Every card in Forces and motion

A non-contact force

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

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.

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.

Drag

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

WhyAir 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

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

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.

80 km/h

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

WhyAverage 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

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

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.

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.

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.

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 velocity changes

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

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?

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.

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.