Stopping distance

The total distance a vehicle travels from the moment a hazard appears until it is fully stopped is the stopping distance. It splits into two parts:

Watch out: a common mix-up is which factors change which part. Tiredness, alcohol, drugs and distraction increase the driver's reaction time, so they increase the thinking distance — not the braking distance.

What changes each distance?

Thinking distance depends on the driver's reaction time and the speed:

• Longer reaction time → longer thinking distance: caused by tiredness, alcohol, drugs, or distraction (e.g. a phone).
• Higher speed → more distance covered before the brakes even come on.

Braking distance depends on the braking force, the mass and the speed, and on the road and vehicle:

• Wet or icy road, or worn tyres / brakes → less grip → longer braking distance.
• Higher speed → far longer braking distance (it grows much faster than thinking distance, because the kinetic energy ∝ v²).
• A heavier or faster vehicle needs a larger braking force to stop in the same distance.

Reaction time is typically 0.2 s – 0.9 s. You can measure it with the ruler-drop experiment.

Thinking distance

Large decelerations are dangerous

In a crash a vehicle stops in a very short time. A large deceleration means a large force on the occupants — enough to cause serious injury.

Safety features work by making the collision take longer, so the deceleration — and therefore the force — is smaller:

• Crumple zones fold and squash, extending the stopping time.
• Seatbelts stretch slightly and stop you being thrown forward; they spread the force and lengthen the stop.
• Airbags inflate and let the head and chest decelerate over a longer time and area.

Misconception: longer collision time does not mean a bigger force — it means a smaller force, because the same momentum change is spread over more time.

Momentum & its conservation

Momentum is mass × velocity. It is a vector (it has direction).

In a closed system (no external forces) momentum is conserved: the total momentum before a collision or explosion equals the total momentum after.

Higher tier

Force equals the rate of change of momentum:

For the same change in momentum, a longer time t gives a smaller force F. That is exactly how crumple zones, seatbelts and airbags protect you.

Force from momentum change

Terminal velocity

When an object falls through a fluid (e.g. a skydiver in air) two forces act: weight pulling down and air resistance (drag) pushing up.

At terminal velocity the forces are balanced, the resultant force is zero, acceleration is zero, and the speed is constant.

At terminal velocity…

Energy resources

We generate most electricity by spinning a turbine connected to a generator. Resources are renewable (won't run out) or non-renewable (finite).

Trade-offs: fossil fuels are reliable and high-output but release CO₂ (climate change) and other pollutants. Nuclear is reliable and CO₂-free in use but produces radioactive waste. Renewables are clean but many (wind, solar, tidal) are intermittent or depend on weather/location.

Trend: over time the UK is shifting away from coal and increasingly towards gas and renewables — renewables make up a growing share of electricity generation.

Renewable or non-renewable?

Tap a resource, then tap the box you think it belongs in. Sort all eight.

Mains electricity & the plug

The UK mains supply is a.c. (alternating current) at 50 Hz and about 230 V. a.c. repeatedly changes direction; a battery gives d.c. (direct current) which flows one way only.

A live wire is dangerous even when the switch is off, because it stays at 230 V relative to earth — touching it gives a path to earth and a shock.

Fuses, earthing & double insulation

• Fuse — a thin wire in the live side. A fault makes the current rise; the fuse heats up and melts, breaking the circuit before the cable overheats.
• Earthing — an earth wire connects a metal case to earth. If a fault makes the case live, a large current flows to earth and blows the fuse, so the case can't stay live.
• Double insulation — appliances with plastic cases have no exposed metal to become live, so they need no earth wire (shown by the ⧈ symbol).

Electrical power links to current and p.d.:

Mains safety

The National Grid

The National Grid moves electricity from power stations to homes. It transmits at very high voltage (hundreds of kV) and low current.

Why high voltage? Power lines have resistance R. The power wasted as heat is I²R. Transmitting at high V means a low current I (since P = VI), and because the loss depends on I², a small current means a much smaller energy loss. This makes transmission efficient.

Misconception: the high voltage is across the lines relative to earth, not "extra current". Higher V → lower I → lower I²R heating loss.

Transformers

A transformer has two coils on an iron core. The turns ratio sets how the p.d. changes:

• Step-up: more turns on the secondary (N_s > N_p) → V goes up.
• Step-down: fewer turns on the secondary → V goes down.

Higher tier

An ideal transformer wastes no power, so power in = power out:

Misconception: transformers only work with a.c. They rely on a changing magnetic field to induce a voltage in the second coil — a steady d.c. current would not work. This is one reason the grid uses a.c.

Transformer ratio

The Solar System & our galaxy

Our Solar System contains the Sun (a star) at the centre, the 8 planets and their moons, plus dwarf planets (e.g. Pluto), asteroids and comets. The Sun and everything orbiting it sit within the Milky Way galaxy — billions of stars bound by gravity.

Misconception: "the Sun is not a star." It is a star — an ordinary main-sequence star that happens to be close to us.

Orbital motion

Planets orbit the Sun, and satellites (natural moons and artificial ones) orbit planets. The force that holds an object in orbit is gravity, pulling it toward the central body.

Higher tier

In a circular orbit the speed is constant but the velocity is always changing (its direction changes). The inward gravitational pull constantly changes the direction of motion without changing the speed. For a stable orbit, if the orbital speed changes the radius must change: a satellite moving faster needs a smaller orbit radius, and a slower one needs a larger radius.

Misconception: orbiting objects are not "force-free." Gravity provides the centripetal force — without it they would fly off in a straight line.

Reasoning about orbits

The life cycle of a star

A star is born when gravity pulls a cloud of dust and gas (a nebula) together into a protostar. When it is hot and dense enough, nuclear fusion of hydrogen begins and it becomes a stable main-sequence star like the Sun.

Stability comes from a balance: the outward pressure from fusion energy exactly balances the inward pull of gravity. What happens at the end depends on the star's mass:

Order a Sun-like star's life

Tap the stages in the correct order, earliest first, for a star like our Sun.

Red-shift & the Big Bang

Light from distant galaxies is shifted toward the red (longer wavelength, lower frequency) end of the spectrum. The further away a galaxy is, the greater its red-shift — so more distant galaxies are receding faster.

This is strong evidence that the universe is expanding — space itself is stretching, carrying galaxies apart. Running the expansion backwards points to the Big Bang: the universe began from a tiny, hot, dense point. The leftover heat is detected today as the cosmic microwave background radiation (CMB), the strongest evidence for the Big Bang.

Misconception: red-shift is not galaxies moving "through" space like cars. It is space itself stretching, so the wavelengths of the light are stretched too.

What red-shift tells us

Seismic waves & Earth's structure

Earthquakes send seismic waves through the Earth. Differences in how they travel reveal the hidden internal structure:

• P waves (primary) are longitudinal. They travel through solids and liquids.
• S waves (secondary) are transverse. They travel through solids only — they cannot pass through liquids.

Because S waves don't reach the far side of the Earth (an S-wave shadow zone), scientists deduced the outer core is liquid. P-wave refraction at boundaries reveals the crust, mantle and core.

Key equations & ideas — P8

You've covered all three parts of OCR Gateway P8 — Physics on the move, Powering Earth and Beyond Earth. Press Finish to see your score.