Three rules for motion
Why does a soccer ball roll after you kick it? Why do you lurch forward when a bus stops? Why does a rocket fly up? Isaac Newton answered these questions in 1687 with three laws of motion. They describe how forces change motion.
Newton’s first law: the law of inertia
An object at rest stays at rest, and an object in motion keeps moving at the same speed in a straight line, unless an unbalanced force acts on it.
Inertia is the tendency of an object to resist changes in its motion. The more mass something has, the more inertia it has.
| Example | What happens |
|---|---|
| A ball on the floor | Stays still until you kick it |
| Bus stops suddenly | Passengers lurch forward because their bodies keep moving |
| Tablecloth trick | Dishes stay put when the cloth is yanked quickly |
| Hockey puck on ice | Glides a long way because there’s very little friction |
Why do things on Earth slow down and stop? Because forces like friction and air resistance act on them. In space, a moving object would keep going forever.
This is why seat belts matter. In a crash, the car stops, but your body keeps moving until something stops it. A seat belt makes sure that something is the belt, not the windshield.
Newton’s second law: force, mass and acceleration
The acceleration of an object depends on the net force acting on it and its mass.
force = mass × acceleration (F = ma)
- Net force is the total force after adding up all the pushes and pulls.
- Acceleration is how quickly speed or direction changes.
What the second law tells us:
| If you… | Then… |
|---|---|
| Push harder (more force) | It accelerates more |
| Push something heavier (more mass) | It accelerates less |
It’s easy to push an empty shopping cart quickly. A full cart is much harder to get moving, because it has more mass.
Example: What force is needed to accelerate a 2 kg ball at 5 m/s²?
F = 2 × 5 = 10 N
Balanced and unbalanced forces
- Balanced forces cancel out (net force = 0). The object’s motion doesn’t change.
- Unbalanced forces don’t cancel. The object accelerates in the direction of the bigger force.
In tug-of-war, if both teams pull equally, the rope doesn’t move. If one pulls harder, the rope moves their way.
Newton’s third law: action and reaction
For every action, there is an equal and opposite reaction.
When one object pushes on another, the second object pushes back with the same size force in the opposite direction. The action force and the reaction force act on different objects.
| Action | Reaction |
|---|---|
| You push back on the ground with your feet | The ground pushes you forward |
| A swimmer pushes water backward | Water pushes the swimmer forward |
| A rocket pushes hot gas down | The gas pushes the rocket up |
| A bird’s wings push air down | Air pushes the bird up |
| You push off a wall in a pool | The wall pushes you away |
Rockets work in space even though there’s no air to push on. The rocket pushes its exhaust gas backward, and the gas pushes the rocket forward.
Why don’t the forces cancel?
Because they act on different objects. When you kick a ball, your foot pushes the ball (and the ball accelerates), and the ball pushes your foot (you feel it). Each force affects a different object.
The three laws together: a bicycle ride
- First law: your bike stays still until you pedal.
- Second law: pedalling harder makes you speed up faster. Carrying a heavy backpack makes it harder.
- Third law: the tires push back on the road, and the road pushes the bike forward.
When you brake, friction is the unbalanced force that slows you down. If you stop suddenly, inertia tries to keep your body moving forward. That’s one reason to wear a helmet!
Common mistakes
- Thinking moving objects need a constant force to keep moving. They only need a force to change their motion.
- Thinking action and reaction forces cancel out. They act on different objects.
- Thinking heavy objects have no inertia when still. Inertia applies whether moving or not.
Try it yourself
- Put a coin on a card on top of a cup. Flick the card away quickly. Where does the coin go? Which law does this show?
- Blow up a balloon, tape it to a straw on a string, and let it go. Explain the motion with the third law.
- Push an empty box and a box full of books across the floor with the same push. Compare.
Quick recap
- First law: objects resist changes in motion (inertia).
- Second law: F = ma; more force means more acceleration, more mass means less.
- Third law: every action has an equal and opposite reaction.
- Unbalanced forces change motion; balanced forces don’t.
- Friction and air resistance are why things slow down on Earth.