Part 1 of 3 · What Forces Do

TaskRead the passage and study the diagram.

A force is a push, a pull or a twist. A force always does one of three things: it can change an object's speed, change its direction, or change its shape by squashing or stretching it. A force can do more than one of these at once.

All forces are measured in Newtons, written N. Forces are measured using a Newtonmeter, sometimes called a spring balance. Inside a Newtonmeter is a spring: the bigger the force pulling on it, the further the spring stretches, and the pointer moves further down the scale.

Here are four important forces. Friction acts whenever an object moves against another object, in the opposite direction to the movement, and makes it harder for things to move. Drag is the resistance force on anything moving through air or water — air resistance and water resistance are both types of drag. Gravity is the force that pulls all objects towards each other; the Earth's gravity pulls everything towards the centre of the Earth. Weight is the force of gravity on an object, measured in Newtons.

Forces come in pairs

• Forces always come in pairs, equal in size and opposite in direction.
• These pairs are called interaction pairs.

Part 2 of 3 · Contact and Non-contact Forces

TaskRead the passage, then complete the table.

Forces can be sorted into two groups. A contact force acts only when an object is touching something else — a surface, air, or water. A non-contact force acts between objects that are not touching at all.

Friction is a contact force, because the two surfaces must be rubbing against each other. Air resistance is also a contact force, because the object must be touching the air.

Gravity is a non-contact force. The Earth pulls on you without touching you, and the Earth pulls on the Moon across a quarter of a million miles of empty space.

TaskStudy the list of forces you will meet in KS3.

TaskSort the forces below into the correct column.

Part 3 of 3 · Drawing Force Arrows and Free Body Diagrams

TaskStudy the two free body diagrams below.

Force arrows show both the direction and the size of a force. The direction of the arrow shows which way the force acts, and the length of the arrow shows how big it is: a longer arrow means a bigger force. A diagram that shows all the forces acting on a single object as arrows is called a free body diagram.

Free body diagrams are useful because they let you see at a glance whether the forces on an object are equal or unequal, and therefore predict how it will move. They also help you to spot interaction pairs — the pairs of equal and opposite forces that always occur together.

TaskComplete these free body diagrams. Add and label the forces acting on each object.

Part 1 of 3 · Deformation and the Reaction Force

TaskRead the passage and study the diagram.

When you apply a force to an object you can change its shape. This is called deformation. You can compress an object, which means squash it, or you can stretch it, which means make it longer without breaking it. When a footballer heads a ball, the forces deform both the ball and the footballer's head.

Deformation explains why you do not fall through your chair. The floor is a solid, and solids are made up of particles arranged in a regular pattern, joined strongly together by chemical bonds. Those bonds behave like tiny springs: when you push on them they push back.

This is what happens when you stand on the floor. Your weight pushes the particles closer together, the bonds between them are compressed, and the compressed bonds push back and support you. That upward push from a surface is called the reaction force.

The same thing happens with a chair. The chair changes shape slightly — it deforms — and this produces the upward force that pushes you back up. The deformation is usually far too small to see, but it is always there.

Part 2 of 3 · Stretching, Extension and Tension

TaskRead the passage, then plan the investigation.

When you exert a force on a spring, the amount it stretches is called the extension. Extension is the change in length, not the total length, so you must always subtract the original length from the new length.

Bungee cords, springs and lift cables all stretch when you exert a force on them. As a bungee jumper falls, the cord stretches further and further. When the cord has stretched as far as it will go, it pulls back and the jumper is pulled upwards again. The force in the stretched cord is called tension.

TaskRecord your results in the table.

Part 3 of 3 · Hooke's Law and the Elastic Limit

TaskStudy the graph, then read the passage.

Hooke's Law states that the extension of an elastic object is directly proportional to the force applied to it. In plain language: if you double the force on the spring, the extension will double. If you treble the force, the extension trebles.

On a graph of extension against force, Hooke's Law produces a straight line that passes through the origin. Passing through the origin matters: it shows that with no force there is no extension. If you plot the total length instead of the extension, the line will not pass through the origin — it will cross the y-axis at the original length of the spring.

Hooke's Law does not go on for ever. If a spring is stretched too far it will not return to its original length when the load is removed, and the graph stops being a straight line. The point where this happens is the elastic limit. Beyond the elastic limit the spring is permanently deformed, and stretching it further will eventually break it.

Part 1 of 3 · What Is Friction?

TaskStudy the diagram, then read the passage.

Your hands might look smooth, but on a microscopic level they have rough, bumpy surfaces. So do wood, glass and plastic — objects are not really as smooth as they look. Friction is a force caused by two surfaces moving over each other, and it happens because those tiny bumps catch on one another.

Whenever an object moves against another object, it feels a frictional force. Friction always acts in the opposite direction to the movement, so it makes it harder for things to move and slows down things that are already moving. Rough surfaces produce more friction than smooth surfaces.

Friction is not always a nuisance. You need friction to walk: the friction between your foot and the ground produces the force that pushes you forwards. The brakes on a bicycle and in a car work only because of friction between the brake pads and the wheel. Friction also lets a pen write on paper and lets a tyre grip the road.

Friction can also be unhelpful. It causes heating, and it wears away things that rub against each other, which is why brake blocks and tyres have to be replaced from time to time. You can reduce friction by lubrication — using a liquid such as oil or grease that stops the surfaces from touching each other directly. Liquids that reduce friction are called lubricants.

Part 2 of 3 · Practical — Investigating Friction

TaskRead the method, then plan and carry out the investigation.

Friction is a force that slows down moving objects or stops objects moving. In this practical you will plan an investigation into how friction depends on one factor: the type of surface, the surface area of the block, or the weight of the block.

Part 3 of 3 · Drag Forces and Streamlining

TaskRead the passage and study the diagram.

Any object moving through water or air will have a resistance force on it that slows it down. Water resistance and air resistance are both types of drag. Drag is bigger when the object moves faster, and bigger on a windy day, because more air particles hit the object each second.

When engineers design new vehicles they try to reduce drag by making them smooth and streamlined. A streamlined shape lets the air flow smoothly around it instead of being pushed roughly out of the way, so the drag force is much smaller. This is why Formula One cars, aeroplanes and racing cyclists all have smooth, tapering shapes.

Animals use streamlining too. A peregrine falcon tucks its wings into its body when it dives. This makes it more streamlined, reducing air resistance, so it reaches its prey in the shortest possible time. Fish and dolphins have smooth tapering bodies for the same reason.

Drag and friction both affect motion in the same way. An object will keep moving at a steady speed in the same direction, or stay still, unless a resultant force acts on it. So if friction or drag forces are acting, you must apply a force just to cancel them out — not to keep the object moving. When the forces cancel exactly, the resultant force is zero and the object is in equilibrium.

Part 1 of 3 · Gravity and Force Fields

TaskRead the passage, then answer the questions.

When a diver jumps off a diving board he moves towards the Earth, because there is a force acting on him. That force is the gravitational force, or gravity. Gravity is a non-contact force, because the diver and the Earth do not need to touch to exert a force on each other.

Remember that forces come in pairs. The Earth pulls on the diver, and the diver pulls on the Earth with an equal and opposite force. The Earth does not noticeably move, because it is so very much more massive than the diver.

The size of the gravitational force between two objects depends on two things: the mass of each object, and how far apart they are. If the mass is larger the force is larger. If the distance is larger the force is smaller.

A field is a special region where something experiences a non-contact force. In a gravitational field a mass experiences a force; in a magnetic field a magnetic material such as iron experiences a force; in an electrostatic field a charged object experiences a force. All three types of field have something in common: as you get further away from the mass, the magnet or the charge, the field gets weaker.

Part 2 of 3 · Mass, Weight and Gravitational Field Strength

TaskRead the passage, then work through the example.

Mass and weight are not the same thing, even though the words are often muddled in everyday speech. Mass is the amount of 'stuff' an object is made of, and it is a measure of how hard it is to get the object moving. Mass is measured in kilograms (kg). Weight is a force — the force of gravity acting on an object — so it is measured in Newtons (N).

The force of gravity on one kilogram is called the gravitational field strength, written g. It is measured in Newtons per kilogram (N/kg). On Earth, g is about 10 N/kg, which means every kilogram of you is pulled down with a force of about 10 N.

TaskComplete the table for a person of mass 55 kg.

Part 3 of 3 · Weight in Space and Staying in Orbit

TaskRead the passage, then answer the questions.

Imagine blasting off from the Earth in a spacecraft. As you move away from the Earth the gravitational field gets weaker. If you stood on bathroom scales inside the spacecraft, the reading would be less than it was on Earth, and it would keep falling as you travelled further out.

The amount of 'you' would not change at all. Your mass stays exactly the same. It is your weight — the force of the Earth on you — that becomes smaller. This is why astronauts appear to float: they are so far from the Earth, and falling freely, that they have almost no weight pressing them against anything.

Isaac Newton worked out that the Earth exerts a force on the Moon, and that this force of gravity keeps the Moon in orbit around the Earth. Remarkably, it is the very same force that acts on an apple and pulls it to the ground. The force does not speed the Moon up; it continuously changes the direction of the Moon's motion, bending its path into a curve around the Earth.

The same idea explains the Earth's own orbit. The Sun exerts a gravitational force on the Earth, and that force keeps the Earth in orbit around the Sun.

Part 1 of 3 · Balanced Forces and Equilibrium

TaskRead the passage and study the diagram.

If two forces act in opposite directions on an object, they work against each other. If the two forces are the same size, nothing changes: the forces are said to be balanced, and we say the object is in equilibrium.

If the forces on an object are balanced — or if no forces act on it at all — then two things can happen. An object that is not moving stays still, or stationary. An object that is already moving carries on moving at exactly the same speed in the same direction. In both cases the resultant force is zero.

This second point catches people out. A car travelling along a motorway at a steady 70 mph has balanced forces on it: the thrust from the engine exactly cancels out the drag and friction. Balanced forces do not mean 'not moving' — they mean 'not changing'.

Part 2 of 3 · Unbalanced Forces and the Resultant Force

TaskRead the passage and study the diagram.

When two forces acting on an object are not equal in size, we say they are unbalanced. Unbalanced forces always cause a change. An object that is not moving starts to move. An object that is already moving changes its speed or its direction.

If more than one force acts on an object, you add the forces together to work out the resultant force. The resultant force is the single force that could replace all the other forces and still have exactly the same effect on the object. For forces along the same straight line, you simply subtract the backward force from the forward force.

In the example, the thrust is 400 N forwards and the drag is 300 N backwards. The resultant force is 400 N − 300 N = 100 N to the right. Because the resultant force is not zero, the car will speed up in that direction.

The driving force is the force pushing a vehicle forwards, usually from an engine. Resistive forces are the ones that act against the motion, such as friction and drag. When the driving force is bigger than the resistive forces the vehicle speeds up; when the resistive forces are bigger the vehicle slows down; and when they are equal the speed stays the same.

TaskComplete the table by calculating each resultant force.

Part 3 of 3 · Practical — Forces Circus

TaskWork round the six stations and complete the table.

Objects feel balanced forces when they move at a steady speed or stay still. If the forces are not equal they are unbalanced, and the speed of the object changes — it may speed up, slow down or change direction. At each station, name two forces acting on the object, give the direction of each, and decide whether the forces are balanced.