OCR Physics Module P5 SPACE FOR REFLECTION
P5a Satellites, gravity and circular motion
Satellitea satellite is an object that orbits a larger object in space
Orbitgravitational force keeps a satellite in orbit
Gravityuniversal force of attraction between masses; decreases as the masses get further apart
Circular motioncircular motion requires a centripetal force and that gravity provides the centripetal force for orbital motion
Circular motion – accelerationartificial satellites are continually accelerating towards the Earth due to the Earth’s gravitational pull, but their
tangential [“straight line”] motion keeps them moving in an approximately circular orbit
Orbital periodtime taken for a satellite to make one complete orbit
Orbit of Moon/Earththe Moon remains in orbit around the Earth and the Earth in orbit around the Sun due to gravitational forces
between them
Orbit of planetsthe orbit period of a planet depends upon its distance from the sun; orbit is not circular but is a slight ellipse
Orbit of cometsthe variation in speed of a periodic comet during its orbit around the sun is caused by its highly elliptical orbit
Gravityuniversal force of attraction between masses; decreases as the masses get further apart
Orbit heightthe orbit of an artificial satellite depends on its height above the Earth’s surface
Satellite orbitorbital period of an artificial satellite increases with height above the Earth’s surface; satellites in lower orbits
travel faster because the gravitational force is stronger
Satellite useheight of orbit of an artificial satellite determines its use
Geostationary orbiting satellitehigh orbit; slower orbit speed; orbits the Earth once in 24 hours around the equator; orbits over ‘fixed point’ on
Earth’s surface (Communications; Weather forecasting; GPS)
Polar orbiting satellitelow orbit; faster orbit speed; orbits the Earth in a few hours over the Poles; covers more area as it orbits the
rotating Earth; (Imaging the Earth’s surface; Military uses [‘Spy satellites])
P5b Vectors and equations of motion
Scalar quantitydirection is not important – e.g. speed, mass
Vector quantitydirection is important – e.g. velocity, force
Parallel vectorscalculate the resultant vector by adding the individual components together
Vectors at right anglescalculate the resultant vector by right angle triangle rule [Pythagoras] – H2 = (A2 +O2)
Speedscalar quantity; measures how fast an object is moving; s=d/t
Velocityvector quantity; speed + distance
Equations of motionsuvat; s= distance (!); u = start speed; v = final speed; a = acceleration: t = time
Use equationsv = u + at; v2 = u2 +2as; s = ut + ½ at2
P5c Projectile motion
Projectileswhen fired in the air; missiles, cannon balls, golf balls, netballs, darts and long-jumpers
Trajectorypath of a projectile; path of an object projected horizontally in the Earth’s gravitational field is curved – parabolic
Object moving horizontallyhas two components of velocity – horizontal and vertical (ignore air resistance)
Horizontal projectionan object projected horizontally in the Earth’s gravitational field, (ignore air resistance): has a constant horizontal
velocity; is accelerating towards the ground so has a steadily increasing vertical velocity
Equations (P5b)use suvat equations for calculations for an object projected horizontally above the Earth’s surface where the
gravitational field is still uniform
Vectorsthe horizontal and vertical velocities of a projectile are vectors
Resultant velocitythe resultant velocity of a projectile is the vector sum of the horizontal and vertical velocities
Forcesignoring air resistance, the only force acting on a ball during the flight is gravity
Downward accelerationprojectiles have a downward acceleration and that this only affects the vertical velocity
Horizontal accelerationfor a projectile there is no acceleration in the horizontal direction (ignore air resistance)
P5d Momentum
Momentumthe greater the mass of an object and/or the greater velocity, the more momentum the object has in that
direction
Momentum – equationmomentum = mass x velocity
Action/reactionevery action has an equal and opposite reaction
Collisionsball struck by an object in sport (e.g. cricket ball and bat) is an example of a collision
Collisions – forceswhen an object collides with another object, the two objects exert an equal and opposite force on each other
Force – equationForce = change in momentum ÷ time
Acceleration and injuriesinjuries in vehicle collision and many sporting injuries are due to a very rapid acceleration of parts of the body
Acceleration and forcea rapid acceleration causes a rapid change in momentum and so a large force is exerted
Safety features in carscrumple zones; seatbelts; airbags; work by increasing the time for change in momentum
Changing momentumspreading the change in momentum over a longer time: reduces the forces required to act; reduces the injury
Conservation of momentummomentum is a property that is always conserved; total momentum is the same before and after the event
Eventssuch as collisions; explosions; recoil; rocket propulsion
Momentum of collisionstotal momentum the same before and after a collision two objects moving in the same direction (including
calculations of mass, speed or momentum)
P5e Satellite Communication
EM spectrum – communicationem waves used to transmit information – microwaves; radio waves
Microwavesinformation transmitted using microwaves to orbiting artificial satellites and then retransmitted back to Earth;
microwaves are sent as a thin beam because they only diffract by a small amount due to their short wavelength
Earth’s atmospherestops some radio frequencies; allows others to pass through; reflects others
Effect of ionosphereradio frequencies below 30MHz are reflected by the ionosphere
Effect of particlesabove 30GHz, rain, dust and other atmospheric effects reduce the strength of the signal due to absorption and
scattering
Radio wavesradio waves have a very long wavelength
Radio/TV receptionaerial for radio/terrestrial TV; ‘dish’ for satellite TV
Diffractionwaves can ‘spread out’ as they pass an object or pass through a gap; amount of diffraction depends upon the
size of the gap and the wavelength of the wave
Diffraction – maximummaximum diffraction occurs when the wavelength equals the size of the gap
Long wave radio waveslong wave radio waves have a very long range because they diffract around hills and over the horizon
FM radioshorter range; only to ‘horizon’
AMmedium wave and long wave radio waves are AM (amplitude modulation); signal transmitted on a carrier wave
that has the signal superimposed on it; information transmitted as variation in amplitude of the wave
FMhigher frequency waves; higher ‘quality’ signal but shorter range; information transmitted as variation in the
frequency of the wave
P5f Nature of waves
Interference of wavesan effect resulting from two waves that overlap
Constructive interferenceareas where the waves add together; patterns of reinforcement
Destructive interferenceareas where the waves subtract from each other; patterns of cancellation
Interference – resultslouder and quieter areas in sound; bright and dark areas in light
Constructive interferencenumber of half wavelengths in thepath difference for two waves from the same source is an even number
Destructive interferencenumber of half wavelengths in the path difference for two waves from the same source is an odd number
Light – as wavesdiffraction of light and its associated interference patterns are evidence for the wave nature of light
Light – diffraction patternsstripes of light and dark; explained by interference
Polarisationelectromagnetic waves are transverse waves and so can be plane polarised – vibrations in one plane
Polaroidblock ‘glare’ from water etc by polarising light being reflected from the surface into one plane
P5g Refraction of waves
Mediuma substance that light passes through
Refractionchange in direction of a wave due to the wave passing from one medium into another
Normalreference line at 90˚ to the point the wave enters the medium; angles measured from the normal line
Change in wave speedrefraction occurs at the boundary between two mediums due to a change in the wave speed
Change in wavelengthchange in speed causes a change in wavelength and may cause a change in direction
Change in directionas a wave enters a denser medium, the wave speed decreases and the wave bends towards the normal
Refractive indexrefractive index is limited to the amount of bending after a boundary; more dense medium has higher refractive
index
Refractive index – equationrefractive index = speed of light in vacuum ÷ speed of light in medium
Snell’s lawrefractive index (n) = sin i ÷ sin r; i = angle of incidence; r = angle of refraction
Reflectionsome, or all, of a light ray can be reflected when travelling from glass, or water, to air
Air to glassa ray of light travelling from air into glass the angle of incidence is usually greater than the angle of refraction
Dispersionwhen white light is refracted into the spectrum colours; blue light is deviated more than red light
Refraction and critical anglerefraction occurs when a ray of light hits a glass/air boundary at an angle less than the critical angle
Total internal reflection (TIR)TIR happens when a ray of light hits a glass/air boundary at an angle greater than the critical angle
TIR and refractive indexTIR can only occur when a ray of light travels from a medium with a higher refractive index into a medium with a
lower refractive index and the angle of incidence is greater than the critical angle
Media and critical angledifferent media have different critical angles because of having different refractive index; the higher the refractive
index of a medium the lower is its critical angle
Critical angle – equationSin c = nr ÷ ni; c = critical angle; ni = refractive index of ‘incident’ medium; nr = refractive index of ‘refraction’
medium
P5h Optics
Convex lens() shape; converging lens; converge parallel rays of light onto a single point (focal point/focus)
Focal lengthof a convex lens as being measured from the centre of the lens to focal point (focus)
‘Fat’ lenseshave short focal lengths
Convex lenses usesmagnifying glass; cameras; projectors
Real imageprojectors and cameras produce real images on a screen/film; image can be projected; image inverted (upside
down)
Ray diagramsfind the position and size of the real image formed by a convex lens by drawing suitable ray diagrams
Focussing – camera images produced by cameras are focussed by moving the lens closer to /further from the film/sensors
Focussing – projectorimages produced by cameras and projectors are focussed by moving the lens closer/further to the object
Virtual imagescannot be projected onto a screen but are the right way up
Magnification formulamagnification = image size ÷ object size