CBSE Class 8 Science Revision Notes Chapter 5 Exploring Forces

Force is a push or pull that arises when two objects interact with each other.
It can change an object’s speed, direction or shape and may act through contact or from a distance.

We apply forces in many daily activities, such as opening a drawer, lifting a school bag, pedalling a bicycle and stopping a moving ball. Some forces need physical contact, while others act even when objects are apart.

These CBSE Class 8 Science Revision Notes Chapter 5 explain force, friction, gravity, weight and buoyant force. The notes follow the current 2026–27 textbook and use everyday examples for easy revision.

Key Takeaways

• Force: A push or pull caused by an interaction between objects.
• Unit: The SI unit of force and weight is newton, written as N.
• Contact forces: Muscular force and friction require contact.
• Non-contact forces: Magnetic, electrostatic and gravitational forces act from a distance.

Revise Class 8 Science Chapter 5 in 30 Minutes

First 10 minutes: Meaning and effects of force, contact and non-contact forces
Next 10 minutes: Friction, magnetism, electrostatic force and gravity
Final 10 minutes: Mass, weight, spring balance, buoyancy and floating

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Force in Class 8 Science Chapter 5 Notes

A force is a push or pull applied to an object.

Examples include:

• Pushing a box
• Pulling a drawer
• Lifting a bag
• Kicking a football
• Stretching a rubber band
• Applying brakes
• Turning a bicycle handle

Force as an Interaction

A force comes into play when at least two objects interact.

For example:

• A hand pushes a table.
• A bat hits a ball.
• A magnet attracts an iron nail.
• Earth pulls a falling fruit.

In each case, one object applies force on another object.

SI Unit of Force

The SI unit of force is the newton.

Its symbol is:

N

The name of the unit is written with a small letter, but its symbol is written with a capital letter.

Effects of Force in Class 8 Science Notes

A force can produce one or more effects.

Force Can Make an Object Move

A stationary object may start moving when a force is applied.

Example:

A football starts moving when it is kicked.

Force Can Stop a Moving Object

A moving object may slow down or stop.

Example:

A fielder stops a moving cricket ball.

Force Can Change Speed

A force may increase or decrease the speed of an object.

Example:

Pedalling harder makes a bicycle move faster.

Applying brakes reduces its speed.

Force Can Change Direction

A force may change the direction in which an object moves.

Example:

A bat changes the direction of a moving ball.

Turning the bicycle handle changes the bicycle’s direction.

Force Can Change Shape

A force may change the shape of an object.

Examples:

• Pressing a balloon
• Stretching a rubber band
• Rolling dough
• Squeezing a sponge

Effects of Force Table

Does Rest Mean No Force Is Acting?

An object at rest may still have forces acting on it.

For example, a book kept on a table is pulled downward by gravity. The table also provides an upward support force.

The object remains at rest because the forces balance each other.

Detailed study of balanced forces is done in higher classes.

Types of Forces in Class 8 Science

Forces are broadly divided into:

• Contact forces
• Non-contact forces

Contact and Non-Contact Forces

Contact Forces in CBSE Class 8 Science Chapter 5 Notes

Contact forces act only when the objects are touching directly or indirectly.

Indirect contact may occur through a rope, stick or another object.

Muscular Force

The force produced by the action of muscles is called muscular force.

Muscles contract and relax while we perform activities.

Examples include:

• Walking
• Running
• Jumping
• Lifting a bag
• Pushing a cart
• Chewing food
• Moving the limbs

Animals also use muscular force.

Examples include:

• A horse pulling a cart
• A bull ploughing a field
• Birds flying
• Fish swimming
• Insects moving

Muscular Force Inside the Body

Muscular force also performs important internal functions.

It helps:

• Move food through the alimentary canal.
• Pump blood through the heart.
• Move the chest during breathing.
• Chew and swallow food.

Friction in Class 8 Science Chapter 5 Notes

Friction is the force that acts when one object moves or tries to move over another surface.

It acts in the direction opposite to motion or attempted motion.

Examples of Friction

• A rolling ball slowly stops.
• A bicycle slows down when pedalling stops.
• Shoes grip the ground.
• Brakes stop a vehicle.
• A book pushed across a table comes to rest.

Direction of Friction

If an object moves towards the right, friction acts towards the left.

If an object tries to move towards the left, friction acts towards the right.

Therefore, friction always opposes relative motion or attempted motion.

Why Does Friction Arise?

Even surfaces that look smooth contain tiny irregularities.

When two surfaces touch:

• Their irregularities interlock.
• This interlocking resists motion.
• Friction is produced.

A rough surface has more prominent irregularities and usually produces more friction.

Effect of Surface Nature

Friction depends on the nature of the surfaces in contact.

Generally:

• Rough surfaces produce more friction.
• Smooth surfaces produce less friction.
• Wet or polished surfaces may provide less grip.

This is why people may slip on wet tiles or ice.

Friction in Air and Water

Friction also acts on objects moving through fluids.

Fluids include:

• Air
• Water
• Other liquids and gases

This friction is often called drag.

Examples include:

• Air resistance on a cyclist
• Water resistance on a swimmer
• Air resistance on an aeroplane
• Water resistance on a ship

Aeroplanes, boats, trains and racing vehicles are designed with suitable shapes to reduce drag.

Non-Contact Forces in Exploring Forces Notes

Non-contact forces act even when two objects are not physically touching.

The main non-contact forces in this chapter are:

• Magnetic force
• Electrostatic force
• Gravitational force

Magnetic Force

The force exerted by a magnet on another magnet or magnetic material is called magnetic force.

It is a non-contact force.

Attraction and Repulsion Between Magnets

Like poles repel:

• North and North
• South and South

Unlike poles attract:

• North and South

Magnetic Materials

A magnet can attract materials such as iron without touching them.

A magnet may also attract or repel another magnet from a distance.

Electrostatic Force in Class 8 Science

When certain materials are rubbed together, electric charges may build up on their surfaces.

These charges are called static charges.

The charged object can attract or repel another object without contact.

Example with a Plastic Scale

When a plastic scale is rubbed with polythene and brought near paper pieces, the paper pieces are attracted.

The scale has acquired a static charge.

Two Types of Charges

Static charges are of two types:

• Positive charge
• Negative charge

Charge Interaction

• Like charges repel.
• Unlike charges attract.
• A charged object may attract some uncharged objects.

Example with Balloons

Two balloons rubbed in the same way acquire similar charges.

They repel each other.

The balloon and the woollen cloth used for rubbing acquire opposite types of charge, so they attract each other.

Definition of Electrostatic Force

The force exerted by a charged object on another charged or uncharged object is called electrostatic force.

It is a non-contact force.

Gravitational Force in Class 8 Science Chapter 5 Notes

The force with which Earth attracts objects towards itself is called gravitational force.

It is also called:

• Force of gravity
• Gravity

Gravitational force is a non-contact force.

Examples

• A fruit falling from a tree
• A coin falling to the floor
• A ball returning after being thrown upward
• Rain falling towards Earth

Important Feature

Gravitational force is always attractive.

Magnetic and electrostatic forces may attract or repel. Gravity only attracts.

Vertical Motion Under Gravity

When a ball is thrown upward:

1. It moves upward.
2. Its speed gradually decreases.
3. It stops momentarily at the highest point.
4. It changes direction.
5. It falls downward.
6. Its speed increases while falling.

Gravity acts downward throughout the motion.

At the Highest Point

At the highest point:

• The ball’s speed is momentarily zero.
• Gravity is still acting on it.
• The ball then begins to fall.

Zero speed at one instant does not mean that no force is acting.

Weight in Class 8 Science Revision Notes Chapter 5

Weight is the force with which Earth pulls an object towards itself.

Since weight is a force, its SI unit is newton.

Unit of weight = N

Why Do Different Objects Have Different Weights?

Different objects contain different amounts of matter.

Earth pulls heavier objects with a greater force.

When different objects are hung from the same spring, they stretch it by different amounts.

The greater the weight, the greater the stretch.

Mass and Weight Class 8 Notes

Mass and weight are related but different.

Mass

Mass is the amount of matter present in an object.

Its common units are:

• Gram
• Kilogram

Mass remains the same at different places.

Weight

Weight is the gravitational force acting on an object.

Its unit is:

• Newton

Weight may change when gravitational force changes.

Difference Between Mass and Weight

Mass and Weight on Other Planets

The mass of an object remains the same on Earth, the Moon or another planet.

Its weight changes because the gravitational pull is different.

For a 1 kg object, approximate values given in the chapter are:

Measuring Weight with a Spring Balance

A spring balance measures force or weight.

It contains:

• A spring
• A hook
• A pointer or indicator
• A graduated scale

When an object is suspended, the spring stretches. The amount of stretch indicates the object’s weight.

Range of a Spring Balance

The range is the minimum-to-maximum value an instrument can measure.

If a spring balance is marked from 0 N to 10 N, its range is:

0 N to 10 N

An object heavier than the maximum value should not be hung from it because the instrument may get damaged.

Smallest Division of a Spring Balance

The smallest division tells us the smallest value the instrument can measure.

Example

Difference between two large marks = 1 N
Number of small divisions = 5

Smallest division:

1 ÷ 5 = 0.2 N

Therefore, the spring balance can measure in steps of 0.2 N.

Steps to Measure Weight

1. Check the range of the spring balance.
2. Check that the pointer starts at zero.
3. Hold the spring balance vertically.
4. Suspend the object from the hook.
5. Allow the object to become steady.
6. Read the value at eye level.
7. Record the weight in newtons.

Force Comparison Table

Buoyant Force in Class 8 Science Chapter 5 Notes

When an object is placed in a liquid, the liquid applies an upward force on it.

This upward force is called:

• Buoyant force
• Upthrust

Example

When an empty closed bottle is pushed under water:

• Water pushes it upward.
• The bottle may rise back to the surface after release.

This shows that the liquid applies an upward force.

Forces Acting on an Object in Water

Two main forces act on an object placed in water:

• Weight acts downward.
• Buoyant force acts upward.

The result depends on the relative size of these forces.

If Weight Is Greater

If downward gravitational force is greater than buoyant force, the object sinks.

If the Forces Balance

If buoyant force becomes equal to the object’s weight, the object floats.

Archimedes’ Principle

Archimedes’ Principle states that an object fully or partly immersed in a liquid experiences an upward force equal to the weight of the liquid displaced by it.

This helps explain floating and sinking.

In Simple Words

An immersed object pushes some liquid out of its place.

The liquid then pushes the object upward.

The amount of upward force depends on the liquid displaced.

Floating and Sinking Class 8 Notes

Objects do not float or sink only because of their size.

A large wooden block may float, while a small metal coin may sink.

Important factors include:

• Weight of the object
• Buoyant force
• Density of the object
• Density of the liquid
• Volume of liquid displaced
• Shape of the object

Pumice Stone

Pumice is a rock that may float on water.

It contains many air-filled pores formed during volcanic activity.

These pores lower its overall density.

Why Does a Mug Feel Lighter in Water?

A mug inside water experiences upward buoyant force.

This force acts against its weight, so the mug feels lighter.

Contact and Non-Contact Force Examples

Common Mistakes in Class 8 Science Chapter 5

Saying Force Exists Without Interaction

A force arises when objects interact, directly or from a distance.

Thinking Every Force Requires Contact

Magnetic, electrostatic and gravitational forces can act without contact.

Saying Friction Always Stops Motion

Friction opposes motion or attempted motion. It may slow an object, provide grip or help control movement.

Confusing Mass with Weight

Mass is measured in grams or kilograms. Weight is measured in newtons.

Believing Gravity Stops at the Highest Point

Gravity continues to act even when a thrown object is momentarily at rest.

Saying Larger Objects Always Sink

Size alone does not decide floating. Density, shape, weight and buoyant force also matter.

Important Terms in Exploring Forces

Quick Revision of Exploring Forces

• Force is a push or pull.
• At least two objects are involved in a force interaction.
• The SI unit of force is newton.
• Force can change speed, direction or shape.
• Contact forces require physical contact.
• Muscular force is produced by muscles.
• Friction opposes motion.
• Rough surfaces generally produce more friction.
• Magnetic force can attract or repel.
• Like electric charges repel.
• Unlike electric charges attract.
• Gravity always attracts.
• Weight is a force measured in newtons.
• Mass remains the same at different places.
• Weight may change with gravity.
• A spring balance measures weight.
• Buoyant force acts upward in a liquid.
• An object sinks when its weight is greater than the buoyant force.
• An object floats when the upward force balances its weight.

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