Chapter-5 Exploring Forces Quick Revision Notes Class 8th Science (Curiosity)

Quick revision notes

Chapter-5 Exploring Forces Quick Revision Notes Class 8th Science (Curiosity)

Class 8 · Science (Curiosity) · Chapter 5 : Exploring Forces · All Board · ENGLISH · 4 views

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Chapter 5 • Class 8 Science

🚴 Exploring Forces

Curiosity — Textbook of Science for Grade 8

📋 Table of Contents

1💪 What is a Force? 2⚡ What Can Force Do? 3🤝 Force as Interaction 4📂 Types of Forces 5⚖️ Weight & Measurement 6🌊 Floating & Sinking 7⚖️ Mass vs Weight 8📸 Quick Revision

💪

5.1 What Is a Force?

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Fig: Push, Pull & Lift

In everyday life we push, pull, lift things all the time. All these are forces!

Pushing a box → Push force
Pulling a rope → Pull force
Lifting a bag → Pull upward force

📌 Key Definition

Force = A push or pull on an object resulting from the object's interaction with another object.
🔢 SI Unit = newton (N) | Symbol = N

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⚡

5.2 What Can a Force Do?

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Fig: Applying force
on different objects

A force applied on an object can:

Make a stationary object move (start from rest)
Change the speed of a moving object (speed up / slow down)
Change the direction of motion
Change the shape of an object (press a balloon 🎈)
Cause some or all of the above effects together!

💡

Important Remember!

Whenever there is a change in speed, direction, or shape of an object → a force is acting on it!
If an object is at rest, forces acting on it are balancing each other.

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🤝

5.3 Forces — Interaction Between Objects

Force always involves at least TWO objects interacting
Example: Hand pushes table → hand & table interact → FORCE!
When interaction stops → force disappears
Both objects experience force from each other

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📂

5.4 Types of Forces

🤲

Contact Forces

Need physical contact between objects to act.

Examples: Muscular Force, Frictional Force

🌀

Non-Contact Forces

Act even without physical contact between objects.

Examples: Magnetic, Electrostatic, Gravitational

💪 Contact Force 1: Muscular Force

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Fig: Muscular force
used by living beings

Force produced due to the action of muscles in our body.

Occurs when muscles contract & elongate
Walking, running, lifting, pushing, jumping = muscular force!
Animals, birds, fish, insects all use it
Heart muscles → pump blood (muscular force inside body!)
It is a contact force

🛑 Contact Force 2: Friction

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Fig: Friction between
two surfaces

When a ball rolls on the ground, it stops on its own. This is because of friction!

Acts when an object moves or tries to move over another surface
Always acts opposite to direction of motion
Arises due to irregularities in surfaces locking into each other
Rough surfaces → more friction
Smooth surfaces → less friction

📌 Key Definition

Friction (Force of Friction) → The force that comes into play when an object moves or tries to move over another surface. It acts in the direction opposite to motion. It is a contact force.

✈️

Friction in Liquids & Gases Too!

Air and water also exert friction on objects moving through them. That's why aeroplanes, ships, and high-speed trains are designed with special streamlined shapes! 🚀

🧲 Non-Contact Force 1: Magnetic Force

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Fig: Magnets
repelling each other

Like poles (N-N, S-S) → Repel each other
Unlike poles (N-S) → Attract each other
Can exert force without touching
Acts on magnetic materials (iron, nickel, cobalt)
Can be both attractive and repulsive

📌 Key Definition

Magnetic Force → The force exerted by a magnet on another magnet or a magnetic material. It is a non-contact force (acts from a distance).

⚡ Non-Contact Force 2: Electrostatic Force

When objects are rubbed together, electrical charges build up → called static charges.

Two kinds of charges: Positive (+) and Negative (−)
Like charges repel (+ repels +, − repels −)
Unlike charges attract (+ attracts −)
Charged comb attracts small paper bits → acts from a distance!
Two balloons rubbed on wool → repel each other

📌 Key Definition

Electrostatic Force → The force exerted by a charged body on another charged or uncharged body. It is a non-contact force. Can be attractive or repulsive.

🌍 Non-Contact Force 3: Gravitational Force

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Fig: Objects always
fall towards Earth

Throw anything up → it always comes back down! Why? Earth attracts everything towards itself!

Always attractive (never repulsive) 🔽
Acts without contact → non-contact force
Object thrown up → speed decreases going up, increases coming down
This up-down motion = Vertical Motion

📌 Key Definition

Gravitational Force / Gravity → The force with which the Earth attracts objects towards itself. It is always an attractive non-contact force. Acts on every object on Earth.

📊 All Forces at a Glance

Force	Type	Attractive/Repulsive	Example
💪 Muscular	Contact	Push/Pull	Lifting a bag
🛑 Frictional	Contact	Opposes motion	Ball rolling to stop
🧲 Magnetic	Non-contact	Both	Compass needle, electromagnet
⚡ Electrostatic	Non-contact	Both	Comb attracting paper bits
🌍 Gravitational	Non-contact	Only Attractive	Apple falling from tree

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⚖️

5.5 Weight and Its Measurement

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Fig: Spring Balance

Weight = force with which Earth pulls an object towards itself
SI unit of Weight = newton (N)
Heavier object → more weight → more stretch in spring
Different objects have different weights
Measured using a Spring Balance 🔩

📌 Key Definition

Weight → The force with which the Earth pulls an object towards itself.
SI Unit = newton (N)
Spring Balance → A device used to measure weight (force). Spring stretches more for heavier objects.

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⚖️

Mass vs Weight — Know the Difference!

Property	⚖️ Mass	🌍 Weight
Definition	Amount of matter in object	Gravitational pull on object
Unit	grams (g) / kilograms (kg)	newton (N)
Changes?	❌ Same everywhere	✅ Changes place to place
On Moon?	Same as on Earth	1/6th of Earth weight

🪐

Weight on Different Planets! (For 1 kg object)

🌍 Earth = 10 N | 🌕 Moon = 1.6 N | 🔴 Mars = 3.8 N | ♀️ Venus = 9 N | 🪐 Jupiter = 25.4 N
Mass stays 1 kg everywhere — only weight changes!

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🌊

5.6 Floating and Sinking

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Fig: Bottle pushed
in water feels upward push

Push an empty bottle into water → feel an upward push! Release → it bounces up. Why?

All liquids push objects upward
This upward force = Buoyant Force / Upthrust
Gravity pulls down, Buoyancy pushes up
Gravity > Buoyancy → object SINKS
Gravity = Buoyancy → object FLOATS

📌 Key Definition

Buoyant Force / Upthrust → The force applied by a liquid on an object in the upward direction. It opposes the gravitational pull on the object.

🏛️

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 displaced liquid weight = object weight → object FLOATS!

🪨

Did You Know? Pumice Rock Floats!

Pumice is a rock formed during volcanic eruptions — it traps tiny air bubbles inside making it less dense than water, so it FLOATS! 🌋

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📸 Quick Revision Snapshots

💪 Force = push or pull on an object due to interaction with another object. SI unit = newton (N).

⚡ Force can make an object move, stop, speed up, slow down, change direction, or change shape.

🤲 Contact Forces: Muscular Force (muscles) & Frictional Force (surfaces in contact).

🌀 Non-Contact Forces: Magnetic, Electrostatic, Gravitational — act from a distance!

🛑 Friction acts opposite to motion. Rough surface = more friction. Smooth = less friction.

🌍 Gravity = always attractive non-contact force. Earth pulls everything towards itself.

⚖️ Weight = gravitational pull in newtons. Mass = amount of matter in kg/g. Mass stays same; weight changes with place.

🌊 Buoyant Force / Upthrust = upward force by liquid. Float when gravity = upthrust; sink when gravity > upthrust.