State the principle of moments.

For rotational forces in equilibrium, the sum of the clockwise moments about a pivot equals the sum of the anti-clockwise moments.

A force of 30 N acts 0.6 m from a pivot and turns it clockwise. Calculate the anti-clockwise force needed 0.9 m from the same pivot to balance it.

The clockwise moment is 30 × 0.6 = 18 N m, so the force = 18 ÷ 0.9 = 20 N.

A child of weight 400 N sits 1.5 m from the pivot of a seesaw. Calculate how far from the pivot a child of weight 500 N must sit on the other side to balance it.

The moment to balance is 400 × 1.5 = 600 N m, so the distance = 600 ÷ 500 = 1.2 m.

A beam has a 40 N force acting 0.3 m to one side of its pivot and a 20 N force acting 0.6 m to the other side. State whether the beam balances and justify your answer.

It balances, because both moments are 12 N m.

Explain what happens to a balanced beam if one of the two forces on it is moved closer to the pivot.

That moment becomes smaller, so the beam rotates in the direction of the larger moment.

Edexcel GCSE Physics 1PH0

Key Concepts of Physics

Topic overview SI units in physics Multiples and sub-multiples of units Converting units, significant figures and standard form

Motion and Forces

Topic overview Scalar and vector quantities Examples of vectors and scalars, and velocity Speed, distance, time and distance-time graphs Acceleration and the equation v squared minus u squared Velocity-time graphs and free fall Measuring speed in the laboratory, and typical everyday speeds Newton's first law, and investigating force, mass and acceleration Newton's second and third laws Weight, mass and gravitational field strength Circular motion and centripetal force Inertial mass and momentum Collisions, and force as the rate of change of momentum Reaction time, stopping distance and large decelerations Factors affecting stopping distance and reaction time Braking distance, speed and work done

Conservation of Energy

Topic overview Gravitational potential and kinetic energy Energy transfer diagrams and conservation of energy Energy stores and closed systems Dissipation and wasteful energy transfer Reducing unwanted energy transfer Efficiency and how to increase it Energy resources and trends in their use

Waves

Topic overview What waves transfer, and the evidence for it Frequency, wavelength, amplitude and period Longitudinal and transverse waves The wave equation Measuring wave speed, and what happens at a boundary Measuring waves, and calculating depth from wave velocity Reflection, refraction, transmission and absorption at a boundary Sound waves, ultrasound and infrasound Uses of ultrasound and infrasound

Light and the Electromagnetic Spectrum

Topic overview Reflection, refraction and total internal reflection Why objects appear coloured Lenses, ray diagrams and images Properties of electromagnetic waves Investigating refraction in glass blocks The groups of the electromagnetic spectrum The continuous spectrum, and what our eyes can detect Absorption, transmission and the velocity of electromagnetic waves Emitting and absorbing thermal radiation Constant temperature and the temperature of the Earth How radiation is generated, and why danger rises with frequency Harmful effects of excessive exposure to radiation Uses of electromagnetic radiation

Radioactivity

Topic overview The atom and the size of atoms Isotopes, atomic number and mass number Relative masses and charges of subatomic particles Electron orbits and energy levels Forming ions, and how the atomic model changed Types of ionising radiation Background radiation and detecting radioactivity Alpha, beta and gamma, and their penetrating power Beta-minus and beta-plus decay Effects of decay on atomic number and mass number Balancing nuclear equations Activity and the becquerel Half-life and the randomness of decay Half-life calculations Uses of radioactivity, and how danger depends on half-life The dangers of ionising radiation Safety precautions, contamination and irradiation Radiotherapy, medical tracers and PET scanners Nuclear power and where nuclear energy comes from Fission of uranium-235 and the chain reaction Controlling a reactor and generating electricity Nuclear fusion and how it differs from fission Why fusion needs extreme conditions

Astronomy

Topic overview Weight and gravitational field strength on other bodies The Solar System and the order of the planets How ideas about the Solar System and ways of observing it have changed Orbits of moons, planets, comets and satellites Gravity, orbital speed and stable orbits The Steady State and Big Bang theories Evidence for an expanding Universe The Doppler effect and red-shift in galaxies The life cycle of a star like the Sun The life cycle of stars more massive than the Sun

Energy – Forces Doing Work

Topic overview Energy stores and energy transfer diagrams Changing a system's energy, and measuring work done Work done and the energy changes it causes Gravitational potential and kinetic energy calculations Dissipation and wasted energy Power as the rate of energy transfer The watt, and calculating efficiency

Forces and Their Effects

Topic overview Contact and non-contact forces Vectors and scalars, and reducing friction Resolving forces and free body diagrams Forces on an object in equilibrium Turning forces and the moment of a force The principle of moments, levers and gears

Electricity and Circuits

Static Electricity

Topic overview Charging by friction, and attraction and repulsion Electrostatic phenomena and earthing Uses and dangers of static electricity Electric fields

Magnetism and the Motor Effect

Topic overview Magnetic poles, permanent and induced magnets Uses of magnetic materials The magnetic field around a bar magnet Plotting compasses and the Earth's magnetic field The magnetic effect of an electric current Solenoids and electromagnets The motor effect and Fleming's left-hand rule The force on a conductor, and how a motor turns

Electromagnetic Induction

Topic overview Inducing a current by moving a magnet Factors affecting an induced potential difference Alternators and microphones How a transformer works The transformer turns-ratio equation The national grid, and step-up and step-down transformers Transformer power calculations Why high-voltage transmission reduces energy loss

Particle Model

Topic overview Kinetic theory and differences in density Calculating density, and investigating it Changes of state and internal energy Specific heat capacity Specific latent heat and thermal insulation Investigating the specific heat capacity of water Gas pressure and the motion of particles Absolute zero and the kelvin scale Compressing gases, and the force a gas exerts Pressure and volume of a fixed mass of gas

Forces and Matter

Topic overview Elastic and inelastic distortion Spring constant, and linear and non-linear behaviour Work done in stretching a spring Atmospheric pressure and pressure in a fluid Force, area and pressure Calculating pressure, and how pressure varies with depth Pressure in liquids: density and depth Upthrust, floating and sinking

Forces and Their Effects

The principle of moments, levers and gears

3 things to know
Paper 2

Overview

The principle of moments with seesaw and beam calculations, how levers make tasks easier and their effort, load and pivot, and how meshed gears transmit a turning effect, including a small gear driving a larger one and low gears on hills.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

How do moments explain balancing a seesaw and the way a lever makes a task easier, and what are the three parts of a lever?

Key term

A force of 12 N acts at right angles 0.25 m from a pivot. State the moment that must act the other way for equilibrium.

Key term

Explain how a heavy person can be balanced by a lighter person on a seesaw.

Questions from this subtopic

Each one with the model answer underneath.

1
State the principle of moments.
Model answer
For rotational forces in equilibrium, the sum of the clockwise moments about a pivot equals the sum of the anti-clockwise moments.
2
A force of 30 N acts 0.6 m from a pivot and turns it clockwise. Calculate the anti-clockwise force needed 0.9 m from the same pivot to balance it.
Model answer
The clockwise moment is 30 × 0.6 = 18 N m, so the force = 18 ÷ 0.9 = 20 N.
3
A child of weight 400 N sits 1.5 m from the pivot of a seesaw. Calculate how far from the pivot a child of weight 500 N must sit on the other side to balance it.
Model answer
The moment to balance is 400 × 1.5 = 600 N m, so the distance = 600 ÷ 500 = 1.2 m.
4
A force of 12 N acts at right angles 0.25 m from a pivot. State the moment that must act the other way for equilibrium.
Model answer
3 N m in the opposite sense.
5
A beam has a 40 N force acting 0.3 m to one side of its pivot and a 20 N force acting 0.6 m to the other side. State whether the beam balances and justify your answer.
Model answer
It balances, because both moments are 12 N m.
6
Explain what happens to a balanced beam if one of the two forces on it is moved closer to the pivot.
Model answer
That moment becomes smaller, so the beam rotates in the direction of the larger moment.

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