1. Force is a push or pull on an object resulting from its interaction with another object.
  2. The SI unit of force is newton (N).
  3. Force can make an object move from rest, change its speed, change its direction of motion, or change its shape.
  4. At least two objects must interact for a force to come into play.
  5. Contact forces require physical contact between objects; examples are muscular force and friction.
  6. Non-contact forces act without physical contact; examples are magnetic, electrostatic, and gravitational forces.
  7. Friction always acts in a direction opposite to the direction of motion or attempted motion.
  8. Friction depends on the nature of the surfaces in contact — greater on rough surfaces.
  9. Magnetic force can be attractive or repulsive; gravitational force is always attractive.
  10. Electrostatic force arises from static charges built up by rubbing certain materials together.
  11. Weight is the force with which the Earth pulls an object; its unit is newton (N).
  12. Mass is the amount of matter in an object; it remains the same everywhere, unlike weight.
  13. The weight of an object on the Moon is one-sixth of its weight on Earth; mass remains unchanged.
  14. Buoyant force (upthrust) is the upward force exerted by a liquid on an object placed in it.
  15. An object floats if the buoyant force equals its weight; it sinks if the gravitational force is greater.

MCQs — 10 Questions with Answers

Q1. What is the SI unit of force?

  • A. Newton (N) ✓
  • B. Kilogram (kg)
  • C. Metre (m)
  • D. Second (s)

Answer: A — The newton is the standard international unit for measuring force; kilogram measures mass, not force.

Q2. Which of the following is an example of a contact force?

  • A. Magnetic force between two magnets
  • B. Gravitational force on a falling object
  • C. Friction between a book and a table ✓
  • D. Force exerted by the Earth on the Moon

Answer: C — Friction requires two surfaces to be in physical contact; the other options are non-contact forces acting at distance.

Q3. A ball rolling on a floor eventually stops. What force is responsible for this?

  • A. Magnetic force
  • B. Gravitational force
  • C. Friction ✓
  • D. Muscular force

Answer: C — Friction between the ball and floor opposes its motion and gradually slows it until it stops.

Q4. Which of the following is NOT an effect of force?

  • A. Change the speed of a moving object
  • B. Change the direction of motion
  • C. Make an object move from rest
  • D. Prevent change in temperature of an object ✓

Answer: D — Forces change motion and shape but do not directly affect temperature; thermal energy changes temperature, not mechanical force.

Q5. Ragini finds it harder to pedal her bicycle uphill than on flat ground. Which forces are responsible for this increased difficulty?

  • A. Only gravitational force pulling downward
  • B. Gravitational force and friction acting together ✓
  • C. Only friction between tyres and road
  • D. Magnetic force and muscular force

Answer: B — Uphill, both gravity (pulling the bike down) and friction (resisting motion) work against the cyclist's muscular force.

Q6. Sonali's bicycle moves faster downhill without her pedalling. What force is pulling the bicycle down?

  • A. Magnetic force
  • B. Frictional force
  • C. Gravitational force ✓
  • D. Muscular force of her legs

Answer: C — Gravity pulls objects downward; although friction opposes motion, gravity is the force accelerating the bicycle down the slope.

Q7. Why is it easier to slip on a wet floor than a dry floor?

  • A. Wet floors are heavier
  • B. Water reduces the friction between shoes and floor ✓
  • C. Wet floors attract magnets
  • D. Gravity is weaker on wet floors

Answer: B — Water lubricates the surface, reducing microscopic irregularities' interlocking, thus decreasing friction and causing slipping.

Q8. Two ring magnets are placed with like poles facing each other. What can we conclude about the magnetic force between them?

  • A. Magnetic force is a contact force because magnets must touch to repel
  • B. Magnetic force is a non-contact force because it acts even without touching ✓
  • C. Magnetic force only acts when magnets are moving
  • D. Magnetic force is the same as frictional force

Answer: B — Magnetic force acts through space without physical contact; like poles repel and unlike poles attract at a distance.

Q9. A book is placed on a rough surface and on a smooth glass surface. On which surface will friction be greater, and why?

  • A. Rough surface because irregularities lock together more tightly ✓
  • B. Smooth surface because glass is hard
  • C. Both surfaces produce equal friction
  • D. It depends only on the weight of the book

Answer: A — Rough surfaces have more pronounced microscopic irregularities that interlock more strongly, creating greater friction than smooth surfaces.

Q10. A swimmer pushes water backward with muscular force to move forward. What does this tell us about forces?

  • A. Only one object is involved in applying force
  • B. Forces always occur when two or more objects interact ✓
  • C. Water cannot exert force on the swimmer
  • D. Muscular force is a non-contact force

Answer: B — The swimmer (one object) interacts with water (another object), demonstrating that force requires interaction between at least two objects.

Flashcards

Define force in one sentence using the word 'interaction'.

A force is a push or pull on an object resulting from the object's interaction with another object.

State the SI unit of force.

The SI unit of force is newton, with symbol N.

List four effects that a force can produce on an object.

Force can make an object move from rest, change its speed, change its direction of motion, or change its shape.

What is friction and in which direction does it act?

Friction is the force between two surfaces in contact that opposes motion and always acts opposite to the direction of motion.

Why does friction arise between two surfaces?

Friction arises due to microscopic irregularities on both surfaces that lock into each other and oppose motion.

Give two examples of contact forces.

Muscular force (lifting, pushing) and friction (object sliding on a table) are contact forces.

What is muscular force and give one example from the human body.

Muscular force is the force produced by muscles contracting and elongating; examples include chewing food or heart contraction for blood circulation.

Why can a magnet exert force on another magnet without touching it?

Magnetic force is a non-contact force that acts through space between two magnetic objects without physical contact.

Does friction depend on the nature of surfaces? Explain briefly.

Yes, friction depends on the nature of surfaces—rough surfaces produce greater friction than smooth surfaces.

If an object is at rest, does it mean no force is acting on it?

No, an object at rest can have multiple forces acting on it that balance each other out.

Important Board Questions

What is force? Give its SI unit. [1 mark]

Define force as push or pull from interaction; state SI unit as newton (N).

Describe the difference between contact forces and non-contact forces with one example each. [2 marks]

Contact forces: require physical contact (friction, muscular force); Non-contact forces: act without touching (magnetic, gravitational). Name one of each.

Why does friction occur between two surfaces? Explain how friction opposes the motion of a sliding object with a real-life example. [3 marks]

Friction arises from microscopic irregularities that lock together; acts opposite to motion direction. Example: ball rolling on ground slows and stops due to friction.

Draw and label a diagram showing the effect of friction on a moving object. Explain why objects stop moving on rough surfaces sooner than on smooth surfaces. List any three everyday effects of friction. [5 marks]

Diagram: show object with friction arrow opposite to motion arrow. Rough surfaces have more irregularities → greater friction → stops sooner. Effects: wear shoes, brake pads, match stick friction.

Assertion-Reason Questions for Exploring Forces

Q1. Assertion (A):

A force always requires interaction between at least two objects.

Reason (R):

A force results from one object pushing or pulling another object; a single isolated object cannot experience a force.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why the assertion is true. The textbook defines force as resulting from object interaction, and logically, two distinct objects are necessary for an interaction to occur. One object alone cannot push or pull itself.

Q2. Assertion (A):

Friction is a contact force that opposes the motion of an object.

Reason (R):

Friction acts between surfaces in contact and always acts in the same direction as the object's motion to speed it up.

Answer: C — A is true but R is false — The assertion is true—friction is indeed a contact force opposing motion (the textbook implies this through examples of rolling balls slowing down). However, the reason is false. Friction acts opposite to the direction of motion, slowing objects down, not speeding them up. The reason contradicts the fundamental nature of friction.

Q3. Assertion (A):

Muscular force is responsible for movement and survival in animals, birds, fish, and insects.

Reason (R):

Muscular force is produced when muscles contract and elongate during physical activities.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both the assertion and reason are true. The textbook states that animals use muscular forces for movement and survival, and explains that muscular force occurs when muscles contract and elongate. However, the reason does not directly explain why muscular force enables survival—it merely describes the mechanism. The reason is true but not the complete explanation for the assertion.

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Case-Based Questions (CBQ) for Exploring Forces

Case Study:

A group of Class 8 students conducted an experiment to understand friction. They took a wooden block and placed it on different surfaces: smooth glass, a wooden table, a rubber mat, and sandpaper. They pushed each block with increasing force until it started moving, and recorded the force required to initiate motion. On glass, 2 N was needed; on the wooden table, 5 N; on the rubber mat, 8 N; and on sandpaper, 12 N. The students noticed that rougher surfaces required greater force to move the block.

Q1. Which surface demonstrated the greatest frictional force opposing the block's motion?

  • A. Smooth glass
  • B. Wooden table
  • C. Rubber mat
  • D. Sandpaper ✓

Answer: D

Q2. What can be concluded about the relationship between surface roughness and frictional force?

  • A. Smoother surfaces create more friction
  • B. Rougher surfaces create more friction ✓
  • C. Friction is independent of surface texture
  • D. Friction depends only on the block's weight, not surface texture

Answer: B

Q3. Why is more muscular force needed to push the block on sandpaper compared to glass?

  • A. Because sandpaper is heavier than glass
  • B. Because friction force between the block and sandpaper is greater due to the rougher surface ✓
  • C. Because gravity is stronger on sandpaper surfaces
  • D. Because the block's weight changes depending on the surface

Answer: B

Q4. If the students wanted to move the wooden block on the rubber mat with minimal effort, what could they do based on the experiment's findings?

  • A. Place the block on a rougher surface like sandpaper
  • B. Use the same rubber mat but push faster
  • C. Increase the block's weight to increase friction
  • D. Place the block on a smoother surface like glass where less frictional force opposes motion ✓

Answer: D

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