Forces & Motion

• Contact forces include friction, drag, tension and normal contact force. Gravitational, electrostatic and magnetic forces act without contact.
• Force is a vector: an arrow's direction shows the force direction and its length can represent magnitude.
• Weight is W = mg and acts through the centre of mass. At fixed g it is directly proportional to mass and is measured with a calibrated newtonmeter.
• Replace forces along one straight line with a single resultant, keeping opposing directions and signs clear.

The punchline: Name the interacting objects, draw directions, then combine the forces.

• Draw each force from the object, label it, and use arrow direction and length to show its vector character.
• Balanced forces give zero resultant; they do not prove the object is stationary—it may move with constant velocity.
• Resolve one force into perpendicular components, or use a scale vector diagram to find a resultant's magnitude and direction.
• Check that your resultant would cause the described change in speed or direction.

The punchline: Choose the object first; only forces acting on that object belong on its diagram.

• Work done is force multiplied by distance moved along the force's line of action. One joule equals one newton-metre.
• Work against friction transfers energy into thermal stores, so the surfaces warm rather than energy disappearing.
• Stretching, bending or compressing a stationary object needs more than one force. Up to the limit of proportionality, F = ke.
• Elastic deformation reverses; inelastic does not fully reverse. Before inelastic deformation, work done on a spring equals its elastic energy increase.

The punchline: Use displacement along the force, and state the energy store that changes.

• Moment = force × perpendicular distance from the pivot to the force's line of action.
• For balance, total clockwise moment equals total anticlockwise moment about the same pivot.
• Levers transmit and can multiply turning effects; meshed gears transmit rotation, reverse direction and trade turning force against speed.
• Use the perpendicular distance, not the sloping length of a lever or spanner.

The punchline: Locate the pivot and line of action before measuring the moment arm.

• Pressure is normal force divided by area. The same force spread over a larger area produces a smaller pressure.
• HT only: liquid pressure p = hρg rises with depth and density; greater pressure below a submerged object can create upward resultant force, or upthrust.
• Floating occurs when upthrust balances weight. If weight is greater than upthrust, the object accelerates downwards.
• Atmospheric pressure comes from air-particle collisions and decreases with altitude because fewer air molecules lie above and the air is less dense.

The punchline: For fluid questions, connect particle collisions or liquid weight to pressure, then pressure to force.

• Distance and speed are scalars. Displacement and velocity are vectors, so they require direction as well as magnitude.
• Velocity is speed in a stated direction. HT only: circular motion can have constant speed but changing velocity because direction continually changes.
• Typical values are about 1.5 m/s walking, 3 m/s running, 6 m/s cycling and 330 m/s for sound in air; real values vary with conditions.
• For constant speed use s = vt; otherwise use total distance divided by total time. Direct measurements of distance and time give speed.

The punchline: If direction matters, use displacement or velocity and state that direction.

• The gradient of a distance–time graph is speed; a horizontal section means stationary and a steeper section means faster motion.
• HT only: draw a tangent to a curved distance–time graph to estimate speed at one instant.
• The gradient of a velocity–time graph is acceleration. A negative gradient means velocity is becoming more negative or less positive—not automatically slowing.
• HT only: the signed area under a velocity–time graph gives displacement; count squares where necessary.

The punchline: Name the axes, calculate the units of gradient or area, then interpret the motion.

• Average acceleration is change in velocity divided by time. For uniform acceleration, use v² − u² = 2as when time is absent.
• Near Earth's surface, free-fall acceleration is about 9.8 m/s² before air resistance becomes important.
• A falling object accelerates while weight exceeds drag. As speed and drag rise, the resultant shrinks; at zero resultant it reaches terminal velocity.
• Separate Physics only: draw and interpret velocity–time graphs for objects reaching terminal velocity.

The punchline: Track how both forces and velocity change through the fall.

• First law: zero resultant keeps an object at rest or moving at constant velocity. A steady vehicle has driving and resistive forces balanced.
• Second law: F = ma. At fixed mass, a ∝ F; at fixed force, a ∝ 1/m. Use ~ when a transport force or acceleration is only an estimate.
• Third law: interacting objects exert equal and opposite forces on each other. The pair acts on different objects, so it does not cancel on one object.
• HT only: inertia is resistance to changing velocity; inertial mass = force/acceleration.

The punchline: Separate balanced forces on one object from a third-law pair on two objects.

• Stopping distance = thinking distance + braking distance. Typical human reaction times are about 0.2–0.9 s.
• Tiredness, alcohol, drugs and distractions can increase reaction time; measure it with repeated ruler-drop or computer tests and examine variation.
• Wet or icy roads and worn tyres or brakes reduce effective braking. Greater starting speed means more kinetic energy must be transferred.
• Large braking forces can overheat brakes, reduce control and increase injury risk. Separate Physics: interpret stopping graphs; HT: estimate road forces.

The punchline: Link each factor to thinking distance, braking distance or both—never just list it.

• Momentum p = mv is a vector, so choose a positive direction and give opposite velocities negative signs.
• For both routes, total momentum before equals total momentum after in a closed system; Combined Science requires qualitative use of collision examples.
• Separate Physics only: complete collision calculations using conservation of momentum.
• Separate Physics HT only: F = Δp/Δt. Increasing stopping time reduces force for the same momentum change, explaining airbags, helmets and crash mats.

The punchline: Set a direction, total the momentum before, then balance the signed total after.