Gravity
Every mass attracts every other mass. Earth's gravity pulls you and nearby objects toward its centre.
1
Start with forces
A force is a push or pull. Forces can change an object's speed, direction, or shape—but only the overall, or net, force changes its motion.
Friction
Surfaces resist sliding past each other. Friction helps shoes grip the ground and brakes stop a bicycle.
Magnetism
Magnets and moving electric charges create magnetic fields that can push or pull at a distance.
2
Newton's three laws of motion
Isaac Newton described three powerful patterns that predict how everyday objects move when forces act on them.
1. The law of inertia
An object at rest stays at rest, and a moving object continues at constant speed in a straight line, unless a net external force acts on it. A rolling ball slows because friction and air resistance act on it.
2. Force changes motion: F = ma
Acceleration equals net force divided by mass. Push harder and acceleration increases. Push a more massive cart with the same force and acceleration decreases.
3. Forces come in pairs
When object A pushes object B, B pushes A back with an equal-sized force in the opposite direction. A rocket pushes exhaust downward; the exhaust pushes the rocket upward. The two forces act on different objects.
3
Experiment with F = ma
A newton (N) measures force, a kilogram (kg) measures mass, and acceleration here is measured in metres per second squared.
Push the cart
Change the force and mass. Predict the acceleration before looking at the answer.
Acceleration = force ÷ mass2.0 m/s²
4
Four more mysteries to chase
Physics stretches from the tiniest particles to the entire universe. These everyday clues open huge doors.
Energy changes form
A roller coaster trades gravitational potential energy for kinetic energy. Energy moves and changes form, while total energy is conserved.
Light carries information
White light contains many wavelengths. Raindrops bend and separate them, allowing us to see a rainbow.
Sound is vibration
A vibrating string pushes nearby air, creating pressure waves. Your eardrum vibrates when those waves arrive.
Circuits need a loop
In a simple circuit, a voltage source can drive electric charge through a complete conducting path.
Orbits are falling
A satellite moves sideways fast enough that as gravity pulls it down, Earth's curved surface falls away beneath it.
Time can surprise us
Einstein showed that measured time depends slightly on speed and gravity. GPS satellites must account for relativity.
Try it safely
Coin, card, and cup
Place a card over an empty cup and a coin on the card. Flick the card sideways. The card moves, while the coin's inertia keeps it nearly in place, so gravity drops it into the cup.
Observe → Predict → Test → Explain
A good experiment changes one thing at a time, repeats the test, and records what actually happened—not only what you hoped would happen.
5
Measure motion precisely
Everyday words such as fast and heavy become scientific when we define measurements and include units.
speed = distance ÷ timeSpeed
A runner covering 100 metres in 20 seconds has an average speed of 5 m/s.
acceleration = change in velocity ÷ timeAcceleration
Acceleration means changing speed, direction, or both. Turning a corner counts even if the speedometer stays steady.
weight = mass × gravitational fieldMass and weight
Mass measures matter and inertia in kilograms. Weight is a gravitational force measured in newtons.
density = mass ÷ volumeDensity
Density compares how much mass is packed into an amount of space. It helps explain floating and sinking.
6
Waves carry energy and information
Waves appear in water, sound, light, radio, earthquakes, and many technologies.
Amplitude
Amplitude describes the size of a wave's disturbance. For sound, a larger amplitude usually means a louder sound.
Wavelength
Wavelength is the distance between matching points on neighbouring waves, such as crest to crest.
Frequency
Frequency counts cycles each second and is measured in hertz. Higher-frequency sound is usually heard as higher pitch.
Reflection
Waves can bounce from boundaries. Reflected sound creates echoes; reflected light lets mirrors form images.
Refraction
A wave can change direction when its speed changes in a new material. This makes a spoon look bent in water.
The spectrum
Radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma radiation are all electromagnetic waves with different frequencies.
7
Electricity needs careful paths
Electric circuits transfer energy through connected components.
Three useful quantities
Current describes the rate of electric charge flow. Voltage describes energy transferred per unit charge. Resistance describes how strongly a component opposes current.
In a series circuit, components share one path. In a parallel circuit, current has branches.
voltage = current × resistance
This relationship is called Ohm's law for components whose resistance remains constant. Only use batteries and child-safe circuits with adult guidance—never wall electricity.
8
Quick knowledge check
Use each question to test a prediction, not just your memory.
If forces are balanced, must an object be stopped?
No. It may be stopped or moving at constant velocity. Balanced forces mean zero acceleration.
Why does a heavy cart accelerate less than a light cart under the same force?
From a = F ÷ m, increasing mass while keeping force fixed reduces acceleration.
Does a rocket push against the ground to rise?
No. It pushes exhaust gases backward, and those gases push the rocket forward with an equal and opposite force.
Can sound travel through empty space?
No. Sound is a mechanical vibration and needs matter. Light is electromagnetic and can travel through vacuum.