Forces and motion

For an object falling through a fluid (e.g. a skydiver), a speed-time graph starts with a steep gradient (large acceleration, close to g) which gradually flattens as drag increases and acceleration decreases, until the line becomes horizontal at terminal velocity — the maximum, constant speed reached once weight and drag are balanced.

If a resultant (non-zero) force acts on an object, it will cause the object to accelerate — changing its speed, its direction, or both.

Required practical — investigating force, mass and acceleration: a trolley sits on a ramp, connected over a pulley to a hanging mass that provides the accelerating force; two light gates on the ramp measure the trolley's velocity as it passes each one, giving its acceleration. To vary the force without changing the total mass being accelerated, masses are moved from the trolley onto the hanger (not added or removed) — repeating this shows acceleration is directly proportional to the resultant force, confirming F = ma.

Inertial mass (HT only) is a measure of how difficult it is to change an object's velocity, defined as inertial mass = force ÷ acceleration — the same relationship as F = ma, rearranged.

• •A rocket taking off: the rocket pushes exhaust gases downward/backward; the gases push back on the rocket with an equal and opposite force, propelling it upward.
• •A book resting on a table: the book's weight pulls down on the table; by Newton's Third Law, the table pushes back up on the book with the normal contact force.

Reaction times vary between people, typically 0.2–0.9 s, and can be measured with a 'ruler drop' test: a ruler is dropped through someone's open fingers, and the distance it falls before being caught is used (with s = ½gt², since it starts at rest) to calculate their reaction time.

When a vehicle brakes, work is done by friction between the brakes and the wheel, transferring the vehicle's kinetic energy into the thermal energy store of the brakes — this is why brakes heat up. A greater speed requires a greater braking force to stop over the same distance, producing a greater deceleration; braking too hard, too often, or from too high a speed risks the brakes overheating, which can lead to a loss of control.