A force can push, pull or twist — but what actually happens depends on the NET force, once every push and pull has been added or cancelled out.
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When more than one force acts on an object, only the NET (combined) force decides what happens to its motion.
📌 Working out the net force
Same direction: forces ADD — two people pushing a cart the same way combine their strength.
Opposite directions: forces SUBTRACT — a tug of war moves toward the stronger side by the difference.
Balanced forces (net force = zero) keep an object at rest OR moving at constant speed, not stop it instantly.
📊 What affects friction
| Factor | Effect on friction |
|---|---|
| Roughness of surfaces | Rougher surfaces → more friction |
| Weight/normal force | Heavier object pressing down → more friction |
| Lubricant (oil, grease) | Reduces friction between surfaces |
| Rolling vs sliding | Rolling friction is much less than sliding friction |
Pressure is force spread over an area — the same force can feel very different depending on how small or large that area is.
Formula: Pressure = Force ÷ Area. A sharp knife cuts more easily than a blunt one because its thin edge concentrates the same force onto a much smaller area, creating far more pressure.
📊 Fluid pressure facts
| Fact | Explanation |
|---|---|
| Liquid pressure increases with depth | More liquid weighs down from above at greater depth |
| Liquids exert pressure equally in all directions | This is why a balloon underwater is squeezed evenly |
| Atmospheric pressure decreases with altitude | Less air above you at higher altitude means less weight pressing down |
Atmospheric pressure is invisible, but it explains everyday things — from why a straw works to why your ears pop in a flight.
✨ Atmospheric pressure at work
Drinking with a straw
you lower pressure inside the straw
Higher outside atmospheric pressure then pushes the liquid up.
A sealed tin can crushed by a vacuum pump
inside pressure removed
Unbalanced outside atmospheric pressure crushes the can inward.
Pro tip: Wide skis or snowshoes spread your weight over a LARGER area, reducing pressure so you do not sink into snow — the opposite trick to a sharp knife, but the same formula.
12 NSO-pattern questions on force, friction and pressure, including two Achievers-level questions.
