Exploring Forces

1. The story that starts the chapter

Sonali and Ragini go cycling on a windy day. They feel the wind pushing them, so they pedal harder. The road is rough in places (hard to pedal) and smooth in others. Coming downhill, their cycles speed up even without pedalling — "something is pulling us down." Every one of these everyday feelings — pushing against wind, struggling on rough roads, rolling downhill — is really about forces. This chapter names and explores them.

2. What is a force? (Section 5.1)

You can move a box by pushing it, pulling it with a rope, or lifting (pulling up) it. In every case you apply a push or a pull. In science, the push or pull applied on an object is called a force.

The push or pull can be applied directly (with your hand) or indirectly (through a stick or rope) — but in all these cases there is some contact involved.

3. What can a force do? (Section 5.2)

Look at everyday actions: opening a drawer, stretching a rubber band, a fielder stopping a ball, kicking a football, applying brakes, rolling a chapati, turning a steering handle. 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;
• or cause some or all of these effects together.

The big idea: whenever there is a change in speed, direction, or shape, a force must be acting — none of these happens on its own.

A step further: If an object is at rest, does that mean no force acts on it? Not necessarily — it can mean the forces on it are balancing each other (you study balanced forces in higher grades).

4. Force is an interaction between objects (Section 5.3)

When you push a table, your hand and the table are two objects interacting. Looking at every action in Table 5.1, you notice forces appear only when at least two objects interact.

A step further: When you push the table, you also feel a force on your hand. The moment you stop pushing, that force disappears. Whenever two objects interact, each feels a force from the other; when the interaction ends, the force ends too.

5. Contact forces (Section 5.4.1)

Many forces need physical contact between objects — directly (hands) or indirectly (a stick or rope). Forces that act only when objects are in contact are called contact forces. The chapter studies two: muscular force and friction.

5a. Muscular force

When we walk, run, lift, push, jump or stretch, the force comes from our muscles. The force produced by the action of muscles is muscular force — it occurs when muscles contract and elongate. Animals, birds, fish and insects all use muscular force to move and survive. Humans have long used the muscular force of animals (bullocks ploughing, a horse pulling a cart).

Ever heard of: Muscular force works inside our body too — it helps us chew food, push it through the alimentary canal during digestion, and the heart muscles expand and contract to circulate blood.

5b. Friction

A rolling ball stops on its own; a cycle slows when you stop pedalling — sooner on a rough road than a smooth one. Some force must be slowing them. The force that comes into play when an object moves, or tries to move, over another surface is the force of friction (friction).

• Friction always acts in the direction opposite to the object's motion (or attempted motion).
• It is a contact force — it arises from two surfaces touching.
• It is caused by tiny irregularities on both surfaces. Even "smooth" surfaces have many minute bumps; when in contact, these lock into each other and oppose sliding.
• Friction is greater on rough surfaces. An object pushed across glass, cloth, wood, tile or sand stops after different distances — so friction depends on the nature of the surfaces in contact.

A step further: Friction acts in liquids and gases too. Air and water exert friction on objects moving through them, which is why aeroplanes, ships, boats and high-speed trains are given streamlined (special) shapes to reduce this friction.

6. Non-contact forces (Section 5.4.2)

Some forces act even when objects are not touching — these are non-contact forces. Three are studied: magnetic, electrostatic and gravitational.

6a. Magnetic force

A magnet attracts magnetic materials. Between two magnets, like poles repel (N–N, S–S) and unlike poles attract (N–S). This attraction and repulsion is a form of push and pull — a force. A magnet can exert this force without touching the other magnet (Activity 5.5: a ring magnet floats above another when like poles face each other). The force exerted by a magnet on another magnet or a magnetic material is magnetic force — a non-contact force.

6b. Electrostatic force

Rub a plastic scale or straw with polythene and bring it near small bits of paper — the paper jumps and sticks to it (Activity 5.6). When certain materials are rubbed together, electrical charges (static charges) build up on their surfaces; such an object is a charged object. A charged object attracts uncharged bits of paper even without contact.

• Two balloons rubbed with wool both get the same kind of charge → they repel (Activity 5.7). Like charges repel.
• A rubbed balloon and the woollen cloth get opposite charges → they attract. Unlike charges attract.
• The two kinds of static charge are called positive and negative.

The force exerted by a charged body on another charged or uncharged body is the electrostatic force — a non-contact force. (When charges move, they form an electric current.)

6c. Gravitational force (gravity)

Throw a ball up — it always comes back down. A fruit falls from a tree; a jumper lands; a dropped object falls. Since all objects fall towards the Earth, the Earth attracts (pulls) them. The force with which the Earth attracts objects towards itself is the gravitational force, also called the force of gravity, or simply gravity.

• Gravity acts without contact → it is a non-contact force.
• Gravity is always attractive — unlike magnetic and electrostatic forces, which can be either attractive or repulsive.
• Thrown straight up, an object slows, stops momentarily at the top, then speeds up coming down — this up-and-down movement under gravity is vertical motion.

7. Weight and its measurement (Section 5.5)

The force with which the Earth pulls an object towards itself is the weight of the object. Weight measures how strongly the Earth pulls an object. Since weight is a force, its SI unit is also the newton (N).

Hang different objects from a spring (Activity 5.9): the spring stretches by different amounts — so the Earth pulls different objects with different forces, i.e. their weights differ. This stretch is the basis of the spring balance.

Tip: the objects you weigh should not exceed the balance's maximum, or it may get damaged.

8. Mass vs weight — a key board distinction

• Mass = the amount of matter in an object; measured in grams (g) or kilograms (kg). Mass is the same everywhere — it does not change with place.
• Weight = the gravitational force with which the Earth (or another planet) pulls an object; measured in newton (N). Weight can change from place to place and is very different on other planets.

Everyday vs science: we casually say "the wheat bag weighs 10 kg," but kg is a unit of mass, not weight. In science we must use the correct term with its correct unit.

9. Floating and sinking (Section 5.6)

If gravity pulls everything down, why do some objects float? Push a closed empty bottle into water — you feel an upward push, and it bounces back up (Activity 5.13). The force applied by a liquid on an object in the upward direction is called upthrust or buoyant force. All liquids apply it.

• On an object in a liquid, gravity pulls down and the buoyant force pushes up.
• If gravitational force > buoyant force → the object sinks.
• If the two forces are equal → the object floats.
• The buoyant force depends on factors such as the density of the liquid (studied later).

Archimedes' Principle (A step further): a body fully or partly immersed in a liquid experiences an upward force equal to the weight of liquid it displaces. If that displaced weight is less than the object's weight → it sinks; if equal → it floats.

Ever heard of: Pumice — a porous volcanic rock full of trapped gas bubbles — is less dense than water, so this rock can float.

10. Quick summary table — the five forces

• Muscular — contact — from muscles; walking, lifting, chewing, bullock ploughing.
• Friction — contact — opposes motion; more on rough surfaces; stops a rolling ball.
• Magnetic — non-contact — magnet on magnet/magnetic material; like poles repel, unlike attract.
• Electrostatic — non-contact — charged body on another body; like charges repel, unlike attract.
• Gravitational — non-contact — Earth's pull; always attractive; gives objects their weight.

11. Common mistakes to avoid

• Saying "no force acts on a resting object" — forces may simply be balanced.
• Calling gravity attractive or repulsive — gravity is always attractive only.
• Mixing up mass (kg, fixed) and weight (N, can change). The Moon changes your weight, not your mass.
• Thinking friction is bad only — it also helps (walking, gripping). It is greater on rough, less on smooth.
• Writing the unit of force as 'kg' — the SI unit of force (and weight) is the newton, N.
• Forgetting that magnetic, electrostatic and gravitational forces act without contact.

12. Quick revision checklist

• Force = push or pull from interaction between objects; unit newton (N).
• Effects: start motion, change speed, change direction, change shape.
• Contact = muscular, friction. Non-contact = magnetic, electrostatic, gravitational.
• Friction opposes motion, greater on rough surfaces; gravity is always attractive.
• Weight = Earth's pull (N, varies); mass = matter (kg, fixed). Spring balance measures weight.
• In liquids: upthrust (buoyant force) up vs gravity down decides floating/sinking.