Science · Physics foundations
Forces & motion
Newton's laws with everyday examples.
Newton's First Law: Objects Keep Doing What They're Doing
An object at rest stays at rest, and a moving object keeps moving, unless a force acts on it. This is called inertia. When a bus stops suddenly, you lurch forward because your body wants to keep moving. Friction (a force between surfaces) and air resistance (a force from air) slow things down in real life.
Newton's Second Law: Force Changes Motion
The bigger the force, the faster an object accelerates (speeds up, slows down, or changes direction). Heavier objects need more force to accelerate. Think of pushing an empty shopping trolley versus a full one. We write this as: force = mass × acceleration.
Newton's Third Law: Forces Come in Pairs
Every action force has an equal and opposite reaction force. When you jump, you push down on the ground and the ground pushes you up with the same force. A rocket pushes hot gas downwards, so the gas pushes the rocket upwards.
Balanced and Unbalanced Forces
When forces on an object are balanced (equal in size, opposite in direction), it stays still or moves at constant speed. Unbalanced forces make objects accelerate. A book resting on a table has balanced forces: gravity pulls down, the table pushes up.
Worked examples
1. A car has a mass of 1000 kg. The engine applies a force of 3000 N. What is the car's acceleration?
- Use the formula: force = mass × acceleration, so acceleration = force ÷ mass.
- Acceleration = 3000 N ÷ 1000 kg = 3 m/s².
- The car accelerates at 3 metres per second every second.
2. Why does a passenger wearing a seatbelt not fly forward when a car brakes suddenly?
- The passenger's body has inertia and wants to keep moving forward (Newton's First Law).
- The seatbelt applies a backwards force on the passenger.
- This force slows the passenger down safely with the car.
3. A swimmer pushes the water backwards with their hands. Explain how this helps them move forward.
- The swimmer applies a backwards force on the water (action force).
- The water pushes the swimmer forwards with an equal force (reaction force, Newton's Third Law).
- This forwards force makes the swimmer accelerate through the water.
Try it yourself
A tennis ball (mass 0.06 kg) is hit with a racket that applies a force of 30 N. Calculate the ball's acceleration.
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