Hook137 mph, 67g, a fireball — and the driver walked away
On 29 November 2020, on the opening lap of the Bahrain Grand Prix, Romain Grosjean's car speared off the track at around 137 mph, hit the steel barrier, and tore in two as it burst into flames. Telemetry later showed the impact peaked at about 67g — his body was decelerated with a force sixty-seven times its own weight. Every serious student of forces should find it astonishing that he climbed out of the fireball with burned hands and nothing worse. The reason is physics that this whole section is about: the barrier, the survival cell and the titanium 'halo' above his head spread that colossal change of momentum over a longer time and a larger area, and in doing so cut the peak force on his body to something a human can survive.
Forces are the reason anything speeds up, slows down, turns, stretches, floats or breaks. P5 is the biggest topic in GCSE Physics because it is the toolkit for the rest of it: vectors and resultants, weight and gravity, work and energy, springs, moments, pressure, the equations of motion, Newton's three laws, stopping distances, and — for Higher tier — momentum and the impulse that saved Grosjean. We will build it in that order, with the equations you are expected to use and the mistakes examiners see every summer.
ModelScalars, vectors and the two families of force
A scalar has size only — mass, temperature, speed, distance, energy. A vector has size and direction — force, weight, velocity, acceleration, displacement, momentum. We draw a vector as an arrow: its length shows the magnitude, its way of pointing shows the direction. That distinction is not pedantry; it is why a car going round a roundabout at a steady 30 mph is still accelerating, because its velocity (a vector) is changing direction even though its speed (a scalar) is not.
Forces come in two families. Contact forces need the objects to touch — friction, air resistance, tension, the normal contact (reaction) force. Non-contact forces act across a gap — gravitational, electrostatic and magnetic forces. Every force is one half of an interaction pair: if you push a wall, the wall pushes back on you with an equal, opposite force. Naming the pair correctly (same type, equal size, opposite direction, acting on different objects) is a recurring exam ask.
ModelWeight, mass and centre of mass
Mass is the amount of matter in an object, a scalar measured in kilograms, and it does not change if you move. Weight is the force of gravity on that mass, a vector measured in newtons, and it depends on the gravitational field strength \(g\) where you are: \[W = mg\] with \(g \approx 9.8\ \text{N/kg}\) on Earth. The same 1 kg bag of sugar weighs about 9.8 N on Earth but only about 1.6 N on the Moon, because \(g\) is smaller there — mass unchanged, weight changed. Weight acts as if concentrated at a single point, the centre of mass, and is measured with a calibrated spring balance (a newtonmeter). Keeping the words separate — mass in kilograms, weight in newtons — is one of the cheapest marks in the paper to win or lose.
MechanismResultant forces and resolving (Higher tier)
When several forces act on an object you can replace them with a single resultant force that has the same effect. Along one line you simply add forces one way and subtract those the other. A free-body diagram — the object as a dot with every force drawn as an arrow — is the tool that keeps this honest, and examiners expect one.
At Higher tier a single force can also be split, or resolved, into two perpendicular components (typically horizontal and vertical), and forces at an angle can be combined using a scale drawing: draw them tip-to-tail to scale, and the closing arrow is the resultant, its length read off with a ruler and its direction with a protractor. A force is in equilibrium when the resultant is zero — the arrows form a closed loop — and the object then stays still or moves at constant velocity.
ModelWork done and the springs that store it
A force does work when it moves its object along the line of the force: \[W = Fs\] where \(W\) is work in joules, \(F\) the force in newtons and \(s\) the distance in metres. One joule is one newton-metre. Work done against friction is transferred to thermal energy — which is why brake discs and rubbed hands get hot.
Stretch or squash a spring and you also do work, stored as elastic potential energy. Up to a point called the limit of proportionality, extension is proportional to force — Hooke's law: \[F = ke\] with \(k\) the spring constant in N/m and \(e\) the extension in metres. A deformation is elastic if the object returns to its original shape when the force is removed, and inelastic if it stays deformed. The energy stored in an elastically stretched spring is \[E_e = \frac{1}{2}ke^{2}\] This is exactly what Required practical 6 measures: hang increasing weights on a spring, record the extension each time, and plot extension against force. A straight line through the origin confirms Hooke's law, and its gradient gives the spring constant; where the line bends over, you have passed the limit of proportionality.
A spring extends by \(0.04\ \text{m}\) when a force of \(6\ \text{N}\) is applied, within its limit of proportionality. Find the spring constant and the elastic potential energy stored.
Spring constant, rearranging \(F = ke\): \[k = \frac{F}{e} = \frac{6}{0.04} = 150\ \text{N/m}\] Energy stored: \[E_e = \frac{1}{2}ke^{2} = \frac{1}{2} \times 150 \times (0.04)^{2}\] \[E_e = \frac{1}{2} \times 150 \times 0.0016 = 0.12\ \text{J}\] Note the extension is squared, so you must square \(0.04\) (giving \(0.0016\)) before multiplying — forgetting to square, or squaring the force instead, is the classic slip that turns 0.12 J into a wrong answer.
MechanismMoments, levers and gears (physics only)
A force that turns something has a moment — a turning effect — given by \[M = Fd\] where \(F\) is the force in newtons and \(d\) is the perpendicular distance from the pivot to the line of the force, in metres, so the moment is in newton-metres. Push a door near the hinge and it barely moves; push at the handle, far from the pivot, and the same force swings it easily.
For a balanced object the principle of moments holds: the total clockwise moment about a pivot equals the total anticlockwise moment. A lever exploits this — a long effort arm lets a small force lift a large load, a force multiplier. Gears do the same job for rotation: a small gear driving a larger one turns more slowly but transmits a bigger moment, which is how low gears give a bike or a car the turning force to climb a hill.
ModelPressure in fluids and in the atmosphere (physics only)
Pressure is force spread over area: \[p = \frac{F}{A}\] in pascals (N/m²). It is why a drawing pin pierces a board — a modest force through a tiny point area is a huge pressure. In a liquid, pressure increases with depth because a deeper point supports a taller column of fluid pressing down: \[p = h\rho g\] where \(h\) is depth, \(\rho\) the density and \(g\) the gravitational field strength. This is why a dam is built thicker at the base and why your ears hurt at the deep end of a pool.
Because pressure rises with depth, the upward push on the bottom of a submerged object is greater than the downward push on its top; the difference is upthrust. An object floats when the upthrust equals its weight (Higher tier). The atmosphere is a fluid too: the weight of the air above you creates atmospheric pressure, and because there is less air above you the higher you climb, atmospheric pressure falls with altitude — the reason aircraft cabins are pressurised and mountaineers gasp for breath.
MechanismDescribing motion — speed, velocity, acceleration and graphs
Distance is a scalar (how far travelled); displacement is a vector (how far, and in which direction, from the start). Speed is a scalar; velocity is speed with a direction. Speed links to distance and time by \[v = \frac{s}{t}\] and acceleration is the rate of change of velocity, \[a = \frac{\Delta v}{\Delta t}\] in m/s². A very useful equation when you do not know the time links the two directly: \[v^{2} - u^{2} = 2as\] where \(u\) is the starting velocity and \(v\) the final velocity.
Graphs make motion visible. On a distance–time graph the gradient is the speed — flat means stationary, steeper means faster, and a curve means changing speed. On a velocity–time graph the gradient is the acceleration and the area under the line is the distance travelled. Reading those two features correctly is worth several marks a paper.
A car accelerates from rest (\(u = 0\)) to \(v = 20\ \text{m/s}\) over a distance of \(s = 50\ \text{m}\). Find its acceleration.
Use the equation that avoids time, \(v^{2} - u^{2} = 2as\), and rearrange for \(a\): \[a = \frac{v^{2} - u^{2}}{2s} = \frac{20^{2} - 0^{2}}{2 \times 50}\] \[a = \frac{400 - 0}{100} = 4\ \text{m/s}^{2}\] Always square the velocities before subtracting — \(20^2 = 400\), not \(2 \times 20\). Writing the formula, substituting, then evaluating in that order is what secures the method marks even if the arithmetic slips.
ModelNewton's three laws and inertia
Newton's first law: if the resultant force on an object is zero, it stays still or keeps moving at constant velocity. A change in motion always needs a resultant force. The tendency of an object to keep doing what it is doing is its inertia.
Newton's second law quantifies that change: \[F = ma\] resultant force equals mass times acceleration. The mass here is the inertial mass — a measure of how hard it is to change an object's velocity (Higher tier). Double the force and you double the acceleration; double the mass and you halve it. Required practical 7 tests exactly this: pull a trolley with a steady force and vary the mass, then keep the mass fixed and vary the force, confirming that acceleration is proportional to force and inversely proportional to mass.
Newton's third law: every action has an equal and opposite reaction, acting on a different object. When you walk, your foot pushes back on the ground and the ground pushes you forward.
CaseStopping distance — where the Highway Code meets physics
The total stopping distance of a vehicle is the thinking distance (travelled during the driver's reaction time, before the brakes are touched) plus the braking distance (travelled while decelerating). Thinking distance grows with speed and with anything that lengthens reaction time — tiredness, alcohol, drugs, distraction. Braking distance grows with speed too, but faster than proportionally, and is worsened by poor road conditions (ice, rain), worn tyres and worn brakes.
Braking transfers the car's kinetic energy to thermal energy in the brakes by the work the braking force does, \(W = Fs\). A larger braking force stops the car in a shorter distance but produces a greater deceleration — and a very large deceleration means a very large force on the vehicle and its occupants, which can cause injury. That trade-off, not just the numbers in the Highway Code table, is what Grosjean's crash shows at the extreme: stop something moving fast in too short a distance and the forces become lethal.
ModelMomentum, conservation and impulse (Higher tier)
Momentum is a property of every moving object: \[p = mv\] mass times velocity, in kg m/s, and it is a vector. In a closed system — no external forces — the total momentum before an event equals the total momentum after: the conservation of momentum. This is how you analyse collisions and explosions; the momenta must balance across the event.
Re-expressing Newton's second law shows why safety features work. Force is the rate of change of momentum: \[F = \frac{m\Delta v}{\Delta t}\] The change in momentum \(m\Delta v\) in a crash is fixed — the car has to stop. But the force depends on how long the stop takes: increase \(\Delta t\) and you decrease \(F\). Crumple zones, airbags, seatbelts, cycle helmets and crash barriers all do the same thing — extend the collision time (and spread the force over a larger area) so the peak force on the body drops. That is the whole reason Romain Grosjean survived 67g: the barrier and survival cell stretched a violent change of momentum over just enough extra time and area to keep him alive.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
For every calculation, write the equation, substitute the numbers with units, then evaluate — the method marks survive an arithmetic slip only if the working is shown. Watch the squared terms: in \(E_e = \frac{1}{2}ke^{2}\) and \(v^{2} - u^{2} = 2as\) you must square the length or velocity before combining, a step examiners report candidates skip.
Keep vector language precise. Distinguish distance from displacement and speed from velocity, and remember that changing direction at constant speed is still acceleration. On graph questions, name which feature you are using: gradient of a distance–time graph is speed; gradient of a velocity–time graph is acceleration; area under a velocity–time graph is distance.
On stopping distances, always split thinking and braking distance and attach the right factors to each — reaction-time factors (tiredness, alcohol, distraction) to thinking distance, and speed and road/tyre/brake condition to braking distance. For Higher-tier momentum questions, quote \(F = \frac{m\Delta v}{\Delta t}\) and argue that safety features increase the collision time to reduce the force; that reasoning, not just the phrase 'crumple zone', is what earns the marks.