Friction, Lift, and Drag Forces Revision Notes | A Level AQA Physics

1. Why Forces in Fluids Matter

Picture a skydiver stepping out of a plane. Gravity pulls them down — yet they do not accelerate forever. Something pushes back. That something is drag, and understanding it (alongside friction and lift) is central to explaining the motion of almost every object you will encounter in mechanics.

These three forces — friction, drag, and lift — are all examples of contact forces that arise whenever surfaces or fluids interact with a moving object. The AQA specification asks you to treat them qualitatively: you need to describe and explain their behaviour, rather than calculate them from first principles. That said, the conceptual depth required at A Level is significant, and examiners will probe your reasoning carefully.

2. Friction — The Force That Opposes Relative Motion

Think about pushing a heavy box across a floor. You apply a horizontal force, but the box resists moving — that resistance is friction. At the microscopic level, even surfaces that look smooth have tiny irregularities. When two surfaces press together, these irregularities interlock, and energy is needed to break those contacts and allow motion.

The magnitude of friction depends on two things: the nature of the surfaces in contact (captured by the coefficient of friction) and the normal contact force pressing the surfaces together. A heavier box on the same floor experiences more friction because the surfaces are pressed together more forcefully.

One important note from the specification: AQA does not require you to distinguish between static friction (the friction preventing an object from starting to move) and dynamic friction (the friction acting once the object is already sliding). You need to know that friction opposes relative motion and that it depends on the surfaces involved — that is the level of treatment required.

• Friction always acts parallel to the surfaces in contact.
• Friction always opposes the direction of relative motion (or the tendency of motion).
• Friction increases when the surfaces are rougher or when the normal contact force is larger.
• Friction converts kinetic energy into thermal energy (heat), which is why surfaces warm up when rubbed together.
• Friction can be useful (brakes, walking, gripping) or a problem (energy loss in engines, wear on surfaces).

Friction is not always a nuisance. Without friction between your feet and the ground, walking would be impossible — your foot would slip backwards rather than pushing you forwards. Tyres grip roads because of friction; brakes slow vehicles by converting kinetic energy to heat through friction between brake pads and discs. Recognising both the useful and problematic roles of friction is exactly the kind of evaluative thinking AQA rewards.

3. Drag — Resistance Through Fluids

When an object moves through a fluid — whether that is air, water, or any other gas or liquid — the fluid exerts a resistive force on it. This force is called drag (or fluid resistance, or air resistance when the fluid is air). Drag always acts in the direction opposite to the object's velocity through the fluid.

Here is the key relationship you must know: air resistance (drag) increases with speed. At low speeds, the drag force is relatively small. As speed increases, drag grows — and it grows rapidly. For many objects, drag is approximately proportional to the square of the speed, meaning doubling the speed roughly quadruples the drag. AQA requires you to know this qualitative relationship, not to derive it mathematically.

Two physical mechanisms contribute to drag. First, the object must push fluid out of its path — this requires force, and by Newton's third law, the fluid pushes back. Second, friction between the fluid and the surface of the object (called skin friction or viscous drag) also resists motion. The shape of the object strongly influences how much drag it experiences: a streamlined shape (like a teardrop or an aerofoil) allows fluid to flow smoothly around it, reducing drag significantly compared to a blunt shape.

The speed-dependence of drag has a profound consequence: terminal velocity. As a falling object accelerates under gravity, its drag increases. Eventually, drag equals the weight of the object. At that point, the resultant force is zero, acceleration is zero, and the object falls at a constant speed — its terminal velocity. This is why a skydiver reaches a maximum speed before opening their parachute, and why opening the parachute (which dramatically increases drag) causes rapid deceleration to a new, much lower terminal velocity.

Notice in the graph above how the speed increases rapidly at first (when drag is small and the resultant force is large), then the rate of increase slows (as drag grows and the resultant force decreases), and finally the speed levels off at terminal velocity (when drag equals weight and the resultant force is zero). This curve shape is a favourite target for AQA 'describe and explain' questions.

4. Lift — The Upward Force from Moving Fluids

Drag acts parallel to the direction of motion (opposing it). Lift acts perpendicular to the direction of motion. For an aircraft wing, this means lift acts vertically upwards while the aircraft moves horizontally — and it is this upward force that keeps the aircraft airborne.

Picture a cross-section of an aircraft wing (an aerofoil). The top surface is curved and longer than the flat or less-curved bottom surface. The aerofoil shape causes the airflow to accelerate over the upper surface of the wing. Faster-moving air exerts lower pressure (this is the qualitative consequence of Bernoulli's principle). The slower air beneath the wing exerts higher pressure. This pressure difference — higher below, lower above — produces a net upward force: lift.

The angle at which the wing meets the oncoming air (the angle of attack) also matters. Tilting the wing so its leading edge points slightly upward deflects air downwards. By Newton's third law, the air pushes the wing upwards. Both the aerofoil shape and the angle of attack contribute to lift in real aircraft.

Lift is not exclusive to aircraft wings. A spinning ball (such as a football or tennis ball) experiences a lift-like force called the Magnus effect: the spin creates a pressure difference across the ball, curving its path through the air. Racing car wings (spoilers) are aerofoils mounted upside-down, deliberately generating downforce — lift directed downwards — to press the tyres harder onto the track and improve grip. These are excellent contexts for AQA 'suggest' questions, where you apply the same principle to a new scenario.

5. Free-Body Diagrams and Combining These Forces

In mechanics, a free-body diagram shows all the forces acting on a single object as arrows, with the length of each arrow representing the magnitude and the direction of the arrow representing the direction of the force. When analysing motion involving friction, lift, and drag, drawing a clear free-body diagram is the first step — and examiners award marks for it.

Worked example: Analysing Forces on a Cruising Aircraft

An aircraft cruises horizontally at constant speed. Identify all forces acting on it, state their directions, and explain what the constant speed tells you about the resultant force.

1. Weight (W) acts vertically downwards through the centre of mass. Thrust (T) acts horizontally forwards from the engines. Drag (D) acts horizontally backwards, opposing motion. Lift (L) acts vertically upwards, perpendicular to the velocity. — At A Level, you must name all four forces and give their directions precisely. 'Upwards' is not enough — 'vertically upwards, perpendicular to the direction of motion' is the exam-standard phrasing.
2. Constant speed (including constant vertical position) means zero vertical acceleration. Therefore: resultant vertical force = 0 So: L = W — Lift exactly balances weight. If lift exceeded weight, the aircraft would accelerate upwards; if weight exceeded lift, it would descend.
3. Constant horizontal speed means zero horizontal acceleration. Therefore: resultant horizontal force = 0 So: T = D — Thrust exactly balances drag. This is why aircraft engines must work continuously — they must overcome drag to maintain speed.
4. Resultant force = 0 in both directions. The aircraft is in equilibrium. — Constant velocity (in any direction) always means zero resultant force by Newton's first law. This is a key reasoning chain examiners look for.

Answer: The aircraft is in equilibrium: lift equals weight (vertical balance) and thrust equals drag (horizontal balance). The resultant force is zero, consistent with constant velocity.

The same reasoning applies to a car travelling at constant speed on a level road. The driving force from the engine equals the total resistive forces (friction at the tyres and air resistance). If the driver increases speed, drag increases (because drag grows with speed), so a greater driving force is needed to maintain the new constant speed. This is why fuel consumption rises sharply at high motorway speeds.

6. Exam Technique and Common Mistakes

Questions on friction, lift, and drag appear most often as 'describe', 'explain', or 'evaluate' questions. The command word tells you exactly what the examiner wants, and matching your response to it is how you collect marks efficiently.

For 'describe' questions about drag or terminal velocity, state what happens in sequence — speed increases, drag increases, resultant force decreases, acceleration decreases, speed approaches terminal velocity. Do not add 'because' clauses; save those for 'explain' questions. For 'explain' questions, every observation needs a scientific reason linked to a principle (Newton's laws, pressure differences, or the speed-dependence of drag).

A particularly common trap in extended-response questions is conflating drag with friction. Friction is a contact force between solid surfaces. Drag is a resistive force from a fluid. They share the feature of opposing motion, but their mechanisms and contexts are different. If a question asks about a car on a road, friction acts at the tyre-road interface; drag (air resistance) acts on the body of the car moving through air. Treating them as the same thing in an 'explain' answer will cost marks.

Across this lesson, the unifying theme is that friction, drag, and lift are all force interactions between objects and their surroundings — whether solid surfaces or fluids — and all are governed by Newton's laws. Mastering the qualitative reasoning for each force, and being precise about direction, mechanism, and the role of speed, gives you the tools to handle any AQA question on this topic.