Pressure, Moments and Levers
Moments, levers and pressure cover the turning effect of a force, moment = force x perpendicular distance from the pivot, the principle of moments for objects in equilibrium, and how levers and gears use moments to multiply force. Pressure in fluids covers pressure = force / area, how pressure in a liquid increases with depth and density, and upthrust, the upward force that lets objects float.
Before you start
Make sure you're comfortable with these topics first:
Method
- For a moments question, identify the pivot, list every force and its perpendicular distance from the pivot, and sort each force's turning effect as clockwise or anticlockwise.
- Apply the principle of moments (total clockwise moment about the pivot = total anticlockwise moment) and rearrange to find the unknown force or distance, keeping all distances in the same unit, usually metres, before substituting.
- For a pressure question, decide whether the situation is a solid pressing on a surface (use pressure = force / area) or a column of liquid (use pressure = height x density x gravitational field strength), and check which unit the answer is required in.
- When an object rests on two supports, remember it is also in vertical equilibrium: the upward support forces must add up to the total downward weight, which can be used as a second equation or a check.
- For an 'explain why' pressure or stability question, refer explicitly to the relationship between force and area, or between depth and density, or to the position of the centre of mass relative to the base, rather than giving a vague answer.
- State the correct unit at every stage of working (newton-metres for moments, pascals or N/m^2 for pressure) and check the final answer is a sensible size for the context.
Worked example
A see-saw is balanced on a central pivot. A child of weight 300 N sits 1.5 m from the pivot on the left-hand side. Calculate the distance from the pivot on the right-hand side at which a second child of weight 250 N must sit for the see-saw to balance.
- Write down the principle of moments: clockwise moment = anticlockwise moment.
- Calculate the moment of the first child (anticlockwise): moment = 300 x 1.5 = 450 Nm.
- Set this equal to the moment of the second child (clockwise): 450 = 250 x d.
- Rearrange and calculate: d = 450 / 250 = 1.8.
- Final answer: the second child must sit 1.8 m from the pivot.
Practice questions
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Q1Write down the equation that links moment, force and distance.Show answer
Answer: Moment = force x perpendicular distance from the pivot.
Q2A force of 40 N is applied 0.25 m from a pivot. Calculate the moment.Show answer
Answer: 10 Nm (40 x 0.25 = 10).
Q3State the equation that links pressure, force and area.Show answer
Answer: Pressure = force / area.
Q4A box of weight 120 N has a base area of 0.40 m^2. Calculate the pressure it exerts on the ground.Show answer
Answer: 300 Pa (120 / 0.40 = 300).
Q5Explain why a rucksack with wide, padded straps is more comfortable to carry than one with thin straps, for the same load.Show answer
Answer: The wide straps spread the same force (the weight of the rucksack) over a larger area, which reduces the pressure exerted on the shoulders, since pressure = force / area, and a larger area gives a smaller pressure for the same force.
Q6State the name of the upward force acting on an object placed in a fluid.Show answer
Answer: Upthrust.
Q7A spanner is used to loosen a bolt. Explain why using a longer spanner makes it easier to loosen the bolt.Show answer
Answer: A longer spanner increases the perpendicular distance from the pivot (the bolt) at which the force is applied; since moment = force x distance, a larger distance produces a larger moment for the same applied force, so less force is needed to produce the moment required to loosen the bolt.
Q8State two factors, other than depth, that affect the pressure at a given point in a liquid.Show answer
Answer: The density of the liquid and the gravitational field strength.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A uniform plank of length 4.0 m rests horizontally on two supports, one at each end, A and B. A crate of weight 600 N is placed 1.0 m from support A. Ignoring the weight of the plank, calculate the upward force provided by support B.
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A drawing pin has a flat head of area 50 mm^2 and a sharp point of area 0.02 mm^2. A student pushes the drawing pin into a piece of wood with a force of 20 N. Calculate the pressure exerted (a) by the flat head, and (b) by the sharp point, giving both answers in N/mm^2.
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A cargo ship floats when empty, but a smaller solid steel block of the same mass sinks in water. Using ideas about upthrust, explain why the ship floats but the solid steel block sinks, even though steel is denser than water.
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See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the pressure, moments and levers worksheet pack - 21 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 15 of GCSE Physics Workbook, the whole course as one free printable PDF.
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