Chapter 5: Exploring Forces — Class 8 Science

⚙ Chapter 5: Exploring Forces

CBSE Class 8 Science — Curiosity Textbook

🎯 Learning Objectives

  • Define force and identify it as a push or pull resulting from interaction between objects.
  • Describe the effects a force can have on an object — change in speed, direction, shape, or state of motion.
  • Explain that forces result from interaction between at least two objects.
  • Classify forces into contact forces (muscular, friction) and non-contact forces (magnetic, electrostatic, gravitational).
  • Explain friction, its causes, and factors affecting it.
  • Distinguish between weight and mass, and use a spring balance to measure weight.
  • Explain buoyant force and Archimedes Principle, and apply them to floating and sinking.
  • Solve numerical problems on force, weight, and buoyancy.

📖 Chapter Index

5.1 What Is a Force?
5.2 What Can a Force Do?
5.3 Forces as Interaction
5.4 Types of Forces
5.4.1 Contact Forces
5.4.2 Non-contact Forces
5.5 Weight and Measurement
5.6 Floating and Sinking
📝 Take Quiz

⚙ 5.1 What Is a Force?
Force push and pull diagram

  • A force is a push or pull on an object resulting from the object’s interaction with another object.
  • The SI unit of force is newton (written with a small ‘n’) and its symbol is N.
  • Forces can be experienced as a push (e.g., pushing a box), a pull (e.g., pulling a rope), or a lift (e.g., lifting a bag).
  • Example from daily life: Wind pushing cyclists, uphill gravity making pedalling harder, brakes stopping a bicycle — all involve force.

🔬 Activity 5.1: Let us explore

  • Take a large cardboard box.
  • Try moving the box in as many different ways as you can — push it, pull it, lift it.
  • Observation: In all ways used to move the box, you had to apply a push or pull. This push or pull is called force.

🔢 Numerical Questions

  1. A force of 50 N is applied on an object. Express this force in terms of its definition. What does 50 N mean physically?
  2. Two students apply forces of 30 N and 20 N on a box from opposite sides. What is the net force and in which direction will the box move?
  3. A person applies 40 N force to push a wall but the wall does not move. Is force being applied? Is there a reaction force?
💡 Remember: Force = Push or Pull. SI unit = Newton (N). At least two objects must interact.

⚙ 5.2 What Can a Force Do to the Objects on Which It Is Applied?
Effects of force on objects

  • The force applied on an object may:
    • Make an object move from rest — e.g., pushing a stationary box.
    • Change the speed of a moving object — e.g., applying brakes slows a bicycle.
    • Change the direction of motion — e.g., hitting a moving ball with a bat.
    • Change the shape of an object — e.g., pressing an inflated balloon.
    • Cause some or all of these effects simultaneously.
  • Examples: Opening a drawer (pull), stretching a rubber band (pull), kicking a football (push), rolling a chapati (push), applying brakes (force opposing motion).

🔬 Activity 5.2: Let us analyse

  • Think of situations where a force (push or pull) is applied.
  • Friend holding your moving bicycle from behind → Pull → Stops or decreases the speed.
  • Hitting a moving ball with a bat → Push → Changes the direction of the moving ball.
  • Pressing an inflated balloon → Push → Change in shape of the balloon.
  • Conclusion: Force can stop a moving object, change its speed, change its direction, or change its shape.

🔢 Numerical Questions

  1. A ball moving at 10 m/s is hit by a bat and its speed becomes 15 m/s in the opposite direction. How many effects of force are demonstrated here?
  2. A rubber band is stretched from 10 cm to 16 cm. What effect of force does this show?
  3. A force of 25 N is applied to start a stationary toy car. After some time, a braking force of 15 N is applied. Describe the sequence of effects of force on the car.
💡 Remember: Force can — Start motion, Stop motion, Change speed, Change direction, Change shape.

⚙ 5.3 Are Forces an Interaction Between Two or More Objects?
Force as interaction between two objects

  • When you push a table, your hand and the table are two objects interacting with each other.
  • At least two objects must interact for a force to come into play.
  • Whenever two objects interact, each object experiences a force from the other. As soon as the interaction ceases, the force disappears.
  • An object at rest does not mean no force is acting — it means the forces acting on it are balancing each other (balanced forces).

🔢 Numerical Questions

  1. Two teams in a tug-of-war apply forces of 400 N each in opposite directions. What is the net force? Will the rope move?
  2. Team A applies 500 N and Team B applies 350 N in a tug-of-war. What is the net force and who wins?
  3. A book of weight 10 N rests on a table. What force does the table apply on the book? In which direction?
💡 Remember: Force requires at least 2 interacting objects. Object at rest = balanced forces, NOT no force.

⚙ 5.4 What Are the Different Types of Forces?
Classification of forces contact and non-contact

  • Forces are broadly classified into two types:
    • Contact Forces — act only when there is physical contact between objects. Examples: Muscular force, Friction.
    • Non-contact Forces — act even without physical contact. Examples: Magnetic force, Electrostatic force, Gravitational force.
  • Contact can be direct (using hands or body parts) or indirect (using a stick or rope).

🔢 Numerical Questions

  1. Classify the following as contact or non-contact force: (a) A book falling from a shelf (b) A boy pushing a cart (c) A magnet attracting a nail from 2 cm away (d) A fielder catching a ball.
  2. Give one example each of contact and non-contact force acting on a bicycle in motion.
💡 Remember: Contact forces need physical touch. Non-contact forces act from a distance.

⚙ 5.4.1 Contact Forces
Muscular force and friction contact forces diagram

  • Muscular Force: The force resulting from the action of muscles is called muscular force. It occurs when muscles contract and elongate. Examples: walking, running, lifting, pushing, jumping, stretching. Animals, birds, fish, and insects use muscular force for movement and survival.
  • Muscular force also works inside our body — it helps chew food, pushes food through the alimentary canal, and the heart muscles pump blood throughout the body.
  • Friction: The force that comes into play when an object moves or tries to move over another surface is called force of friction (or simply friction).
  • Friction always acts in a direction opposite to the direction of motion or attempted motion.
  • Friction arises due to the irregularities on the two surfaces in contact. Even smooth surfaces have minute irregularities that lock into each other.
  • Friction is greater on rough surfaces and less on smooth surfaces.
  • Air, water, and other liquids also exert friction on objects moving through them. This is why aeroplanes, ships, and high-speed trains have streamlined shapes.

🔬 Activity 5.3: Let us investigate

  • Take an object with a flat base (lunch box or geometry box) and place it on a table or floor.
  • Gently push it and observe. Does it stop after travelling some distance?
  • Push it in the opposite direction. Does it stop again?
  • Observation: The object stops due to a force (friction) acting between its surface and the table surface, in the direction opposite to its motion.

🔬 Activity 5.4: Let us explore

  • Repeat Activity 5.3 but place the same object on different surfaces: glass, cloth, wood, ceramic tile, and sand.
  • Does the object stop after travelling the same distance on all surfaces?
  • Observation: The object stops at different distances on different surfaces. Friction is greater on rough surfaces (sand, cloth) and less on smooth surfaces (glass, tile).
  • Conclusion: The force of friction depends upon the nature of the surfaces in contact.

🔢 Numerical Questions

  1. A box is pushed with a force of 60 N on a rough floor. Friction acts with a force of 40 N. What is the net force on the box? In which direction does it move?
  2. A ball rolls on surface A and stops after 5 m. On surface B, it stops after 2 m. Which surface has more friction? Why?
  3. A cyclist applies 80 N of pedalling force. Friction from road = 30 N, air resistance = 10 N. What is the net forward force?
💡 Remember: Friction opposes motion. More rough surface = more friction. Friction acts between surfaces in contact.

⚙ 5.4.2 Non-contact Forces
Non-contact forces magnetic electrostatic gravitational

  • Magnetic Force: The force exerted by a magnet on another magnet or a magnetic material is called magnetic force. Like poles repel; unlike poles attract. A magnet can exert force from a distance without contact — hence it is a non-contact force.
  • Electrostatic Force: When two objects of certain materials are rubbed together, static charges build up. A charged object can attract uncharged objects without contact. The force exerted by a charged body on another charged or uncharged body is called electrostatic force. Like charges repel; unlike charges attract.
  • Gravitational Force: The force with which the Earth attracts objects towards itself is called gravitational force (or force of gravity or simply gravity). It is always attractive — unlike magnetic or electrostatic force. It is a non-contact force.
  • An object thrown upward slows down, stops momentarily at the top, then falls back — this is due to gravitational force acting downward throughout.

🔬 Activity 5.5: Let us test (Magnetic Force)

  • Take two ring magnets and a wooden stick. Insert one ring magnet onto the stick held vertically.
  • Insert the second magnet above it with like poles facing each other.
  • Observation: The second magnet floats above the first — showing repulsion between like poles without contact. This is magnetic force acting as a non-contact force.
  • Reverse the poles — the second magnet is attracted downward (unlike poles attract).

🔬 Activity 5.6: Let us experiment (Electrostatic Force)

  • Rub a plastic scale or straw vigorously with polythene.
  • Bring it close to small pieces of paper on a table without touching.
  • Observation: The paper pieces get attracted towards the charged plastic scale — showing electrostatic force acting without contact.

🔬 Activity 5.7: Let us experiment (Electrostatic — Charges)

  • Inflate two balloons and rub both with a woollen cloth. Hang them close together.
  • Observation: The similarly charged balloons repel each other (like charges repel).
  • Bring the woollen cloth near one charged balloon.
  • Observation: The balloon and cloth attract each other (unlike charges attract). The two kinds of static charges are positive and negative.

🔬 Activity 5.8: Let us observe (Gravitational Force)

  • Take a ball and throw it vertically upwards. Does it come down?
  • Throw it again harder. Does it still fall back down?
  • Observation: Every object thrown up always comes back down. The Earth attracts (pulls) all objects towards itself — this is gravitational force. It is always attractive and acts without contact.

🔢 Numerical Questions

  1. Two magnets repel each other with a force of 0.5 N when kept 2 cm apart. What type of poles are facing each other? If the distance is increased, how does the force change?
  2. A charged balloon attracts small pieces of paper from a distance of 3 cm. What force is this? Can this force work without contact?
  3. A ball is thrown upward with a force of 50 N. Gravity acts downward with 10 N on it. What is the net force while going up? What happens to the speed?
💡 Remember: Magnetic and Electrostatic — can attract or repel. Gravitational — always attractive. All three act without contact.

⚙ 5.5 Weight and Its Measurement
Spring balance weight measurement newton unit

  • Weight is the force with which the Earth pulls an object towards itself. It is measured in newton (N).
  • Mass is the amount of matter in an object. It is measured in grams (g) or kilograms (kg). Mass remains the same everywhere.
  • Weight varies from place to place as gravitational force varies. On Moon, weight is about 1/6th of weight on Earth.
  • A spring balance is used to measure weight. It has a spring fixed at one end with a hook at the other. When an object is hung, the spring stretches — the amount of stretch gives the weight in newtons.
  • The Earth pulls different objects with different forces — heavier objects stretch the spring more.
  • Weight comparison across celestial bodies (for 1 kg mass): Earth = 10 N, Moon = 1.6 N, Mars = 3.8 N, Venus = 9 N, Jupiter = 25.4 N.

🔬 Activity 5.9: Let us explore

  • Take a spring and objects of different masses (pencil box, tiffin box, small stone).
  • Hang one end of the spring from a nail. Hang each object one by one from the other end.
  • Observation: The spring stretches by different amounts for different objects. Heavier objects stretch the spring more. This shows Earth pulls different objects with different forces — weight of different objects is different.

🔬 Activity 5.10 & 5.11: Reading the Spring Balance

  • Look at the spring balance carefully. Note the maximum weight (range). Example: 0 to 10 N.
  • Note the number of divisions between two bigger marks (e.g., 5 divisions between 0 and 1 N).
  • Smallest division = 1 N ÷ 5 = 0.2 N. So the spring balance can read minimum 0.2 N.

🔬 Activity 5.12: Let us measure

  • Suspend objects one by one from the hook of a spring balance.
  • Read the weight carefully from the scale in newtons.
  • Record: Pencil Box — measure weight (N). Partially filled water bottle — measure weight (N).
  • Note: Objects should not be heavier than the maximum range of the spring balance to avoid damage.

🔢 Numerical Questions

  1. A spring balance has a range of 0 to 20 N with 10 divisions between each 1 N mark. What is the smallest weight it can measure?
  2. An object weighs 60 N on Earth. What is its weight on the Moon? (Moon’s gravity = 1/6th of Earth)
  3. A student weighs 500 N on Earth. Calculate: (a) His mass on Earth (b) His weight on Moon (c) His mass on Moon. (g on Earth = 10 N/kg)
  4. A spring balance reads 0.2 N per division. An object causes 15 divisions of stretch. What is the weight of the object?
💡 Remember: Mass = same everywhere (kg). Weight = varies with gravity (N). Weight on Moon = Weight on Earth ÷ 6.

⚙ 5.6 Floating and Sinking
Floating sinking buoyant force upthrust gravitational force water

  • The force applied by a liquid on an object in the upward direction is known as upthrust or buoyant force.
  • When an object is placed in a liquid: Gravitational force acts downward; Buoyant force acts upward.
  • If gravitational force > buoyant force → object sinks.
  • If gravitational force = buoyant force → object floats.
  • Buoyant force depends on the density of the liquid.
  • Archimedes’ Principle: When an object is fully or partially immersed in a liquid, it experiences an upward force equal to the weight of the liquid it displaces.
  • If weight of liquid displaced < weight of object → object sinks. If weight of liquid displaced = weight of object → object floats.
  • Pumice is a rock formed during volcanic eruptions that can float on water because it is porous and less dense than water.

🔬 Activity 5.13: Let us investigate

  • Take an empty bottle with its lid closed tightly and a bucket full of water.
  • Push the bottle into the water. Do you feel an upward push?
  • Release the bottle. Does it bounce back up to the surface?
  • Observation: Water applies an upward force (buoyant force / upthrust) on the bottle. When released, the buoyant force is greater than the gravitational force on the empty bottle, so it floats.

🔢 Numerical Questions

  1. An object weighs 50 N in air. When immersed in water, it experiences an upthrust of 30 N. What is the apparent weight in water? Does it sink or float?
  2. An object weighs 40 N in air. When placed in water, it displaces water weighing 40 N. Does the object sink or float?
  3. A ship made of steel floats on water but a solid steel ball sinks. Explain using buoyant force.
  4. An object weighs 80 N in air and 60 N when fully immersed in water. Calculate the upthrust (buoyant force).
💡 Remember: Upthrust = Weight of liquid displaced (Archimedes). Float: Buoyant force ≥ Weight. Sink: Buoyant force < Weight.

📝 Chapter 5 Practice Quiz — 25 Questions

5.1 What Is a Force?

LOTS

What is a force? What is its SI unit?
Show Answer
A force is a push or pull on an object resulting from its interaction with another object. The SI unit of force is newton (N).
Medium

Give three examples of force from daily life.
Show Answer
Pushing a box, pulling a rope, kicking a football, lifting a bag, applying brakes on a bicycle (any three).
HOTS

A person pushes a wall with 50 N force but the wall does not move. Is force applied? Explain.
Show Answer
Yes, 50 N force is applied on the wall. The wall applies an equal and opposite reaction force of 50 N on the person. The forces are balanced so the wall does not move.
HOTS

Why do cyclists find it harder to ride against the wind than with the wind?
Show Answer
Wind pushes against the cyclist (opposing force). Riding against wind means the cyclist must apply more muscular force to overcome both friction and wind force. Riding with wind means wind force aids the cyclist, reducing the effort needed.

5.2 What Can a Force Do?

LOTS

List four effects a force can have on an object.
Show Answer
Force can: (1) Make an object move from rest. (2) Change speed. (3) Change direction of motion. (4) Change shape of an object.
Medium

A ball is hit by a bat and changes direction. Which effect of force is shown?
Show Answer
Change in direction of motion. The bat applies force on the moving ball, causing it to move in a new direction.
HOTS

A rubber ball and a clay ball are dropped from the same height. The rubber ball bounces but clay ball does not. Explain using effects of force.
Show Answer
When the rubber ball hits the floor, force changes its direction (it bounces back). Clay ball deforms permanently — force changes its shape instead of direction. Clay absorbs the force by changing shape; rubber restores its shape and redirects the force.
HOTS

Can force cause change in shape without change in motion? Give an example.
Show Answer
Yes. When you press a lump of clay, it changes shape but does not move from its place. The force changes the shape without causing any motion.

5.3 Forces as Interaction

LOTS

How many objects are needed for a force to come into play?
Show Answer
At least two objects must interact for a force to come into play.
Medium

A book rests on a table. Is any force acting on the book? Explain.
Show Answer
Yes. Gravity pulls the book downward with 10 N (its weight). The table pushes the book upward with 10 N (normal force). Both forces are equal and opposite — the book remains at rest due to balanced forces.
HOTS

When you push a wall, you feel a force on your hand too. Why does this force disappear when you stop pushing?
Show Answer
Forces exist only when two objects interact. When you push the wall, interaction occurs and both feel a force. The moment you stop pushing, the interaction ceases and the force disappears.
HOTS

Two teams in a tug-of-war pull with equal force. Why does the rope not move even though force is applied?
Show Answer
The forces are equal and opposite — they balance each other. Net force = 0. According to the concept of balanced forces, an object does not move when net force is zero.

5.4 Types of Forces

LOTS

What is the difference between contact and non-contact forces?
Show Answer
Contact forces act only when there is physical contact between objects (e.g., muscular force, friction). Non-contact forces act even without physical contact (e.g., magnetic, electrostatic, gravitational force).
Medium

Classify: (a) Friction (b) Gravity (c) Muscular force (d) Magnetic force.
Show Answer
(a) Friction — Contact. (b) Gravity — Non-contact. (c) Muscular force — Contact. (d) Magnetic force — Non-contact.
HOTS

A crane uses an electromagnet to lift scrap iron. Which type of force does it use? Why is this better than using a hook?
Show Answer
Electromagnetic (magnetic) force — a non-contact force. It is better than a hook because the magnet can pick up large amounts of scattered scrap iron without needing to attach each piece individually. It can also be switched off to release the load.
HOTS

Can an object experience both contact and non-contact forces at the same time? Give an example.
Show Answer
Yes. A bicycle in motion experiences friction (contact force between tyres and road) and gravity (non-contact gravitational force pulling it down) simultaneously.

5.4.1 Contact Forces

LOTS

What is friction? In which direction does it act?
Show Answer
Friction is 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.
Medium

Why do aeroplanes and high-speed trains have streamlined shapes?
Show Answer
Air exerts friction on objects moving through it. Streamlined shapes are designed to reduce air friction (air resistance), allowing aeroplanes and trains to move faster with less energy consumption.
HOTS

A ball rolls further on a tile floor than on a carpet. Explain why using the concept of friction.
Show Answer
The carpet has a rougher surface with more irregularities than the tile floor. Greater roughness means greater friction. More friction decelerates the ball faster, making it stop sooner on carpet than on the smoother tile floor.
HOTS

Friction is both useful and harmful. Give one example of each.
Show Answer
Useful: Friction between vehicle tyres and road allows vehicles to brake and stop safely. Without friction, vehicles would skid. Harmful: Friction between machine parts causes wear and tear, generates heat, and wastes energy as heat rather than useful work.

5.4.2 Non-contact Forces

LOTS

What is gravitational force? Is it always attractive?
Show Answer
Gravitational force is the force with which the Earth attracts objects towards itself. Yes, gravity is always attractive — unlike magnetic or electrostatic force which can be repulsive too.
Medium

What are static charges? How are they produced?
Show Answer
Static charges are electrical charges that build up on the surface of objects when two objects of certain materials are rubbed together. The object that acquires static charges is called a charged object.
HOTS

The Moon does not fall onto the Earth even though gravity attracts it. Why?
Show Answer
The Moon is moving in an orbit around the Earth. The gravitational force of Earth acts as a centripetal force that keeps the Moon in its circular orbit rather than pulling it straight down. The Moon’s orbital speed balances the gravitational pull.
HOTS

Two charged balloons rubbed with woollen cloth repel each other. What does this tell us about the charges on them?
Show Answer
Since both balloons were rubbed with woollen cloth in the same way, they acquired similar (like) charges. Like charges repel each other. The woollen cloth acquires the opposite type of charge from the balloons.

5.5 Weight and Its Measurement

LOTS

What is the difference between mass and weight?
Show Answer
Mass is the amount of matter in an object — measured in kg, same everywhere. Weight is the gravitational force pulling the object — measured in N, varies from place to place.
Medium

How does a spring balance work to measure weight?
Show Answer
When an object is hung from the hook of a spring balance, the spring stretches due to the object’s weight. The amount of stretching is indicated on the scale in newtons, giving the weight of the object.
HOTS

An astronaut weighs 600 N on Earth. What is his weight on the Moon and on Mars? (Moon gravity = 1/6 of Earth, Mars gravity = 3/8 of Earth)
Show Answer
Weight on Moon = 600 ÷ 6 = 100 N. Weight on Mars = 600 × 3/8 = 225 N. Mass remains 60 kg everywhere.
HOTS

Why is it incorrect to say the weight of a wheat bag is 10 kg in scientific terms?
Show Answer
In science, weight is a force measured in newtons (N), not kilograms. 10 kg is the mass of the wheat bag. Its weight on Earth = 10 kg × 10 N/kg = 100 N. Saying weight = 10 kg mixes up mass and weight.

5.6 Floating and Sinking

LOTS

What is buoyant force? When does an object float?
Show Answer
Buoyant force (upthrust) is the upward force applied by a liquid on an immersed object. An object floats when buoyant force equals or exceeds its weight (gravitational force).
Medium

State Archimedes’ Principle.
Show Answer
When an object is fully or partially immersed in a liquid, it experiences an upward force (buoyant force) equal to the weight of the liquid it displaces.
HOTS

A steel ship floats but a solid steel ball sinks. Explain using buoyant force and density.
Show Answer
A ship is hollow, displacing a large volume of water. The weight of water displaced equals the ship’s weight, so buoyant force = weight and it floats. A solid steel ball is dense and displaces very little water. Buoyant force is less than its weight, so it sinks.
HOTS

Why does an object feel lighter when immersed in water compared to in air?
Show Answer
In water, the object experiences an upward buoyant force in addition to the downward gravitational force. This reduces the net downward force (apparent weight). So the object feels lighter in water than in air where no buoyant force acts.