2.3 Forces & Energy
Forces cause objects to speed up, slow down, change direction, or change shape. Energy is the ability to do work and comes in many forms. Together, forces and energy explain how everything in the universe moves and changes.
Key Idea
Forces and energy are closely linked. A force can transfer energy from one object to another. When you kick a ball, the force of your foot transfers kinetic energy to the ball, making it move.
What You Will Learn
- The difference between contact and non-contact forces
- How gravity works and the difference between mass and weight
- What friction is and how it affects motion
- The different types of energy and how energy is transferred
- The law of conservation of energy
Types of Forces
A force is a push or pull that can change an object's speed, direction, or shape. Forces are measured in newtons (N).
Contact vs Non-Contact Forces
| Contact Forces | Non-Contact Forces |
|---|---|
| Require physical touching | Act at a distance — no touching needed |
| Friction, air resistance, tension, normal force, applied force | Gravity, magnetism, electrostatic force |
Key Forces to Know
| Force | Type | Description |
|---|---|---|
| Gravity | Non-contact | Pulls objects with mass toward each other; pulls everything toward Earth's centre |
| Friction | Contact | Acts between surfaces that slide past each other; opposes motion |
| Air resistance | Contact | A type of friction between a moving object and air |
| Normal force | Contact | The support force a surface pushes back with (e.g., a table holding up a book) |
| Magnetism | Non-contact | Attracts or repels magnetic materials without touching |
| Applied force | Contact | A force applied by a person or object (e.g., pushing a door) |
Balanced and Unbalanced Forces
- Balanced forces: Forces are equal and opposite → object stays still or moves at constant speed (no change in motion).
- Unbalanced forces: One force is larger → the object accelerates (speeds up, slows down, or changes direction).
Example
A book sitting on a table: gravity pulls it down and the normal force pushes it up. These forces are balanced, so the book stays still. If you push the book sideways with a force greater than friction, the forces become unbalanced and the book moves.
Gravity & Weight
Gravity is a non-contact force that attracts all objects with mass toward each other. On Earth, gravity pulls everything toward the centre of the planet.
Mass vs Weight
| Feature | Mass | Weight |
|---|---|---|
| What it measures | The amount of matter in an object | The force of gravity on an object |
| Unit | Kilograms (kg) | Newtons (N) |
| Changes with location? | No — always the same | Yes — depends on gravity |
| Measured with | Balance / scales | Newton meter (spring balance) |
On the Moon: g = 1.6 N/kg (about 1/6 of Earth)
Example: A person with a mass of 60 kg:
Weight on Earth = 60 × 10 = 600 N
Weight on Moon = 60 × 1.6 = 96 N
Their mass (60 kg) stays the same in both places.
Friction
Friction is a contact force that acts between two surfaces when they slide (or try to slide) past each other. It always acts in the opposite direction to motion.
Key Facts About Friction
- Friction opposes motion — it slows things down or prevents them from starting to move.
- Friction depends on: (1) the roughness of the surfaces and (2) the force pushing the surfaces together.
- Rough surfaces = more friction; smooth surfaces = less friction.
- Friction converts kinetic energy into heat energy (that is why you feel warmth when you rub your hands together).
Friction: Helpful or Harmful?
| Helpful Friction | Harmful Friction |
|---|---|
| Shoe grip on the ground (stops slipping) | Moving parts in machines wear out |
| Brakes stopping a car | Wastes energy as heat |
| Writing with a pencil on paper | Slows down vehicles (air resistance) |
Reducing Friction
Friction can be reduced by using lubricants (oil, grease), making surfaces smoother, using rollers or wheels, or creating a more streamlined shape to reduce air resistance.
Types of Energy
Energy is the ability to do work or cause change. It exists in many different forms and can be transferred from one form to another.
The Main Energy Stores
| Energy Type | Description | Example |
|---|---|---|
| Kinetic energy | Energy of a moving object | A rolling ball, a running person |
| Gravitational potential energy | Energy stored due to height above the ground | A book on a shelf, a roller coaster at the top |
| Elastic potential energy | Energy stored in a stretched or compressed object | A stretched rubber band, a compressed spring |
| Chemical energy | Energy stored in chemical bonds | Food, batteries, fuels (petrol, wood) |
| Thermal (heat) energy | Energy related to the temperature of an object | A hot cup of tea, the Sun |
| Light energy | Energy carried by electromagnetic waves | Sunlight, a lamp, a laser |
| Sound energy | Energy carried by vibrations through a medium | Music from speakers, a clap |
| Electrical energy | Energy carried by moving electric charges | A battery powering a torch, mains electricity |
| Nuclear energy | Energy stored in the nucleus of an atom | The Sun (nuclear fusion), nuclear power plants |
Speed, Distance, and Time
Speed = 150 ÷ 2 = 75 km/h
Speed vs Velocity
- Speed = how fast an object is moving (no direction mentioned).
- Velocity = speed in a specific direction (e.g., “75 km/h north”).
Energy Transfer & Conservation
Energy cannot be created or destroyed — it can only be transferred from one form to another. This is called the law of conservation of energy.
Energy Transfer Examples
| Situation | Energy Transfer |
|---|---|
| Dropping a ball from a height | Gravitational PE → Kinetic energy |
| A torch is switched on | Chemical energy → Electrical energy → Light + Heat |
| Rubbing hands together | Kinetic energy → Thermal (heat) energy |
| Eating food and running | Chemical energy → Kinetic energy + Thermal energy |
| A wind turbine generating electricity | Kinetic energy (wind) → Electrical energy |
| Stretching a rubber band and releasing it | Elastic PE → Kinetic energy |
Law of Conservation of Energy
Energy cannot be created or destroyed. It can only be transferred from one form to another or moved from one place to another. The total amount of energy in a closed system always stays the same.
Wasted Energy
In most energy transfers, some energy is “wasted” as thermal (heat) energy that spreads out into the surroundings. For example, a light bulb converts electrical energy into light, but also produces heat. The heat is not useful — it is wasted energy. More efficient devices waste less energy.
Simple Machines
Simple machines make work easier by changing the size or direction of a force. They do not create energy — they transfer it.
| Machine | How It Helps | Example |
|---|---|---|
| Lever | Multiplies force around a pivot point | Seesaw, crowbar, scissors |
| Inclined plane (ramp) | Reduces the force needed by increasing the distance | Wheelchair ramp, slide |
| Pulley | Changes the direction of force; can multiply force | Flagpole, crane, blinds |
| Wheel and axle | Reduces friction and multiplies force | Doorknob, bicycle wheel |
Practice Q&A
Attempt each question before revealing the model answer.
Flashcard Review
Tap each card to reveal the answer. Try to answer from memory first.
Practice Test
20 questions covering forces, gravity, friction, energy types, and energy transfer.