Chapter 5

Exploring Forces

5.1 What Is a Force?

Think of all the ways you could move a heavy cardboard box: you might push it, pull it with a rope, or lift it up and carry it. In every single case, you are applying a push or a pull.

In science, that is all a force is — a push or a pull applied on an object. Nothing more mysterious than that.

5.2 What Can a Force Do to the Objects on Which It Is Applied?

We apply forces all day without noticing — opening a drawer, stretching a rubber band, kicking a football, braking a bicycle, rolling a chapati, turning a steering handle.

What does a force actually do? A force acting on an object can:

• make a resting object start moving;
• change the speed of an object that is already moving;
• change its direction of motion;
• change its shape;
• or bring about several of these effects at once.

So here's a handy rule: whenever something speeds up, slows down, changes direction, or changes shape, you can be sure a force is acting on it.

5.3 Are Forces an Interaction Between Two or More Objects?

When you push a table, your hand (one object) acts on the table (another object). Look closely at any force and you will always find at least two objects taking part — a force never appears on its own. It comes from the interaction between two objects, and the instant that interaction stops, the force vanishes. Remarkably, each object feels a force from the other at the very same time.

Put formally, a force is a push or pull on an object that results from its interaction with another object. Its SI unit is the newton, written with a small 'n' and given the symbol N.

5.4 What Are the Different Types of Forces?

Forces fall into two broad families, depending on whether or not the two objects have to touch: contact forces and non-contact forces.

5.4.1 Contact forces

Some forces act only when two objects are physically in touch — directly, through our hands or body, or indirectly, through a stick or rope. Forces of this kind are called contact forces.

Muscular force

When you walk, run, lift, jump or stretch, it is your muscles that produce the force moving you. The force produced by the action of muscles, as they contract and relax, is called muscular force. Animals, birds, fish and insects all rely on it to move and survive, and for a very long time humans have harnessed the muscular force of bullocks and horses for ploughing and transport.

Friction

A rolling ball, or a bicycle you stop pedalling, slows down and halts on its own — sooner on a rough road than a smooth one. The hidden force responsible is friction: the force that comes into play when one surface moves, or even tries to move, over another.

Friction always acts opposite to the direction of motion, which is exactly why it slows things down. It arises because even surfaces that look perfectly smooth carry countless tiny irregularities that lock into one another — and rougher surfaces, with more of them, produce more friction.

5.4.2 Non-contact forces

Other forces can act even when the objects are nowhere near touching. These are called non-contact forces.

Magnetic force

A magnet can draw a piece of iron towards it, or push and pull another magnet, with no contact at all — like poles (North–North) repel, while unlike poles (North–South) attract. This force exerted by a magnet on another magnet or on a magnetic material is the magnetic force, and because it reaches across a gap, it is a non-contact force.

Electrostatic force

Rub a plastic scale hard with a piece of polythene and hold it near tiny bits of paper — the paper leaps up and clings to it, though nothing touched! Rubbing certain materials together builds up static charges on their surfaces, and such a charged object can attract — or repel — other objects from a distance.

There are two kinds of charge, called positive and negative: like charges repel one another, unlike charges attract. The force between charged bodies (or between a charged and an uncharged body) is the electrostatic force — another non-contact force.

Gravitational force

Throw a ball upward as hard as you like, and it always falls back down — because the Earth pulls every object towards itself. This pull is the gravitational force, also called the force of gravity, or simply gravity.

It acts without any contact, so it is a non-contact force; and unlike magnetic or electrostatic forces, gravity is always an attractive pull — never a push. An object moving straight up or down under this pull is said to undergo vertical motion.

5.5 Weight and Its Measurement

The force with which the Earth pulls an object towards itself is called the object's weight. Since weight is a force, it is measured in the same unit as force — the newton (N). Hang different objects from a spring, one at a time, and it stretches by different amounts: the Earth pulls heavier objects harder, so their weight is greater.

This stretching is exactly how a spring balance works — an object hung from its hook stretches a spring, and a scale marked in newtons reads off the weight directly. Before trusting any instrument, always check its range and its smallest division; for a typical school spring balance the range is 0 to 10 N, with each small mark standing for 0.2 N.

5.6 Floating and Sinking

Push an empty, tightly-capped bottle down into a bucket of water and you feel it shove back against your hand; let go, and it bounces up to the surface. Water — and in fact every liquid — pushes upward on any object placed in it. This upward push is called upthrust, or the buoyant force.

So an object in water feels two forces at once: gravity pulling it down, and the buoyant force pushing it up. If gravity wins, the object sinks; if the two balance, it floats. This is why a small coin sinks while a large wooden block floats.