A-Level Physics: Moments, Equilibrium and Motion with Drag

Many A level mechanics questions are not about how things accelerate but about why they stay still, or why they stop accelerating. This material brings together the parts of year-12 mechanics that deal with balanced forces and turning effects, and with motion through air and liquids, where drag grows with speed until a terminal velocity is reached.

The quiz (12 questions) begins with moments. You take moments about a support to find the force on a plank carrying a person, balance a uniform beam whose own weight acts beyond the pivot, and calculate the torque of a couple applied to a steering wheel. A toppling question asks at what tilt angle a tall block turns over, using the vertical line through its centre of mass. You then resolve forces: two cables at different angles supporting a sign, a block sliding down a rough slope at constant velocity, and a box pulled up a slope against friction.

The second half moves to drag. You decide what happens when a skydiver at terminal velocity opens a parachute, find the drag on a ball bearing in oil when upthrust is included, and calculate the acceleration of a sphere at a quarter of its terminal speed when drag is proportional to speed. Two energy questions close the set: the top speed of a car when the resistive force grows with the square of speed and power = force × velocity, and the efficiency of a motor lifting a load.

The flashcards (12 cards) cover the definitions and rules you need to recall: moment of a force, the principle of moments, couples and their torque, the conditions for equilibrium, centre of mass and stability, what drag depends on, terminal velocity, power = Fv, efficiency and the components of weight on a slope.

The written work (8 questions) asks for longer, structured answers: support forces on a bridge as a vehicle crosses, why a couple alone cannot produce equilibrium, why a lorry on a sideways slope may topple, the tension in a picture string, the full motion of a skydiver with a velocity–time graph described in words, the power and energy used by a cyclist, finding the centre of mass of a card with a plumb line, and measuring the terminal velocity of a ball bearing in oil.

The topics correspond to the mechanics content common to A level Physics specifications in England (for example, AQA section 3.4 on moments, equilibrium, terminal speed, power and efficiency). All questions are original practice written by Zestly.

• Apply the principle of moments to beams and planks on supports and pivots
• Calculate the torque of a couple and state the two conditions for equilibrium
• Use the centre of mass to predict toppling
• Resolve forces for objects on slopes and bodies held by cables
• Explain terminal velocity with drag and upthrust, and calculate power = Fv and efficiency

Practice material written by Zestly, based on the DfE GCE AS and A level subject content for physics (DFE-00356-2014), mechanics: moments, equilibrium, motion with drag, work, energy and power.