Define the moment of a force about a point.

The moment of a force about a point is the product of the force and the perpendicular distance from that point to the line of action of the force.

What must be stated alongside the size of a moment, and why?

The sense of the moment must be stated as clockwise or anticlockwise, because two moments of the same size in opposite senses cancel rather than add.

Name the two factors that determine the turning effect of a force about a point.

The turning effect depends on the magnitude of the force and on the perpendicular distance from the point to the line of action of that force.

Why is the perpendicular distance used in calculating a moment rather than the distance from the pivot to the point where the force is applied?

Only the component of the force at right angles to the line joining the pivot to the point of application produces turning, and using the perpendicular distance from the pivot to the line of action gives exactly that turning effect.

CCEA A-Level Physics 1210

Physical Quantities, Scalars and Vectors

Topic overview Physical quantities and SI base units Prefixes and standard form Scalars and vectors Resolving vectors and finding resultants Coplanar forces in equilibrium

Moments and Equilibrium

Topic overview Moment of a force Centre of gravity The principle of moments Determining unknown masses using moments

Linear Motion and Dynamics

Topic overview Displacement, velocity and acceleration The equations of motion for uniform acceleration Measuring the acceleration of free fall with light gates Displacement–time and velocity–time graphs Projectile motion Projectile calculations

Newton's Laws of Motion

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Linear Momentum and Impulse

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Work, Energy and Power

Topic overview Work done and energy transfer Calculating work done, including forces at an angle Gravitational potential energy Kinetic energy Conservation of energy and energy exchanges Work done by a resultant force and change in kinetic energy Power, efficiency and energy use in society

Current, Charge and Potential Difference

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Resistance and Resistivity

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Internal Resistance and Potential Dividers

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Waves

Topic overview Transverse and longitudinal waves Amplitude, period, frequency and wavelength Phase and phase difference Reading wave graphs The wave equation Polarisation The electromagnetic spectrum

Refraction

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Lenses

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Superposition, Interference and Diffraction

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Quantum Physics

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Wave-Particle Duality

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Astronomy

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Practical Skills and Data Analysis

Topic overview Choosing and using measuring instruments Precision, accuracy and recording measurements Using the cathode ray oscilloscope to measure voltage and frequency Tables, graphs and presenting results Linearising a relationship and using the gradient Absolute and percentage uncertainty Combining uncertainties and uncertainty on a graph Random and systematic error, reliability and refining an experiment

Deformation of Solids

Topic overview Hooke's law and the spring constant Elastic and plastic deformation and the elastic limit Stress, strain and the Young modulus Measuring the Young modulus of a wire Strain energy and force–extension graphs Choosing materials: strength, stiffness and cost

Thermal Physics

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Uniform Circular Motion

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Simple Harmonic Motion

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The Nucleus and Radioactive Decay

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Nuclear Energy, Fission and Fusion

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Gravitational Fields

Topic overview Fields of force and field lines Gravitational field strength Newton's law of universal gravitation The field strength of a point mass Circular motion of planets and satellites Kepler's third law Geostationary satellites

Electric Fields

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Capacitors

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Magnetic Fields

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Charged Particles and Accelerators

Topic overview Charged particles in a uniform electric field Charged particles in a magnetic field Circular paths in a magnetic field Comparing electric and magnetic deflection The synchrotron Relativistic mass increase Antimatter and annihilation

Fundamental Particles

Topic overview Fundamental particles The four fundamental forces Classifying particles Quarks and the structure of hadrons Conservation laws Beta decay in the quark model

Moments and Equilibrium

Moment of a force

3 things to know
Unit AS 1

Overview

Recall questions on the definition of a moment and its equation, why the unit is the newton metre and not the joule, why the perpendicular distance is used, the moment of a force through the pivot, the spanner and door handle examples, and calculations with a force at an angle to the lever.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Give the unit of a moment and explain why it is not written as a joule even though it is also a newton metre.

Key term

State the equation for the moment of a force, defining each symbol and its unit.

Key term

State the moment of a force whose line of action passes through the pivot.

Questions from this subtopic

Each one with the model answer underneath.

1
Define the moment of a force about a point.
Model answer
The moment of a force about a point is the product of the force and the perpendicular distance from that point to the line of action of the force.
2
State the equation for the moment of a force, defining each symbol and its unit.
Model answer
The moment is M = Fd, where M is the moment in newton metres (N m), F is the force in newtons (N) and d is the perpendicular distance in metres (m) from the point to the line of action of F.
3
Give the unit of a moment and explain why it is not written as a joule even though it is also a newton metre.
Model answer
The unit is the newton metre (N m); the joule is reserved for energy, which is a force multiplied by a distance moved in the direction of the force, whereas a moment uses a distance measured at right angles to the force.
4
What must be stated alongside the size of a moment, and why?
Model answer
The sense of the moment must be stated as clockwise or anticlockwise, because two moments of the same size in opposite senses cancel rather than add.
5
Name the two factors that determine the turning effect of a force about a point.
Model answer
The turning effect depends on the magnitude of the force and on the perpendicular distance from the point to the line of action of that force.
6
Why is the perpendicular distance used in calculating a moment rather than the distance from the pivot to the point where the force is applied?
Model answer
Only the component of the force at right angles to the line joining the pivot to the point of application produces turning, and using the perpendicular distance from the pivot to the line of action gives exactly that turning effect.

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