CIE A Level Physics
CIE A Level Physics Forces, Density and Pressure Exam Questions
Free CIE A Level Physics exam questions for Forces, Density and Pressure, with a mark scheme for every pack. Covers types of force, moments, couples, equilibrium, centre of gravity, density, pressure in fluids and upthrust.
Forces, Density and Pressure exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About CIE A Level Physics Forces, Density and Pressure
Forces, density and pressure is Topic 4 of the Cambridge International AS & A Level Physics (9702) syllabus. It covers turning effects of forces, the conditions for equilibrium, density, and pressure in liquids and gases including the origin of upthrust. It builds on vectors from Topic 1 and Newton's laws from Dynamics. Examiners often set moments problems on beams and bridges, ask you to draw a closed vector triangle for three forces in equilibrium, and test the derivation and use of Δp = ρgΔh.
What you need to know
- Turning effects The moment of a force is force × perpendicular distance from the pivot to the line of action. A couple is a pair of equal, opposite forces whose lines of action do not coincide; torque of a couple = one force × perpendicular distance between them.
- Equilibrium For a body in equilibrium the resultant force is zero and the resultant moment about any point is zero (principle of moments). Three coplanar forces in equilibrium form a closed triangle.
- Centre of gravity The weight of a body can be treated as acting at a single point called the centre of gravity.
- Density and pressure Density ρ = m/V. Pressure p = F/A, measured in pascals (N m^-2).
- Fluid pressure The pressure difference in a fluid is Δp = ρgΔh, derived from the weight of a column of fluid acting on its base area.
- Upthrust Upthrust arises because pressure on the lower surface of a submerged object is greater than on the upper surface. Archimedes' principle: upthrust F = ρgV, equal to the weight of fluid displaced.
Common mistakes in Forces, Density and Pressure
- Using the distance along the beam instead of the perpendicular distance when the force is at an angle. Resolve the force or find the perpendicular distance.
- Forgetting the weight of the beam itself, acting at its centre of gravity.
- Using the density of the object in F = ρgV for upthrust. Use the density of the fluid that is displaced.
- Giving Δp = ρgΔh as the total pressure at depth. It is the extra pressure due to the liquid; add atmospheric pressure for the total.
- Stating that a couple produces a resultant force. A couple produces only a turning effect, with zero resultant force.
Try three Forces, Density and Pressure exam-style questions
1. A uniform beam of length 2.0 m and weight 50 N is hinged at one end and held horizontal by a vertical string at the other end. A load of 30 N hangs 0.50 m from the hinge. Calculate the tension in the string. (3 marks)
Show the mark scheme answer
Taking moments about the hinge: T × 2.0 = 50 × 1.0 + 30 × 0.50; T × 2.0 = 65; T = 32.5 N.
2. Calculate the pressure due to the water at a depth of 12 m. Density of water = 1000 kg m^-3. (2 marks)
Show the mark scheme answer
Δp = ρgΔh = 1000 × 9.81 × 12; Δp = 1.18 × 10^5 Pa.
3. Explain how upthrust on a submerged object arises. (2 marks)
Show the mark scheme answer
Pressure in a fluid increases with depth; so the pressure and force on the lower surface are greater than on the upper surface, giving a resultant upward force.
Forces, Density and Pressure FAQs
What does Cambridge AS & A Level Physics forces, density and pressure cover?
It covers moments, couples and torque, conditions for equilibrium, centre of gravity, density, pressure, Δp = ρgΔh and upthrust.
Which papers is this topic examined in?
It is an AS Level topic assessed in Paper 1 and Paper 2, and moments and density measurements often feature in practical work.
What equations do I need for forces, density and pressure?
Moment = Fd, torque of a couple = Fd, ρ = m/V, p = F/A, Δp = ρgΔh and upthrust F = ρgV.
