Exploring Forces – Class 8 Science Chapter 5 Notes & NCERT PDF

Class 8ScienceChapter 5NCERT book: Curiosity - Science Class 8

Notes and a simple summary of Chapter 5 of the NCERT Class 8 Science book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 5: Exploring Forces

Probe and Ponder

Why is it harder to pedal a bicycle uphill than on flat ground?

Why is it easier to slip on a wet surface?

Why do we feel "light" just after a swing reaches its highest point and starts coming down?

Introduction

On a windy day, Sonali and Ragini go cycling. Riding against the wind, they must pedal harder; rough patches of road make pedalling difficult; they ring their bells and turn their handles to change direction near a herd of sheep; and coming down the hill, their bicycles speed up without pedalling. "It seems something is pulling us downhill!" says Sonali. Every part of their ride involves forces.

Key Concepts

5.1 What is a force?

Moving a large cardboard box by pushing, pulling or lifting (Activity 5.1) shows that we always apply a push or a pull.

Generally, the push or pull applied on an object is called force.

5.2 What can a force do?

A force applied on an object may:

make an object move from rest;

change the speed of a moving object;

change the direction of motion;

change the shape of an object.

Examples: a friend holding your moving bicycle from behind (pull → slows it), hitting a moving ball with a bat (push → changes direction), pressing an inflated balloon (push → changes shape). An object at rest may have forces acting on it that balance each other.

5.3 Forces are interactions

At least two objects must interact for a force to come into play.

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 (N).

When you push a table, your hand also feels a force; it disappears when the interaction stops.

5.4.1 Contact forces

Forces that act only when objects are in physical contact (directly or through a stick or rope).

Muscular force — force due to the action of muscles (walking, lifting, jumping). Animals use it to move; humans have used the muscular force of animals for tasks. Inside our body it helps chew food, push food through the alimentary canal, and pump blood.

Friction — the force that comes into play when an object moves or tries to move over another surface. It acts opposite to the direction of motion.

- A box pushed on a table stops after some distance because of friction (Activity 5.3).

- Friction arises from tiny irregularities in surfaces that lock into each other.

- The same box stops at different distances on glass, cloth, wood, tile and sand (Activity 5.4): friction depends on the nature of the surfaces and is greater on rough surfaces.

- Air and water also exert friction, so aeroplanes, ships, boats and high-speed trains are given special shapes to reduce it.

5.4.2 Non-contact forces

Forces that act without contact.

Magnetic force — force exerted by a magnet on another magnet or a magnetic material. Two ring magnets on a stick with like poles facing make the upper magnet float (Activity 5.5).

Electrostatic force — force exerted by a charged body on another charged or uncharged body.

- A plastic scale rubbed with polythene attracts small bits of paper (Activity 5.6). Rubbing builds up static charges.

- Two balloons rubbed with a woollen cloth repel each other, but a rubbed balloon and the woollen cloth attract (Activity 5.7).

- Like charges repel; unlike charges attract. The two kinds of charge are positive and negative; the rubbing and rubbed objects get opposite charges.

- Moving charges make up an electric current.

Gravitational force (gravity) — the force with which the Earth attracts objects towards itself. It is always attractive, unlike magnetic and electrostatic forces, which can attract or repel.

- A ball thrown straight up slows down, stops momentarily at the top, then falls back with increasing speed — vertical motion under gravity.

5.5 Weight and its measurement

The force with which the Earth pulls an object towards itself is called the weight of the object. The SI unit of weight is newton (N).

A spring stretches by different amounts for different objects (Activity 5.9): the Earth pulls different objects with different forces.

A spring balance measures weight. In the book's balance (Fig. 5.13) the range is 0 to 10 N. There are 5 small divisions between 0 N and 1 N, so one division = 1 N ÷ 5 = 0.2 N (its least count).

Never hang an object heavier than the maximum the balance can measure — it may get damaged.

Mass vs weight

Mass is the amount of matter (grams or kilograms) — it is the same everywhere.

Weight is the gravitational pull — it can change from place to place and planet to planet.

A 1 kg object weighs about 10 N on Earth, 1.6 N on the Moon, 3.8 N on Mars, 9 N on Venus and 25.4 N on Jupiter.

Saying "the wheat bag weighs 10 kg" is everyday language; scientifically, 10 kg is its mass.

5.6 Floating and sinking

A closed empty bottle pushed into water bounces back up (Activity 5.13): water pushes it upward.

The upward force applied by a liquid on an object is called upthrust or buoyant force.

As the book explains it: if the gravitational force is more than the buoyant force, the object sinks; if the two are equal, it floats. Buoyant force depends on the density of the liquid.

Archimedes' Principle: an object fully or partially immersed in a liquid experiences an upward force equal to the weight of the liquid it displaces.

Pumice, a volcanic rock full of trapped gas bubbles, is less dense than water and floats.

Worked Examples

Least count of a spring balance: 1 N between big marks ÷ 5 divisions = 0.2 N.

Mass and weight on the Moon: a 1 kg object has mass 1 kg everywhere, but its weight changes from 10 N on Earth to 1.6 N on the Moon.

Activities in the Chapter

5.1 Moving a box: every way of moving it is a push or a pull.

5.3 / 5.4 Sliding a box: it stops because of friction, sooner on rough surfaces.

5.5 Floating ring magnets: a magnet exerts force without contact.

5.6 / 5.7 Charged scale and balloons: like charges repel, unlike charges attract.

5.8 Throwing a ball up: it always returns — gravity.

5.9–5.12 Spring and spring balance: measure weights, find range and least count.

5.13 Bottle in water: upthrust pushes it up.

Key Terms

Force (बल) — a push or pull from an interaction

Newton, N (न्यूटन) — SI unit of force and weight

Contact force (संपर्क बल)

Non-contact force (असंपर्क बल)

Muscular force (पेशीय बल)

Friction (घर्षण)

Magnetic force (चुंबकीय बल)

Static charge (स्थिर आवेश)

Electrostatic force (स्थिरवैद्युत बल)

Gravitational force / gravity (गुरुत्वाकर्षण बल)

Weight (भार)

Mass (द्रव्यमान)

Spring balance (कमानीदार तुला)

Upthrust / buoyant force (उत्प्लावन बल)

Real-life Connections

We slip on wet or polished floors because friction there is low.

Brakes, grip on shoes and tyre treads all depend on friction.

Rubbing a balloon on your hair makes it pull strands of hair — electrostatic force.

A mug feels lighter inside a bucket of water because of upthrust.

Common Misconceptions

"A force is only needed to start motion." — Force is also needed to change speed, direction or shape.

"An object at rest has no force on it." — The forces on it may be balanced.

"Mass and weight are the same." — Mass stays the same; weight depends on gravity.

"Gravity can repel." — Gravitational force is always attractive.

Keep the Curiosity Alive — Hints

Muscular — child lifting a bag; magnetic — compass needle pointing North; friction — cricket ball stopping before the boundary; gravitational — fruit falling from a tree; electrostatic — balloon attracting hair.

A force is always required to change speed (True); friction does not increase a rolling ball's speed (False); charged objects do exert force on each other (False statement).

A ball thrown upward: gravity acts downward at every stage (upward motion, downward motion and at the top).

On the Moon, weight becomes one-sixth because the Moon's gravity is weaker — mass stays the same.

💡 Key Learning Points

✓A force is a push or pull resulting from an interaction between objects; its SI unit is the newton (N).
✓Force can start motion, change speed or direction, or change shape.
✓Contact forces (muscular, friction) need touch; non-contact forces (magnetic, electrostatic, gravitational) act at a distance.
✓Weight is the Earth's pull on an object, measured in newtons with a spring balance; mass stays the same everywhere but weight can change.
✓Liquids exert an upward buoyant force (upthrust); Archimedes' Principle relates it to the weight of liquid displaced.

👨‍🏫 Teaching Tips

→Retell Sonali and Ragini's bicycle ride and ask students to spot every force in it before starting the chapter.
→Let students fill Table 5.1 with their own push/pull examples and classify the effect of each force.
→Slide the same notebook on glass, cloth, wood, tile and sand and have groups rank the stopping distances.
→Do the floating ring magnet and charged balloon activities side by side to compare magnetic and electrostatic forces.
→Hand out spring balances and have students find range and least count before any weighing, as in Activities 5.10–5.11.
→Use the Earth/Moon/Mars/Venus/Jupiter weights for 1 kg to discuss why mass stays constant but weight changes.
→Organise the 'Friction — a necessity or a problem?' class debate from the chapter.
→Push a closed empty bottle into a bucket so every student feels upthrust.

📋 Assessment Questions

Can the student list the four effects a force can have on an object?
Can the student classify given forces as contact or non-contact and give an example of each?
Can the student explain friction and why it is greater on rough surfaces?
Can the student find the least count of a spring balance and distinguish mass from weight?
Can the student explain why some objects float and others sink using buoyant force?

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