State the equation for the work done by a constant force acting along the line of motion.

W = Fx, where W is the work done, F is the force and x is the distance moved in the direction of the force.

How is the work done calculated when the force is not along the line of motion?

W = Fx cos θ, where θ is the angle between the force and the direction of motion.

Why does the cos θ term appear when a force acts at an angle to the motion?

Only the component of the force along the direction of motion does work, and that component is F cos θ.

How much work is done by a force acting at right angles to the direction of motion, and why?

None, because cos 90° = 0, so the force has no component along the direction of motion.

WJEC AS-Level Physics 1420QS

Basic Physics

Topic overview Units and quantities Scalars, vectors and density Moments, centre of gravity and equilibrium Prefixes, derived units and unit conversion Adding coplanar vectors and the triangle of forces Resolving forces on an inclined plane Taking moments: beams, ladders and stability

Kinematics

Topic overview Describing motion Equations of motion and free fall Projectile motion Non-uniform acceleration and curved motion graphs Projectiles launched at an angle Measuring speed, acceleration and g in the laboratory

Dynamics

Topic overview Newton's second law and momentum Force and Newton's laws Impulse and the force–time graph Collisions and explosions in one dimension Free body diagrams and connected bodies Newton's third law pairs and everyday forces

Energy Concepts

Topic overview Work and power Conservation of energy and efficiency The work–energy relationship Gravitational potential energy and energy interchange Elastic potential energy and the force–extension graph Power, velocity and the efficiency of machines

Solids Under Stress

Topic overview Stress, strain and the Young modulus Stress–strain graphs and the behaviour of materials Measuring the Young modulus, and material properties Hooke's law and elastic strain energy Crystalline, amorphous and polymeric solids Dislocations, ductile and brittle fracture

Using Radiation to Investigate Stars

Topic overview Stellar spectra and black bodies Wien's law, Stefan's law and stellar properties The inverse square law and stellar distance Finding the radius and temperature of a star Multiwavelength astronomy

Particles and Nuclear Structure

Topic overview Quarks, leptons and hadrons The four interactions and conservation laws The first generation of quarks and leptons Antiparticles and annihilation Baryons, antibaryons and mesons Deducing quark compositions from charge Beta decay and quark flavour change Testing a reaction against the conservation rules

Conduction of Electricity

Topic overview Current, charge and the ampere Drift velocity and the equation I = nAvq The coulomb and the charge on an electron Conductors, insulators and other charge carriers The free-electron model and the speed of conduction

Resistance

Topic overview Calculating resistance from resistivity Potential difference, Ohm's law and power Resistivity, temperature and superconductivity The volt: energy transferred per unit charge Current–voltage characteristics of non-ohmic components Measuring resistance and resistivity in the laboratory The heating effect of a current and choosing a power equation Thermistors and how a semiconductor's resistance falls with temperature Conductance, conductivity and comparing materials

D.C. Circuits

Topic overview Kirchhoff's laws, and resistors in series and parallel Potential dividers, emf and internal resistance Cells, resistivity, power and energy Working out currents and potential differences in resistor networks Sensor circuits with thermistors and LDRs Internal resistance, power transfer and short-circuit current Ammeters, voltmeters and their effect on the circuit

The Nature of Waves

Topic overview What a wave is, and the wave equation Describing a wave: peaks, compressions and phase Polarisation and plane-polarised waves Displacement–time and displacement–position graphs Wavefronts and rays Phase difference between two points on a wave

Wave Properties

Topic overview Energy transfer and the wave equation Transverse and longitudinal waves, path difference and intensity Diffraction and superposition Two-source interference and the diffraction grating Stationary waves How much a wave spreads at a gap or an obstacle Coherence and the conditions for observable interference Young's double slit and λ = ay/D Measuring wavelength with a diffraction grating Measuring the speed of sound with stationary waves

Refraction of Light

Topic overview What refraction is and why it happens Refractive index and Snell's law Total internal reflection and optical fibres The wave model of refraction and n₁v₁ = n₂v₂ Snell's law at a boundary between two media Optical fibres: core, cladding and pulse broadening

Photons

Topic overview Photon energy and the electronvolt The photoelectric effect Stopping potential and the Planck constant The photoelectric effect and photon energies Line spectra and energy levels Demonstrating the photoelectric effect The electromagnetic spectrum and typical photon energies Electron diffraction and the de Broglie relationship Radiation pressure

Lasers

Topic overview Stimulated emission and the properties of laser light Population inversion, pumping and the laser cavity Why a two-level system cannot give a population inversion Three-level and four-level laser systems The semiconductor laser and its uses

Mathematical Requirements (Appendix C)

Topic overview Significant figures and arithmetic means Uncertainty and how uncertainties combine Algebraic symbols and proportionality Rearranging and solving equations Small angle approximations Tangents and instantaneous rates of change Areas under curves and numerical methods Reading a tangent on a motion graph Straight-line graphs and y = mx + c What a gradient and an intercept represent Linearising a relationship to find a constant Log plots of exponential decay Choosing what to plot to find a constant Logarithmic scales and orders of magnitude Working with base-ten logarithms The shapes of common graphs Inverse and root relationships on a graph Areas, volumes and Pythagoras Geometry applied to physical situations Substituting into physical equations Trigonometry and right-angled triangles Resolving a force into components Radians, arc length and force diagrams Random processes and translating between representations Mean values and periodic functions Symbols for approximation and comparison

How Science Works (Appendix D)

Topic overview Apparatus and taking measurements Working safely with radioactive sources Planning an investigation Analysing and evaluating evidence Uncertainty, repeatability and drawing a conclusion

Energy Concepts

6 subtopics
18 things to know
AS Unit 1

What Energy Concepts covers

Energy Concepts is examined on AS Unit 1 of WJEC Physics, Motion, Energy and Matter. 85 recall questions across 6 subtopics.

Where it is assessed

AS Unit 1

Motion, Energy and Matter.

Check the WJEC specification ↗

The 6 subtopics

Each one has its own page. WJEC AS Physics students sit this topic in the first year.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

State the equations for the three forms of mechanical energy.

Key term

What is power?

Key term

What is the SI unit of work, and what is it equivalent to in base units?

Key term

Define each symbol in the relationship Fx = ½mv² − ½mu², giving its unit.

Key term

Describe the interchange of energy during one complete swing of a pendulum.

Key term

Why does the power needed to drive a car at a steady speed rise so sharply as the speed increases?

Key term

State the principle of conservation of energy.

Key term

Write down the relationship between the work done by the resultant force on a body and the change in the body's kinetic energy.

Key term

Why is only the change in gravitational potential energy ever calculated near the Earth's surface, and never an absolute value?

Key term

Give the expression for the energy stored in a spring obeying Hooke's law in terms of the stretching force and the extension.

Key term

Why is the energy stored in a spring ½Fx rather than Fx?

Key term

State the equation for the power developed by a force acting on a body moving at constant velocity, defining each symbol with its unit.

Key term

Define the work done by a constant force acting along the line of motion.

Key term

State the equation for the gravitational potential energy gained by a mass raised through a height.

Key term

What does it mean for the work done on a body to be negative?

Numbers

Calculate the gravitational potential energy gained by a 65 kg climber ascending 320 m, taking g as 9.81 N kg⁻¹.

Numbers

Calculate the energy stored in a spring of spring constant 240 N m⁻¹ stretched by 8.0 cm.

Key term

Why does the useful output power of a vehicle at a steady speed equal the total resistive force multiplied by that speed?

Take Energy Concepts further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

Work vs Power
Work: the product of a force and the distance moved along the line of that force. Power: the rate at which energy is transferred.
Gravitational potential energy vs Elastic potential energy
Gravitational potential energy: the energy stored by raising a mass through a height. Elastic potential energy: the energy stored by stretching or compressing a spring.

Questions from this topic

Each one with the model answer underneath.

1
Define the work done by a constant force acting along the line of motion.
Model answer
Work done is the product of the force and the distance moved in the direction of the force.
2
State the equation for the work done by a constant force acting along the line of motion.
Model answer
W = Fx, where W is the work done, F is the force and x is the distance moved in the direction of the force.
3
What is the SI unit of work, and what is it equivalent to in base units?
Model answer
The joule (J), equivalent to a newton metre, or kg m² s⁻² in base SI units.
4
How is the work done calculated when the force is not along the line of motion?
Model answer
W = Fx cos θ, where θ is the angle between the force and the direction of motion.
5
Why does the cos θ term appear when a force acts at an angle to the motion?
Model answer
Only the component of the force along the direction of motion does work, and that component is F cos θ.
6
How much work is done by a force acting at right angles to the direction of motion, and why?
Model answer
None, because cos 90° = 0, so the force has no component along the direction of motion.

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Energy Concepts revision questions

Is Energy Concepts examined at AS in WJEC Physics?: Yes. It sits in AS Unit 1, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.
What does Energy Concepts cover in WJEC AS-Level Physics?: 6 subtopics: work and power, conservation of energy and efficiency, the work–energy relationship, gravitational potential energy and energy interchange, elastic potential energy and the force–extension graph and power, velocity and the efficiency of machines. Each one has its own page with questions and model answers.
How is Energy Concepts organised?: By the WJEC 1420QS specification, subtopic by subtopic, so it matches the order you are taught it in.

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