Force and Pressure
In one line
A force is a push or a pull, and pressure is how much force presses on each bit of area.
Key points
- A force is a push or a pull on an object. Forces happen when objects interact.
- The SI unit of force is the newton (N).
- A force can start or stop motion, change an object's speed, change its direction, or change its shape.
- Forces in the same direction add up. Forces in opposite directions subtract, like in tug of war.
- Contact forces need touching: muscular force and friction.
- Non-contact forces act from a distance: magnetic, electrostatic and gravitational force.
- Pressure = force acting on a unit area. The smaller the area, the bigger the pressure for the same force.
- Liquids and gases also press on the walls of their containers. Liquid pressure increases with depth.
The formula
Pressure = Force ÷ Area
Force is in newtons (N) and area in square metres (m²), so pressure is in N/m², also called the pascal (Pa).
Worked exampleA box weighing 100 N rests on the floor on a face of area 0.5 m². Pressure = 100 ÷ 0.5 = 200 Pa. Stand it on a smaller face of 0.25 m² and the pressure doubles to 400 Pa.
Types of force
| Force | Contact? | Example |
|---|---|---|
| Muscular force | Yes | Lifting a bag, kicking a ball |
| Friction | Yes | A rolling ball slowly stops on the ground |
| Magnetic force | No | A magnet pulls iron pins from a distance |
| Electrostatic force | No | A rubbed balloon attracts small bits of paper |
| Gravitational force | No | A mango falls from a tree to the ground |
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Everyday examples of pressure
- Sharp knives cut easily: the thin edge has a tiny area, so pressure is high.
- School bags have wide straps: the weight spreads over a bigger area, so it presses less on your shoulders.
- Dams are thicker at the bottom: water pressure is greatest at the bottom.
- Water spurts out faster from a lower hole in a bottle than from a higher one, because pressure is greater lower down.
- A sucker sticks to a smooth wall: pressing out the air leaves the air outside (atmospheric pressure) to push it in place.
Important words
- Newton (N)
- The SI unit of force.
- Pascal (Pa)
- The SI unit of pressure. 1 Pa = 1 N/m².
- Atmospheric pressure
- The pressure of the air around us, pressing on everything.
- Friction
- A force that opposes motion between two surfaces in contact.
Common mistakes
- Mixing up force and pressure. Same force, smaller area means more pressure, but the force itself doesn't change.
- Forgetting direction. Force has both size and direction. Two equal forces in opposite directions cancel out.
- Thinking gravity needs contact. Gravity is a non-contact force. It acts even when nothing touches.
Quick check
Revise in 30 seconds
| Force | Push or pull; unit newton (N) |
|---|---|
| Effects | Changes speed, direction, shape; starts or stops motion |
| Contact | Muscular, friction |
| Non-contact | Magnetic, electrostatic, gravitational |
| Pressure | Force ÷ Area; unit pascal (Pa) |
| Liquids | Pressure increases with depth |