A force is a push or a pull, and it always comes from two objects interacting. Forces change motion, squash and stretch things, and act through fluids as pressure.
Speed is how far something travels in a given time: \(\text{speed} = \frac{\text{distance}}{\text{time}}\), usually in metres per second.
Average speed over a journey uses the total distance and the total time, even if the object stopped or changed speed along the way. Typical speeds: walking about 1.5 m/s, cycling about 6 m/s, a car in town about 13 m/s.
A distance–time graph shows a journey. A sloping straight line means constant speed, and the steeper the line the faster the motion. A horizontal line means the object is stationary.
A curve means the speed is changing: curving upwards means speeding up, curving flatter means slowing down.
How fast something appears to move depends on where you watch it from. Two trains travelling the same way at 30 m/s and 20 m/s have a relative speed of only 10 m/s, so if the faster one is behind it closes the gap at 10 m/s.
Travelling in opposite directions, the speeds add instead: two cars at 12 m/s and 8 m/s close at 20 m/s.
A force is a push or a pull, and it always arises from an interaction between two objects. There is no such thing as a single object exerting a force on nothing.
Contact forces need the objects to touch. Non-contact forces act across a gap: gravity, magnetism and static electricity all pull or push without contact.
Force is measured in newtons (N) with a newtonmeter, which contains a spring. On Earth a 1 kg mass has a weight of about 10 N.
Hanging masses on a spring and measuring how much it stretches for each one is the standard way of investigating how force and extension are related.
Forces are drawn as arrows. The length shows the size and the direction the arrow points shows the direction of the force.
Along one line, forces in the same direction add and forces in opposite directions subtract, giving the resultant. If the resultant is zero the forces are balanced; if not, they are unbalanced.
When opposing forces are balanced an object is in equilibrium and its motion does not change. For example, a weight hanging still on a stretched spring: the spring's upward pull exactly matches the weight pulling down.
Equilibrium does not mean at rest. An object moving at a steady speed in a straight line is also in equilibrium.
A force is needed to start something moving, to stop it, to speed it up, to slow it down or to change its direction. Without an unbalanced force, motion carries on unchanged.
What the change looks like depends on the size of the force and its direction. A force along the direction of travel speeds the object up; one against it slows the object down; one across it changes the direction.
Friction acts between two surfaces in contact and opposes their relative movement. Drag is the resistance of a fluid, either air resistance or water resistance, and it grows as the object goes faster.
Both transfer energy to thermal stores, so rubbed hands and bicycle brakes get warm. Streamlining reduces drag, so a cyclist's helmet is smooth and pointed.
For a spring, extension is directly proportional to the force applied, provided it is not stretched too far. That is Hooke's law, and it gives a force–extension graph that is a straight line through the origin.
Double the force and the extension doubles. Past the limit of proportionality the line curves and the two are no longer proportional; past the elastic limit, further out still, the spring no longer returns to its original length. Work done in stretching is stored in the spring's elastic potential store, which is released when it springs back.
A moment is the turning effect of a force about a pivot, and it depends on both the force and how far from the pivot it acts.
A long spanner therefore turns a stiff nut more easily than a short one, and a door handle is fitted at the edge rather than next to the hinge.
Pressure is the force acting on each unit of area, at right angles to the surface: \(\text{pressure} = \frac{\text{force}}{\text{area}}\).
The same force spread over a larger area gives a smaller pressure, which is why wide tyres stop a tractor sinking into a field and why a drawing pin has a sharp point at one end and a broad head at the other.
Pressure in a liquid increases with depth, because there is more liquid above pressing down. Dams are therefore built thicker at the base.
Because the pressure on the bottom of a submerged object is greater than on the top, there is a resultant upward force called upthrust. If the upthrust equals the object's weight there is no resultant vertical force, so the object can float at rest or stay suspended in the fluid. If the weight is greater it sinks.
Air has weight, and atmospheric pressure is caused by the weight of the air above a surface.
As you go higher there is less air above you, so the pressure falls. That is why aircraft cabins are pressurised and why mountaineers at altitude find breathing difficult.
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