Key term
OCR GCSE Physics A J249
Electricity
Topic overviewCharge, and producing static electricityTransfer of electrons, and electric fieldsCurrent as a rate of flow of chargeCharge, current and timeSeries and parallel circuits, and circuit symbolsCurrent, potential difference and resistanceResistors whose resistance changes, and investigating themLinear and non-linear circuit elements from graphsNet resistance in series and parallelCircuits for measurement, and electrical power
Energy
Topic overviewConservation of energy, and energy storesEnergy changes on heating, and calculating themEnergy of a moving body, a spring and a raised objectDissipation, and energy transfer in domestic devicesPower ratings, and calculating efficiencyReducing unwanted energy transfer, and the rate of cooling
Forces
Topic overviewMeasuring distance and time, and calculating speedConverting units, and ratesVectors and scalars in motionDistance-time and velocity-time graphsAverage speed, and the equations of uniform motionHow objects interact, and forces as vectorsNewton's first lawNewton's second lawForce and the time taken to change velocityWork done, and stored energyPower, and Newton's third lawStretching, bending and compressing; elastic and plasticForce and extension, linear and non-linearThe spring constant, and work done in stretchingGravitational fields, weight and free fallCalculations with weight and gravitational field strengthTurning forces and the moment of a forceLevers and gearsPressure in fluids, and the force on a surface
Global Challenges
Topic overviewTypical speeds and everyday accelerationsReaction time, stopping distance and the dangers of large decelerationsEstimating stopping distances and the forces in a collisionEnergy resources, and trends in their useThe national grid, and step-up and step-down transformersWhy the national grid is efficient, and power in a transformerMains electricity: a.c., d.c. and the domestic supplyLive, neutral and earth wires, and electrical dangerRed-shift and the Big BangThe formation of the Sun, and radiation from bodiesThe solar system
Magnetism and Magnetic Fields
Matter
Topic overviewHow the atomic model changed, and the structure of the atomThe size of atoms, and defining densityDensity of the states of matter, and density calculationsConservation of mass in changes of stateHeating a system, and specific heat capacityCalculations with specific heat capacity and latent heatGas molecules, temperature and pressureCompressing a gas, and changing its volumeAtmospheric pressure and why it varies with height
Radioactivity
Topic overviewProtons, neutrons and isotopesUnstable nuclei and the emissions they give outBalancing nuclear equationsElectron energy levels, and radiation from atomsHalf-life and the randomness of decayPenetration of alpha, beta and gammaContamination and irradiation, and half-life as a hazardUses of nuclear radiationNuclear fission and fusion
Waves in Matter
Topic overviewAmplitude, wavelength, frequency and periodThe wave equationTransverse and longitudinal wavesSound crossing a boundary; reflection, transmission and absorptionModelling waves with ripples and soundElectromagnetic waves as transverse, and energy transferFrequency and wavelength across the spectrumWhat our eyes detect, and light as an electromagnetic waveUses of electromagnetic wavesHazards of ultraviolet, X-rays and gamma raysRay diagrams for reflection and refractionHow colour arises
Forces
Force and the time taken to change velocity
- 2 things to know
- Paper 1
Overview
Acceleration as change in velocity over time, the resultant force needed to change the velocity of a car or a ball in a given time, how fast a go-kart moves after a force has acted, and why crumple zones and padded landing mats lower the force by lengthening the time taken to stop.
What you need to know
Answer each one before you reveal it.
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Key term
A car of mass 900 kg speeds up from 5 m/s to 20 m/s in 6 s. What resultant force acts on it?
Questions from this subtopic
Each one with the model answer underneath.
- 1
How is acceleration related to a change of velocity and the time it takes?
Model answer
Acceleration is the change in velocity divided by the time taken for the change.
- 2
A car of mass 900 kg speeds up from 5 m/s to 20 m/s in 6 s. What resultant force acts on it?
Model answer
The acceleration is 15 ÷ 6 = 2.5 m/s², so the force is 900 × 2.5 = 2250 N.
- 3
Why does taking longer to bring a moving object to rest reduce the force acting on it?
Model answer
The same change of velocity spread over a longer time gives a smaller acceleration, and force is mass × acceleration.
- 4
How does a crumple zone reduce the force on the people inside a car during a crash?
Model answer
It lengthens the time over which they slow down, which lowers the acceleration and so lowers the force.
- 5
A ball of mass 0.15 kg is brought to rest from 20 m/s in 0.05 s. What average force acts on it?
Model answer
The acceleration is 20 ÷ 0.05 = 400 m/s², so the force is 0.15 × 400 = 60 N.
- 6
For a fixed resultant force acting for a fixed time, what happens to the change in velocity if the mass is doubled?
Model answer
It is halved, because the acceleration is halved.
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