Forces and motion

A non-contact force

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

为什么Contact 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

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

为什么One 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.

Drag

提示A cyclist crouches to cut it down; a parachute is built to make as much of it as possible.

为什么Air resistance and water resistance are both forms of it. It grows as an object speeds up, which is why racing cars are streamlined and why a falling skydiver eventually stops getting any faster.

8 cm

提示Double the pull and the spring answers in the same proportion.

为什么Hooke'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.

80 km/h

提示Add the two legs together first and work with the totals — never average the two separate rates.

为什么Average speed is always total distance ÷ total time, never the mean of the separate speeds. Here that is 40 km in 30 minutes, and 40 ÷ 0.5 h = 80 km/h. The two legs are 90 km/h and 60 km/h, whose mean is 75 km/h — a different answer, because the coach spends twice as long on the first leg as on the second.

They are balanced

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

为什么Balanced 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

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

为什么Forces 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

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

为什么Pressure = 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

提示It is why you feel lighter standing in a swimming pool.

为什么Water 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 velocity changes

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

为什么Acceleration 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

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

为什么Moment = 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

提示Divide the push by the mass being shifted.

为什么F = 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.