Class 8 Science Exploring Forces Notes | CBSE Force and Pressure Chapter Explained

SK Classes

Class 8 Science Premium Notes

Chapter: Exploring Forces

Force • Balanced Force • Unbalanced Force • Contact Force • Non-Contact Force • Mass • Weight • Buoyancy • Density

Board Exam + Foundation Level Preparation

Subject: Science
Class: 8
Board: CBSE
Institute: SK Classes
Purpose: Concept clarity, exam preparation, quick revision and competency-based learning.

Chapter Overview

In this chapter, students learn about force and its effects in daily life. Force is involved whenever we push, pull, lift, throw, stop, stretch, compress, walk, swim or even stand on the ground.

What students will learn

• Meaning of force
• Effects of force on objects
• Balanced and unbalanced forces
• Contact forces and non-contact forces
• Muscular force, friction and tension
• Gravitational, magnetic and electrostatic forces
• Mass, weight, density, buoyant force and upthrust
• Floating and sinking of objects

Exam Tip: This chapter is the foundation of higher physics topics like motion, pressure, gravitation, work, energy and machines.

Important Keywords

Keyword	Meaning
Force	A push or pull acting on an object.
Interaction	Action between two objects that produces force.
Contact Force	A force that acts only when objects touch each other.
Non-Contact Force	A force that acts without physical contact.
Muscular Force	Force produced by muscles.
Friction	A force that opposes motion between two surfaces.
Tension	Pulling force transmitted through a rope, string or cable.
Balanced Force	Forces that cancel each other and do not change motion.
Unbalanced Force	Forces that do not cancel and change motion.
Gravitational Force	Force of attraction between objects due to their masses.
Magnetic Force	Force exerted by a magnet.
Electrostatic Force	Force between charged objects.
Mass	Amount of matter present in an object.
Weight	Force with which Earth attracts an object.
Density	Mass per unit volume of a substance.
Buoyant Force	Upward force exerted by liquid or gas on an object.
Upthrust	Another name for buoyant force.

1. Force and Its Effects

What is Force?

Definition: A force is a push or pull acting on an object.

Examples of Push

• Pushing a door
• Kicking a football
• Pressing a switch
• Pushing a table

Examples of Pull

• Pulling a drawer
• Pulling a rope
• Drawing water from a well
• Pulling a school bag

NCERT Alert: A force arises due to interaction between two objects. Force cannot exist alone.

Effects of Force

• Force can move a stationary object.
• Force can stop a moving object.
• Force can change the speed of an object.
• Force can change the direction of motion.
• Force can change the shape of an object.

FORCE │ ├── Can move a stationary object ├── Can stop a moving object ├── Can increase speed ├── Can decrease speed ├── Can change direction └── Can change shape

2. Balanced and Unbalanced Forces

Balanced Forces

When two or more forces acting on an object are equal in magnitude and opposite in direction, they are called balanced forces.

Student A → [ BOX ] ← Student B 50 N 50 N Net Force = 0 Result: Box does not move

Features of Balanced Forces

• Forces are equal in magnitude.
• Forces act in opposite directions.
• Net force is zero.
• They do not change the state of motion.
• They may change the shape of an object.

Unbalanced Forces

When forces acting on an object are unequal and do not cancel each other, they are called unbalanced forces.

Student A → [ BOX ] ← Student B 80 N 30 N Net Force = 50 N towards right Result: Box moves right

Balanced Force	Unbalanced Force
Forces are equal and opposite.	Forces are unequal.
Net force is zero.	Net force is not zero.
No change in motion.	Changes motion.
Example: Equal tug of war.	Example: One team wins tug of war.

Common Mistake: Balanced force does not mean no force is acting. It means forces are acting but cancelling each other.

3. Contact Forces

Forces that act only when two objects are in physical contact are called contact forces.

Examples of Contact Forces

• Muscular force
• Friction
• Tension
• Buoyant force

Muscular Force

The force applied by muscles is called muscular force.

• Lifting a school bag
• Writing with a pen
• Pulling a bucket
• Kicking a football

Exam Tip: Muscular force is a contact force because our body must touch the object to apply force.

Friction

Friction is the force that opposes the motion of an object. It acts between two surfaces in contact.

Motion of object → Friction ←

Useful Effects of Friction

Activity	How Friction Helps
Walking	Prevents slipping.
Writing	Helps pencil grip paper.
Braking	Helps vehicles stop.
Holding objects	Helps grip objects.

Harmful Effects of Friction

• Causes wear and tear of shoes and tyres.
• Produces heat in machines.
• Wastes energy.
• Slows down moving objects.

Ways to Reduce Friction

• Using lubricants like oil and grease
• Using ball bearings
• Polishing surfaces
• Streamlining vehicles

Tension Force

Tension is the pulling force transmitted through a string, rope, cable or wire when it is stretched.

Hand pulls rope upward ↑ │ Tension │ [Bucket] ↓ Weight

Common Mistake: Tension does not always act upward. It acts along the length of the rope or string.

4. Non-Contact Forces

Forces that act without physical contact between objects are called non-contact forces.

FORCES │ ├── Contact Forces │ ├── Muscular Force │ ├── Friction │ ├── Tension │ └── Buoyant Force │ └── Non-Contact Forces ├── Gravitational Force ├── Magnetic Force └── Electrostatic Force

Gravitational Force

Gravitational force is the force of attraction between two objects due to their masses.

• A stone falls down when dropped.
• Rain drops fall towards Earth.
• Fruits fall from trees.
• We stay on the ground because Earth pulls us.

NCERT Alert: Gravitational force is always attractive in nature.

Magnetic Force

The force exerted by a magnet is called magnetic force. A magnet can attract magnetic materials like iron, nickel and cobalt.

Poles	Effect
Like poles	Repel each other
Unlike poles	Attract each other

Electrostatic Force

The force between charged objects is called electrostatic force. It may be attractive or repulsive.

Activity: Rub a plastic comb on dry hair and bring it near small paper pieces. The paper pieces get attracted due to electrostatic force.

5. Mass and Weight

Mass

Mass is the amount of matter present in an object.

• Mass remains constant everywhere.
• Its SI unit is kilogram.
• It does not depend on gravity.

Weight

Weight is the force with which Earth attracts an object.

Weight = Mass × Acceleration due to gravity
W = m × g

Mass	Weight
Amount of matter in an object.	Force of gravity on an object.
SI unit is kilogram.	SI unit is newton.
Constant everywhere.	Changes from place to place.
Does not depend on gravity.	Depends on gravity.
Scalar quantity.	Vector quantity.

Exam Tip: Mass is matter. Weight is gravity’s pull.

6. Buoyant Force and Upthrust

When an object is placed in a liquid, the liquid exerts an upward force on the object. This upward force is called buoyant force.

Important: Buoyant force is also called upthrust.

↑ Buoyant Force / Upthrust [ Object ] ↓ Weight

Condition	Result
Buoyant force is greater than or equal to weight	Object floats
Weight is greater than buoyant force	Object sinks

Examples of Buoyancy

• Ships float on water.
• Wood floats in water.
• Iron nail sinks in water.
• Life jackets help swimmers float.
• Balloons float in air due to upthrust of air.

7. Density

Density tells how closely packed the matter is in a substance.

Density = Mass / Volume
ρ = m / V

Object	Observation	Reason
Wood	Floats	Density is less than water.
Iron nail	Sinks	Density is greater than water.
Oil	Floats on water	Oil is less dense than water.
Stone	Sinks	Density is greater than water.

Common Mistake: Heavy objects do not always sink. Floating depends mainly on density and buoyant force.

8. Pressure

Pressure is the force acting per unit area.

Pressure = Force / Area
P = F / A

If the same force acts on a smaller area, pressure is more. If the same force acts on a larger area, pressure is less.

Situation	Explanation
Sharp knife cuts easily	Small area produces more pressure.
Broad school bag straps are comfortable	Larger area reduces pressure.
Camel walks easily on sand	Broad feet reduce pressure.
Nail has pointed tip	Small area creates more pressure.

NCERT Line-by-Line Important Points

1. A force is a push or pull.
2. Force is due to interaction between two objects.
3. Force can change the state of motion of an object.
4. Force can change the shape of an object.
5. Forces may be contact or non-contact.
6. Muscular force is a contact force.
7. Friction is a contact force.
8. Friction always opposes motion.
9. Magnetic force can act without contact.
10. Gravitational force is attractive in nature.
11. Electrostatic force acts between charged objects.
12. Mass remains constant everywhere.
13. Weight changes from place to place because it depends on gravity.
14. Buoyant force acts upward.
15. Upthrust and buoyant force mean the same thing.
16. Density is mass per unit volume.
17. Pressure is force per unit area.
18. A force acting on a smaller area produces greater pressure.

Important Formulae

Formula	Meaning
Net Force = Larger Force - Smaller Force	Used when two opposite unequal forces act.
W = m × g	Weight is gravitational force.
Density = Mass / Volume	Density tells compactness of matter.
ρ = m / V	Symbolic form of density.
Pressure = Force / Area	Pressure depends on force and area.
P = F / A	Symbolic form of pressure.

Quick Comparison Tables

Contact Force vs Non-Contact Force

Basis	Contact Force	Non-Contact Force
Contact needed?	Yes	No
Acts from distance?	No	Yes
Examples	Muscular force, friction, tension	Gravitational, magnetic, electrostatic

Gravitational vs Magnetic vs Electrostatic Force

Force	Acts Between	Nature	Example
Gravitational	Any two masses	Always attractive	Apple falling
Magnetic	Magnets/magnetic materials	Attractive or repulsive	Magnet attracting iron
Electrostatic	Charged objects	Attractive or repulsive	Comb attracting paper

Board Exam Important Questions

Very Short Answer Questions

1. Define force.
2. Give two examples of push.
3. Give two examples of pull.
4. What is a contact force?
5. What is a non-contact force?
6. Name two contact forces.
7. Name two non-contact forces.
8. What is friction?
9. What is muscular force?
10. What is tension?
11. What is gravitational force?
12. What is magnetic force?
13. What is electrostatic force?
14. Define mass.
15. Define weight.
16. What is buoyant force?
17. What is upthrust?
18. Define density.
19. Write the formula for pressure.
20. Write the SI unit of force.

Short Answer Questions

1. Write three effects of force.
2. Why is force said to be due to interaction?
3. Explain balanced forces with an example.
4. Explain unbalanced forces with an example.
5. Why is muscular force called a contact force?
6. How does friction help us in daily life?
7. Write two harmful effects of friction.
8. What is tension? Give two examples.
9. Why is gravitational force called a non-contact force?
10. Give one activity to show electrostatic force.
11. Differentiate between mass and weight.
12. Why does a plastic bottle pushed into water come up again?
13. Why does an iron nail sink in water?
14. Why does wood float on water?
15. Why are school bag straps made broad?

Long Answer Questions

1. Explain the effects of force with suitable examples.
2. Differentiate between contact and non-contact forces.
3. Explain friction. Write its useful and harmful effects.
4. Explain gravitational, magnetic and electrostatic forces.
5. Explain mass, weight and density with formulae.
6. What is buoyant force? Explain floating and sinking using examples.
7. Explain balanced and unbalanced forces with diagrams.
8. Explain pressure and its relation with area using daily-life examples.

Case Study Based Questions

Case Study 1: Football Match

During a football match, a player kicks a stationary ball. The ball starts moving. Another player stops the ball with his foot. Later, the goalkeeper changes the direction of the ball by pushing it away.

1. Which force starts the motion of the ball?
2. Which effect of force is shown when the ball starts moving?
3. Which effect of force is shown when the goalkeeper changes the direction of the ball?
4. Is muscular force a contact force or non-contact force?
5. Name the force that slows down the ball on the ground.

Case Study 2: Magnet and Iron Pins

A student brings a bar magnet near iron pins lying on a table. The pins get attracted towards the magnet even without touching it.

1. Name the force acting between magnet and iron pins.
2. Is this force contact or non-contact?
3. Name two materials attracted by magnets.
4. Can magnetic force be attractive and repulsive?
5. What happens when like poles of two magnets are brought together?

Case Study 3: Floating and Sinking

A teacher puts a wooden block, an iron nail and a plastic ball in water. The wooden block and plastic ball float, but the iron nail sinks.

1. Which upward force acts on objects placed in water?
2. What is another name for buoyant force?
3. Why does the iron nail sink?
4. Why does the wooden block float?
5. Which property helps us decide whether an object will float or sink?

Assertion-Reason Questions

Options:

A. Both Assertion and Reason are true, and Reason is the correct explanation.
B. Both Assertion and Reason are true, but Reason is not the correct explanation.
C. Assertion is true, but Reason is false.
D. Assertion is false, but Reason is true.

1. Assertion: Friction is a contact force.
   Reason: Friction acts only when two surfaces are in contact.
2. Assertion: Weight remains the same everywhere.
   Reason: Weight depends on gravitational force.
3. Assertion: A magnet can attract iron pins without touching them.
   Reason: Magnetic force is a non-contact force.
4. Assertion: Balanced forces always change the state of motion.
   Reason: Balanced forces have zero net force.
5. Assertion: A sharp knife cuts better than a blunt knife.
   Reason: Smaller area produces greater pressure.

Competency-Based Questions

Q1. Why are tyres of vehicles made rough?

Answer: Tyres are made rough to increase friction between the tyres and road. This prevents slipping and helps the vehicle move safely.

Q2. Why does a camel walk easily on sand?

Answer: A camel has broad feet. Broad feet increase the area of contact and reduce pressure on sand. Therefore, the camel does not sink easily.

Q3. Why do we slip on a wet floor?

Answer: Water reduces friction between our feet and the floor. Due to less friction, we may slip.

Q4. Why does a stone fall down when released?

Answer: The Earth attracts the stone towards itself due to gravitational force.

Q5. Why does oil float on water?

Answer: Oil floats on water because oil is less dense than water.

Common Mistakes Students Make

Mistake: Writing force as only push.
Correction: Force can be a push or a pull.

Mistake: Confusing mass and weight.
Correction: Mass is amount of matter. Weight is gravitational force.

Mistake: Thinking friction is always harmful.
Correction: Friction is useful in walking, writing, braking and gripping.

Mistake: Writing that heavy objects always sink.
Correction: Floating and sinking depend on density and buoyant force, not only on weight.

Memory Tricks

Concept	Memory Trick
Effects of Force	SDS Shape: Speed, Direction, State of motion, Shape
Contact Forces	MFT: Muscular force, Friction, Tension
Non-Contact Forces	GEM: Gravitational, Electrostatic, Magnetic
Mass vs Weight	Mass = Matter, Weight = Gravity’s Pull
Density	Dense means packed.

Quick Revision Sheet

Must-Remember Points

1. Force is a push or pull.
2. Force needs interaction.
3. Force can change motion, speed, direction and shape.
4. Balanced forces have zero net force.
5. Unbalanced forces change motion.
6. Muscular force, friction and tension are contact forces.
7. Gravitational, magnetic and electrostatic forces are non-contact forces.
8. Friction opposes motion.
9. Mass remains constant; weight changes with gravity.
10. Buoyant force acts upward.
11. Density decides floating and sinking.
12. Pressure increases when area decreases.

Final Practice Section

Very Short Answer

1. Define force.
2. Name the force that opposes motion.
3. Name the force exerted by muscles.
4. What is the SI unit of force?
5. What is upthrust?
6. Write the formula for density.
7. Write the formula for pressure.
8. Name one non-contact force.
9. Name one contact force.
10. What is the direction of friction?

Short Answer

1. Explain balanced force with an example.
2. Explain unbalanced force with an example.
3. Why does a rolling ball stop after some time?
4. Why does a magnet attract iron pins without touching them?
5. Why does weight change from place to place?
6. Why does a ship float on water though it is heavy?
7. How does friction help us walk?
8. Why are sharp tools more effective in cutting?

Answer Key

Very Short Answer Key

1. Force is a push or pull acting on an object.
2. Friction.
3. Muscular force.
4. Newton.
5. Upward force exerted by liquid or gas on an object.
6. Density = Mass / Volume.
7. Pressure = Force / Area.
8. Gravitational force / magnetic force / electrostatic force.
9. Muscular force / friction / tension.
10. Opposite to the direction of motion.

Assertion-Reason Answer Key

1. A
2. D
3. A
4. D
5. A

End of Chapter | SK Classes | Class 8 Science | Exploring Forces

```