Forces and motion

A non-contact force

HintGravity still pulls a dropped ball even though nothing is holding it.

The whyContact forces need surfaces to meet — friction, air resistance, a push on a door. Gravity, magnetism and static electricity reach across a gap, so they still work in the vacuum of space.

The newton

HintThe unit shares its name with the physicist who explained why apples fall.

The whyOne newton is roughly the downward pull on a small apple. Its symbol is a capital N, written large because the unit is named after a person.

Friction

HintIt is why a sledge stops on grass but keeps sliding on ice.

The whyIt always opposes the sliding and turns some of the movement energy into heat. It is also what lets you walk, grip a pencil and stop a bicycle.

8 cm

HintDouble the pull and the spring answers in the same proportion.

The whyHooke's law says the extension is proportional to the force, so doubling the load doubles the stretch — as long as you stay within the spring's limit.

5 metres per second

HintDivide the distance covered by the time it took.

The whySpeed = distance ÷ time, so 100 ÷ 20 = 5. Written properly the unit is m/s, which reads as the number of metres travelled in every second.

They are balanced

HintThe engine drives it forward while drag holds it back, and neither wins.

The whyBalanced does not mean there are no forces — it means they cancel, so nothing about the motion changes. A still object stays still and a moving one keeps the same speed and direction.

25 N

HintBoth pushes point the same way, so they work together rather than against each other.

The whyForces along a line add when they point the same way and subtract when they oppose. The resultant is the single force that would have exactly the same effect as all of them together.

area

HintA drawing pin and your thumb can take the same push with very different results.

The whyPressure = force ÷ area, measured in newtons per square metre, also called pascals. A sharp point concentrates the same force into a tiny patch, which is exactly why it pierces.

Upthrust

HintIt is why you feel lighter standing in a swimming pool.

The whyWater presses harder on the bottom of an object than on its top, and that difference pushes it upwards. An object floats when this push has grown to match its weight.

How quickly speed changes

HintA sports car and a milk float can both reach 30 mph — one simply takes far longer.

The whyAcceleration is the rate at which velocity changes, so slowing down and turning a corner both count. A steady 30 mph in a straight line is zero acceleration.

The distance from the pivot

HintA long spanner shifts a stiff nut that a stubby one cannot — what has changed?

The whyMoment = force × perpendicular distance to the pivot, measured in newton metres. Gripping twice as far out halves the force you need for the same turning effect.

3 m/s²

HintDivide the push by the mass being shifted.

The whyF = ma rearranges to a = F ÷ m, so 12 ÷ 4 gives the answer. The unit reads as metres per second, gained every second — the same trolley pushed twice as hard picks up speed twice as quickly.