Class 8 › Science › Chapter 5
Exploring Forces
Push and pull, effects of force, contact and non-contact forces (muscular, friction, magnetic, electrostatic, gravitational), weight and its measurement, and upthrust.
1. What Is a Force?
Force: a push or pull on an object resulting from its interaction with another object. SI unit: newton (symbol N; written with a small 'n').
- Moving a cardboard box by pushing, pulling, lifting or carrying always needs a push or a pull.
- Everyday examples: opening a drawer, stretching a rubber band, a fielder stopping a ball, kicking a football, braking a bicycle, rolling a chapati, turning an autorickshaw's handle.
2. Effects of a Force
| A force may... | Example |
|---|---|
| Make an object move from rest | Pushing a box, kicking a football |
| Change the speed of a moving object (increase, or stop / decrease) | Friend pulling your moving bicycle from behind to stop it |
| Change the direction of motion | Hitting a moving ball with a bat |
| Change the shape of an object | Pressing an inflated balloon, stretching a rubber band |
| Cause some or all of these effects together | – |
- Whenever there is a change in speed, direction or shape, a force is acting; none of these happens without a force.
- A step further: an object at rest does not mean no force acts on it; it means the forces on it are balancing each other (balanced forces: higher grades).
3. Force as an Interaction Between Objects
- A force arises only when at least two objects interact (e.g. your hand and a table).
- A step further: when two objects interact, each experiences a force from the other (you feel a force on your hand when you push the table). When the interaction ends, both forces vanish.
Types of forces.
4. Contact Forces and Muscular Force
Contact forces: forces that act only when there is physical contact between the objects, directly (hands) or indirectly (a stick or rope).
- Muscular force: the force caused by the action of muscles (they contract and elongate) in walking, running, lifting, pushing, jumping, stretching.
- Animals, birds, fish and insects use muscular force to move and survive; humans have long used the muscular force of animals for tasks (ploughing, carrying loads).
- Inside the body: muscular force helps us chew, push food through the alimentary canal, and the heart's muscles circulate blood.
5. Friction
Friction: the force that comes into play when an object moves or tries to move over another surface. It always acts in the direction opposite to the motion (or attempted motion). It is a contact force.
- Why a rolling ball or a free-wheeling bicycle stops: friction between the surfaces (a push on a box makes it stop after sliding a distance).
- Cause: even smooth-looking surfaces have minute irregularities that lock into each other and oppose motion.
- Depends on the nature of the surfaces (Activity 5.4: an object stops at different distances on glass, cloth, wood, tile, sand): friction is greater on rough surfaces; smoother surfaces such as ice or a polished floor give less friction, so we slip more easily.
- Fluids: air, water and other liquids also exert friction on objects moving through them, so aeroplanes, ships, boats and high-speed trains have specially shaped bodies to reduce it.
- Friction is both a necessity (walking, gripping, braking) and a problem (wear, wasted energy).
6. Non-contact Forces
Non-contact forces: forces whose effect can be experienced even when objects are not in contact.
Magnetic force
Magnets and magnetic materials
Force exerted by a magnet on another magnet or a magnetic material. Like poles repel, unlike poles attract. Two ring magnets on a stick with like poles facing: the upper one floats (Activity 5.5). Can be attractive or repulsive.
Electrostatic force
Charged bodies
Force exerted by a charged body on another charged or uncharged body. Rubbing certain materials builds up static charges on their surfaces. Can be attractive or repulsive.
Gravitational force
Pull of the Earth
Force with which the Earth attracts objects (also called gravity). It is always attractive, unlike magnetic and electrostatic forces.
Electrostatic force: activities
Activity 5.6: a plastic scale or straw rubbed with polythene picks up small paper pieces without touching them. The rubbed object is charged and attracts uncharged objects.
Activity 5.7: two balloons rubbed with a woollen cloth and hung close repel each other (similar charges). A rubbed balloon and the woollen cloth attract each other (opposite charges).
Conclusion: there are two kinds of static charge, positive and negative; like charges repel, unlike charges attract. Both the rubbing and the rubbed object get charged, with opposite kinds.
- A step further: when charges move, they form an electric current in a circuit (lamp glowing, heating and magnetic effects).
- Gravity and motion: a thrown ball goes up, slows down, stops momentarily at the top, then falls with increasing speed; objects dropped or thrown upward move in a straight vertical path (vertical motion under gravity).
7. Weight and Its Measurement
Weight: the force with which the Earth pulls an object towards itself. It is a force, so its SI unit is the newton (N).
- Hanging different objects from a spring stretches it by different amounts: the Earth pulls different objects with different forces, i.e. different weights.
- Spring balance: a spring fixed at one end with a hook at the other; the stretch shows the weight, read on a scale in newton (another scale gives mass in grams, valid on Earth).
- Do not hang objects heavier than the maximum reading: the balance may be damaged.
Reading a spring balance (Activities 5.10–5.12)
Range: maximum weight it can measure (in the textbook figure, 0 to 10 N).
Smallest reading (least count) = value between two big marks ÷ number of divisions between them. Example: 1 N ÷ 5 = 0.2 N.
Always check range and least count of the instrument before using it; they differ between balances.
8. Mass vs Weight
| Mass | Weight | |
|---|---|---|
| Meaning | Amount of matter in an object | Gravitational force on the object (by the Earth or another planet) |
| Unit | gram (g), kilogram (kg) | newton (N) |
| Changes with place? | No, same everywhere | Yes: varies slightly on Earth, a lot on other planets |
| Object of mass 1 kg | Earth | Moon | Mars | Venus | Jupiter |
|---|---|---|---|---|---|
| Weight | 10 N | 1.6 N | 3.8 N | 9 N | 25.4 N |
- Weight on the Moon is about one-sixth of that on Earth because the Moon's gravitational pull is weaker; the mass stays the same.
- Mass can be found indirectly from weight with a spring balance, or by comparing with known masses on a beam balance.
- Everyday language: 'the wheat bag weighs 10 kg' really means mass 10 kg; in science, use the correct term and unit.
9. Floating and Sinking
Upthrust (buoyant force): the upward force applied by a liquid on an object placed in it. Activity 5.13: a closed empty bottle pushed into water feels an upward push and bounces back up. A mug feels lighter inside water for the same reason.
U = upthrust (up), W = weight / gravitational force (down).
- Sinks if the gravitational force is more than the buoyant force; floats if the two forces are equal.
- The buoyant force depends on the density of the liquid (density: later chapter).
- Archimedes' Principle (step further): an object fully or partly immersed in a liquid feels an upward force equal to the weight of the liquid it displaces. If the displaced liquid's weight is less than the object's weight, it sinks; if equal, it floats.
- Pumice (volcanic rock) floats: it has many trapped gas bubbles, making it light and porous (less dense than water).
10. Quick Revision
| Force | Type | Key point |
|---|---|---|
| Muscular | Contact | Caused by muscles |
| Friction | Contact | Opposes motion; more on rough surfaces |
| Magnetic | Non-contact | Attracts or repels; like poles repel |
| Electrostatic | Non-contact | Between charged bodies; like charges repel |
| Gravitational | Non-contact | Always attractive; gives weight |
| Upthrust | Contact (liquid) | Upward push by a liquid; decides float or sink |
Exercise hints
From 'Keep the curiosity alive'
Q1: muscular → (b); magnetic → (e); frictional → (a); gravitational → (c); electrostatic → (d). Q2: (i) True, (ii) False (friction slows the ball), (iii) False. Q3: the balloons repel (similar charges). Q7: smooth surfaces such as ice or polished floors give less friction. Q9: the Moon's gravity is weaker so weight changes; mass does not.
My reasoning (not textbook answers)
Q4, Q5, Q6
Q4: the coin's weight is more than the upthrust so it sinks; the wooden block floats because upthrust equals its weight (the block is less dense than water). Q5: gravity acts downward in all three cases; air friction, if counted, acts opposite to the motion. Q6: release from a lower point or use a rougher surface to stop before A; release higher or use a smoother surface to cross A.
Common mistakes:
• Saying a force can act on a single object alone; it needs an interaction between at least two objects.
• Saying friction helps an object keep moving; it opposes motion.
• Mixing up mass (kg) and weight (N).
• Saying mass changes on the Moon; only weight changes.
• Saying gravity can repel; gravitational force is always attractive.
• Writing the unit as 'Newton'; it is newton (N).
• Saying things float only because they are light; it depends on upthrust vs weight (density).
• Saying a force can act on a single object alone; it needs an interaction between at least two objects.
• Saying friction helps an object keep moving; it opposes motion.
• Mixing up mass (kg) and weight (N).
• Saying mass changes on the Moon; only weight changes.
• Saying gravity can repel; gravitational force is always attractive.
• Writing the unit as 'Newton'; it is newton (N).
• Saying things float only because they are light; it depends on upthrust vs weight (density).