Forces are everywhere
A force is a push or a pull. Forces can make things start moving, stop, speed up, slow down, change direction or change shape. Forces are measured in newtons (N), named after the scientist Isaac Newton. Holding an apple takes about 1 N of force.
Contact and non-contact forces
| Contact forces (must touch) | Non-contact forces (act at a distance) |
|---|---|
| Pushing a door | Gravity |
| Friction | Magnetism |
| Air resistance | Static electricity |
| A tow rope pulling a car |
Gravity
Gravity is a pulling force between all objects. The bigger an object’s mass, the stronger its pull. Earth is huge, so its gravity pulls everything toward its centre.
- Gravity makes objects fall.
- Gravity keeps the Moon orbiting Earth and Earth orbiting the Sun.
- Your weight is the force of gravity pulling on you.
The Moon has weaker gravity than Earth, only about one-sixth as strong. A person who weighs 600 N on Earth would weigh about 100 N on the Moon. Their mass, the amount of matter in them, stays the same.
Friction
Friction is a force that resists motion when surfaces rub together. It always acts in the opposite direction to motion.
| More friction | Less friction |
|---|---|
| Rough surfaces (sandpaper, gravel) | Smooth surfaces (ice, polished floors) |
| Heavy objects pressing down | Lighter objects |
| Rubber soles | Oil or grease between parts |
Friction is helpful when we walk, brake a bike or strike a match. It’s unhelpful when it wears out shoes and engine parts. Air resistance is friction with air. Parachutes use it to slow a fall.
Magnetism
Magnetism pulls or pushes on certain metals, like iron, nickel and cobalt.
- Every magnet has a north pole and a south pole.
- Opposite poles attract (N–S).
- Like poles repel (N–N or S–S).
Earth itself is a giant magnet. That’s why a compass needle points north.
Balanced and unbalanced forces
Usually more than one force acts on an object.
- Balanced forces are equal in size and opposite in direction. They cancel out, so the object’s motion doesn’t change. A book resting on a table has gravity pulling down and the table pushing up equally.
- Unbalanced forces are not equal. They cause a change in motion. In tug-of-war, if one team pulls harder, the rope moves toward them.
| Situation | Forces | Result |
|---|---|---|
| Tug-of-war: both teams pull with 500 N | Balanced | Rope doesn’t move |
| One team pulls 600 N, the other 500 N | Unbalanced (100 N to one side) | Rope moves toward the stronger team |
| A parked car | Balanced | Stays still |
| A car accelerating | Unbalanced | Speeds up |
Motion
To describe motion, we use:
- position: where something is
- speed: how fast it moves (distance ÷ time)
- direction: which way it’s going
Example: A cyclist travels 300 m in 60 s. Speed = 300 ÷ 60 = 5 m/s.
Common mistakes
- Thinking a moving object needs a force to keep moving. On a frictionless surface it would keep going forever; on Earth, friction slows it.
- Mixing up mass and weight. Mass is how much matter; weight is gravity’s pull.
- Thinking heavier objects fall faster. Without air resistance, all objects fall at the same rate.
Try it yourself
- Use a rubber band stretched between two fingers to pull a book across a table, then a table covered in a towel. Which stretches the rubber band more? Why?
- Drop a flat sheet of paper and a crumpled sheet at the same time. Which lands first? Explain.
- Draw arrows showing the forces on a book resting on a desk.
Quick recap
- A force is a push or pull, measured in newtons.
- Gravity pulls objects toward each other; weight is gravity’s pull on you.
- Friction resists motion; air resistance is friction with air.
- Magnets attract opposites and repel likes.
- Balanced forces cause no change in motion; unbalanced forces do.