FreeLesson 1 of 36 min read
Newton's three laws of motion in everyday life
Learn what Newton's three laws of motion say and spot them at work in buses, trolleys, footsteps and rockets.
Every time you push a shopping trolley, brake on a bike or jump off a step, you are using physics described by Isaac Newton's three laws of motion. They explain why things start moving, why they stop and why they turn. Once you understand them, you can predict what will happen in almost any everyday situation involving forces.
These laws are an established model that works extremely well at everyday speeds and sizes. For things moving close to the speed of light, or for tiny particles like electrons, scientists use other theories. For cars, balls and people, Newton is all you need.
First law: things keep doing what they are doing
An object stays still, or keeps moving at a steady speed in a straight line, unless a resultant force acts on it. A resultant force is the overall, unbalanced force left after you add up all the forces. The tendency of objects to keep their motion is called inertia.
Think of a bus braking suddenly. The brakes push on the bus, so it slows. Nothing has pushed on you yet, so your body carries on forward until the seatbelt or a handrail stops you. That lurch is your inertia showing.
Second law: force, mass and acceleration
When there is a resultant force, the object accelerates: it speeds up, slows down or changes direction. The law links three quantities: resultant force = mass x acceleration, or F = m a. Force is in newtons (N), mass in kilograms (kg) and acceleration in metres per second squared (m/s^2).
So a bigger force gives a bigger acceleration, and a bigger mass gives a smaller acceleration for the same force.
Example: pushing a trolley
An empty trolley has a mass of 20 kg. You push with a resultant force of 40 N.
a = F / m = 40 / 20 = 2 m/s^2
Now the trolley is loaded to 80 kg and you push with the same 40 N.
a = 40 / 80 = 0.5 m/s^2
Four times the mass gives one quarter of the acceleration.
Third law: forces come in pairs
When object A pushes or pulls on object B, B pushes or pulls on A with a force of the same size in the opposite direction. The two forces act on different objects, so they never cancel each other out.
Everyday force pairs:
- Walking: your foot pushes the ground backwards; the ground pushes you forwards.
- Swimming: your hands push water backwards; the water pushes you forwards.
- Rockets: the engine pushes exhaust gas downwards; the gas pushes the rocket upwards.
- Leaning on a wall: you push the wall; the wall pushes you back just as hard.
Putting the laws together
To analyse any situation, pick one object, draw arrows for every force on it, and find the resultant. If it is zero, use the first law: the motion does not change. If it is not zero, use the second law to find the acceleration. Use the third law to find the partner forces on other objects.
Next time you see something speed up, slow down or turn, ask: what is the resultant force, and what is pushing back? In the next lesson you will use these laws to explain a ball rolling down a ramp.
Key takeaways
- Without a resultant force, an object keeps its speed and direction.
- Resultant force equals mass times acceleration, so heavier objects are harder to accelerate.
- Forces come in equal and opposite pairs that act on different objects.
Check your understanding
Written with AI help. Spotted a mistake? Tell us.