Dynamics — OCR A-Level Physics

The newton is the SI unit of force, defined as the force that gives a mass of one kilogram an acceleration of one metre per second squared. In base units this is written 1 N = 1 kg m s⁻², which follows directly from F = ma. Because the newton is a derived unit, it can always be broken down into kilograms, metres and seconds, which is useful for checking that equations are homogeneous. For example, a resultant force of 5 N acting on a 2 kg mass produces an acceleration of 2.5 m s⁻², and the unit of that result, m s⁻², matches the base-unit form of the newton divided by the kilogram.

Weight is the gravitational force acting on an object, calculated as W = mg, where m is mass in kilograms and g is gravitational field strength in newtons per kilogram (N kg⁻¹). On Earth, g ≈ 9.81 N kg⁻¹, so a 2.0 kg mass has weight W = 2.0 × 9.81 = 19.62 N. Weight is a vector directed towards the centre of the gravitating body. Mass is a scalar measure of inertia, constant everywhere, while weight varies with g. In an MCQ, you may be asked to calculate weight, identify units, or distinguish mass from weight. Always convert grams to kilograms before substituting. The equation applies to a uniform gravitational field, which is a good approximation near a planet's surface.

Tension is the pulling force transmitted through a string, rope or cable when it is stretched. Normal contact force acts perpendicular to a surface where two objects touch. Upthrust is the upward force exerted by a fluid on an object immersed in it, due to pressure differences. Friction opposes relative motion or the tendency of relative motion between surfaces in contact. In MCQs, you may identify which force acts in a given situation, or match descriptions to terms. For example, a book on a table experiences a normal contact force upwards; a submerged ball experiences upthrust; a box sliding on a floor experiences friction; a hanging mass experiences tension in the string. Recognise the direction and origin of each force.

A free-body diagram shows a single object isolated from its surroundings, with all external forces acting on it represented as arrows. Each arrow's length indicates magnitude and its direction shows the force's direction. The object is drawn as a simple shape or dot. Forces to include: weight, normal contact force, tension, friction, upthrust, applied forces. Do not include forces the object exerts on other things. For example, a block on a rough slope has weight downwards, normal contact force perpendicular to the slope, and friction along the slope. In MCQs, you may select the correct diagram or identify a missing force. Ensure arrows start from the object and are labelled.

Motion under constant force involves constant acceleration, described by the equations of motion (suvat): v = u + at, s = ut + ½at², v² = u² + 2as, s = ½(u + v)t. In one dimension, motion is along a straight line; in two dimensions, motion can be split into independent perpendicular components, often horizontal and vertical. For projectile motion, horizontal acceleration is zero (constant velocity) and vertical acceleration is g downwards. For example, a ball thrown horizontally: horizontal distance x = uₓt, vertical drop y = ½gt². In MCQs, you may calculate a quantity using suvat, resolve vectors, or identify independence of components. Always define a positive direction and use consistent signs.

Drag is a contact force that opposes the motion of an object through a fluid, where a fluid is any liquid or gas. It acts along the line of motion but in the opposite direction to velocity, so it always removes kinetic energy from the object. Unlike solid friction, drag depends strongly on speed and on the shape and size of the object. For example, a skydiver falling at 50 m s⁻¹ experiences far greater drag than the same skydiver moving at 5 m s⁻¹. Drag arises because the object must push fluid aside and because fluid layers slide past one another, producing viscous forces. In this section you treat drag as a resistive force to be included in free-body diagrams and in Newton's second law calculations.

For an object moving through air, the size of the drag force depends on several factors. Speed is the most important: drag increases as speed increases, and at higher speeds drag grows roughly with the square of speed. The cross-sectional area of the object also matters, because a larger area pushes more air aside. The shape of the object affects how smoothly air flows around it, so a streamlined shape reduces drag compared with a blunt shape. The density of the air is another factor, since denser air provides more resistance. Surface roughness can also influence drag by changing the flow near the surface. In calculations you may be told which factors are held constant so that you can isolate the effect of one variable.

When an object falls in a uniform gravitational field with drag present, its acceleration is not constant. At release, speed is zero so drag is zero and the object accelerates at g. As speed rises, drag increases, reducing the resultant force and therefore the acceleration. The object reaches terminal velocity, a constant maximum speed, when drag equals weight and the resultant force is zero. Before terminal velocity, the object accelerates at a continuously decreasing rate. A velocity–time graph shows a curve with its maximum gradient at t=0, which then continuously flattens as terminal velocity is approached. The same analysis applies to an object projected upwards, where weight and drag both act downwards.

This row is a guided reading entry for section 3.2.1 Dynamics. The statement is empty, so there is no assessed content to teach here. Use this row to direct learners to the relevant specification statements (a), (b) and (c) on drag, factors affecting drag in air, and motion of falling objects with drag. Learners should read the specification text alongside their textbook or class notes, then check that they can define drag, list the factors affecting drag in air, and explain terminal velocity using the balance of weight and drag. No marks are awarded for this row; it is a study guide only.

Terminal velocity is the constant velocity reached by a falling object when the resultant force on it is zero. As an object falls through a fluid, its weight acts downwards while drag and upthrust act upwards. Drag increases with speed, so the resultant downward force decreases and acceleration falls. Eventually drag plus upthrust equals weight, the resultant force is zero, and velocity stays constant. For a skydiver, terminal velocity is reached before the parachute opens; opening the parachute increases drag, so the skydiver decelerates to a new, lower terminal velocity. The key idea is that zero resultant force means zero acceleration, not zero velocity.

To determine terminal velocity in a fluid, release an object into a tall column of fluid and measure its velocity after it has had time to reach constant speed. A common method uses a tall measuring cylinder of glycerol or oil with a ball bearing or small sphere. Mark two horizontal lines a known distance apart near the bottom of the column, where the sphere is already moving at terminal velocity. Use a stopwatch or light gates to measure the time taken to travel between the lines, then calculate velocity as distance divided by time. Repeat and average to reduce random error. The sphere must be small and the fluid deep enough that terminal velocity is reached before the timing section.