Motion, forces and energy
Physical quantities and measurement
Every physical quantity is a number with a unit. Measure carefully, and measure many small things together to reduce errors. Scalars have size only; vectors have size and direction, so two vectors at right angles combine by Pythagoras.
≈ 25 minSpeed, velocity and acceleration
Speed is distance per unit time; velocity is speed in a given direction. Acceleration is the rate of change of velocity. On a distance–time graph the gradient is speed; on a speed–time graph the gradient is acceleration and the area underneath is the distance travelled.
≈ 35 minMass, weight and density
Mass is how much matter something contains, in kilograms, and it's the same everywhere. Weight is the gravitational force on that mass, , in newtons, and it changes from planet to planet. Density is mass per unit volume; objects less dense than a liquid float in it.
≈ 30 minForces and motion
If the forces on an object are balanced, it stays at rest or keeps moving at constant velocity. An unbalanced (resultant) force makes it accelerate: . Friction and air resistance oppose motion, which is why a falling object reaches a terminal velocity. Moving in a circle needs a force towards the centre.
≈ 35 minSprings and Hooke's law
A spring stretches in proportion to the load on it, , up to the limit of proportionality. The spring constant tells you how stiff it is. Extension is measured from the original length, and a load–extension graph is a straight line through the origin until the limit is passed.
≈ 25 minTurning effect and centre of gravity
A force can turn an object about a pivot: its moment is force perpendicular distance from the pivot. When an object balances, the total clockwise moment equals the total anticlockwise moment. An object's weight acts at its centre of gravity; a low centre of gravity and wide base make it stable.
≈ 30 minMomentum and impulse
Momentum is mass times velocity, , and it has a direction. In any collision or explosion with no external force, the total momentum before equals the total after. A force changes momentum: , so spreading a change over a longer time means a smaller force.
≈ 30 minEnergy, work and power
Energy is stored in kinetic, gravitational, elastic, chemical, thermal, nuclear and other stores, and transferred by forces, heating, electrical currents and waves. It's never created or destroyed. Work done is energy transferred by a force, . Power is the rate of transfer, . Efficiency is the fraction of input energy that ends up useful.
≈ 35 minEnergy resources
Most of our energy comes, directly or indirectly, from the Sun: fossil fuels, biofuels, wind, waves, hydroelectric and solar. Tides, geothermal and nuclear fuel are the exceptions. Renewable resources won't run out; non-renewable ones will. Every resource is a trade-off between cost, reliability, scale and pollution.
≈ 25 minPressure
Pressure is force per unit area, , measured in pascals ( Pa N/m²). The same force on a smaller area gives a bigger pressure. In a liquid, pressure increases with depth and density: .
≈ 25 min