Forces cause objects to speed up, slow down, change direction or change shape. Energy cannot be created or destroyed — it only changes form. These two ideas underpin all of Sec 1–4 Physics and Combined Science.
A force is a push or a pull. Forces are measured in Newtons (N) using a spring balance (newton meter). Forces have both magnitude (size) and direction — they are vector quantities.
| Force | Description | Direction | Example |
|---|---|---|---|
| Gravity / Weight | Attractive force between masses; Earth pulls all objects towards its centre | Always downward (towards Earth's centre) | Ball falling, weight of a book |
| Normal force | Contact force from a surface pushing back on an object resting on it | Perpendicular (at 90°) to the surface, away from surface | Table pushing up on a book |
| Friction | Force that opposes relative motion between two surfaces in contact | Opposite to the direction of motion or intended motion | Brakes slowing a bicycle |
| Air resistance (drag) | Friction force from air acting on a moving object | Opposite to direction of motion | Parachute slowing a fall |
| Upthrust (buoyancy) | Upward force from a fluid on a submerged object | Upward | Object floating in water |
| Elastic force | Force from a stretched or compressed spring/elastic material | Towards equilibrium (restoring) | Compressed spring pushing back |
| Magnetic force | Attraction or repulsion between magnets or between magnet and magnetic material | Along the line between poles | Magnet attracting iron filings |
| Mass | Weight | |
|---|---|---|
| Definition | Amount of matter in an object | Gravitational force acting on the object |
| Unit | Kilogram (kg) | Newton (N) |
| Changes with location? | No — constant everywhere | Yes — less on the Moon (weaker gravity) |
| Measured with | Balance | Spring balance / newton meter |
Weight (N) = Mass (kg) × Gravitational field strength (N/kg)
On Earth: g ≈ 10 N/kg, so a 5 kg object weighs 50 NExam trap: Never write "the mass of the object is 50 Newtons" — mass is in kg, weight is in N. This is one of the most common unit errors in Sec 1 papers.
| Situation | What happens | Example |
|---|---|---|
| Balanced forces (resultant = 0 N) | Object stays still, OR continues at constant speed in a straight line | Book resting on table; car moving at constant speed on a straight road |
| Unbalanced forces (resultant ≠ 0 N) | Object accelerates (speeds up, slows down, or changes direction) | Rocket launching; car braking |
The resultant force is the single force that has the same effect as all the forces combined. For forces along the same line: add forces in the same direction, subtract forces in opposite directions.
For a spring or elastic material, the extension is directly proportional to the force applied — as long as the elastic limit is not exceeded.
Force (N) = Spring constant (N/m) × Extension (m)
F = k × e — the spring constant k measures stiffness; higher k = stiffer springA graph of force vs extension is a straight line through the origin (up to the elastic limit). Beyond the elastic limit, the spring is permanently deformed.
Friction arises from the interlocking of microscopic surface irregularities when two surfaces are in contact. Even surfaces that look smooth have tiny bumps at the microscopic level.
| Friction is useful when… | Friction is harmful when… |
|---|---|
| Walking (shoe grips the ground) | Machine parts wearing out |
| Car tyres gripping the road | Engine overheating due to friction between parts |
| Brakes slowing a vehicle | Reducing efficiency of moving parts |
| Writing with a pen on paper | Ropes and cables fraying |
Pressure is the force applied per unit area. The same force spread over a smaller area creates greater pressure.
Pressure (Pa) = Force (N) ÷ Area (m²)
Units: Pascals (Pa) = N/m². Also commonly expressed in N/cm² for Sec 1.Key principle: to increase pressure, decrease the area. To decrease pressure, increase the area. The force stays the same in both cases.
Energy of motion. Any moving object has KE. More mass and/or more speed → more KE.
Energy stored due to height above a reference point. Higher up → more GPE.
Energy stored in a stretched or compressed spring or elastic material.
Energy related to the temperature and movement of particles.
Energy stored in chemical bonds. Released during chemical reactions (e.g. burning, digestion).
Energy carried by light waves (electromagnetic radiation).
Energy carried by vibrations through a medium.
Energy carried by moving electric charges.
Energy stored in atomic nuclei. Released in nuclear reactions (fission/fusion).
| Device / Situation | Energy transformation |
|---|---|
| Burning candle | Chemical energy → Light energy + Thermal energy |
| Solar panel | Light energy → Electrical energy |
| Electric motor | Electrical energy → Kinetic energy (+ Thermal energy wasted) |
| Loudspeaker | Electrical energy → Sound energy |
| Microphone | Sound energy → Electrical energy |
| Ball thrown upward | Kinetic energy → Gravitational PE (as it rises) |
| Ball falling | Gravitational PE → Kinetic energy (+ some thermal from air resistance) |
| Photosynthesis in a plant | Light energy → Chemical energy (stored in glucose) |
| Car engine | Chemical energy (fuel) → Kinetic energy + Thermal energy + Sound energy |
Conduction and convection both require matter (a medium). Radiation does not — it is how the Sun's heat reaches us through the vacuum of space.
Energy cannot be created or destroyed. It can only be transferred from one object to another or transformed from one form to another. The total amount of energy in a closed system always remains constant.
When energy seems to "disappear" (e.g. a bouncing ball that bounces lower each time), it has been transformed into thermal energy (heat) and sound — not destroyed.
In most real-world devices, not all input energy is converted to the desired output — some is always "wasted" as thermal energy (heat) due to friction or resistance.
Efficiency (%) = (Useful energy output ÷ Total energy input) × 100%
A perfectly efficient machine would be 100% — impossible in practice due to friction and heat losses.Example: A light bulb converts 20 J of electrical energy into 4 J of light and 16 J of heat. Efficiency = (4 ÷ 20) × 100% = 20%.
A Sankey diagram shows energy transfers visually. The width of each arrow is proportional to the amount of energy. The main arrow shows input energy; branches show where energy goes (useful outputs and wasted outputs).
All the output arrows in a Sankey diagram must add up to the total width of the input arrow — this represents conservation of energy.
A box of mass 10 kg rests on a table. The base of the box has an area of 0.5 m².
(a) Calculate the weight of the box. (g = 10 N/kg)
(b) Calculate the pressure the box exerts on the table.
(c) If the box is placed on its side so the contact area is 0.25 m², what happens to the pressure? Calculate the new pressure.
A student winds up a toy car (a spring-loaded mechanism) and releases it. (a) What energy transformation occurs when the spring is wound up? (b) What energy transformation occurs as the car moves across the floor? (c) The car eventually slows down and stops. Has energy been destroyed? Explain.
A parachutist jumps from a plane. At first, they accelerate downwards. After some time, they reach a constant speed called "terminal velocity". (a) What two forces act on the parachutist? State their directions. (b) Explain why the parachutist accelerates at first. (c) Explain why the parachutist eventually reaches terminal velocity.