Physics P5

Uses of satellites:

• monitoring weather and climate, 
• communications,
•  space research, 
• spying, 
• navigation

Communication satellites stay over the same point on earth

• stay above same point and orbit with the earth
• geostationary satellites
• ideal for telephone and tv satellites as they are always in the same place to transfer signals

weather and spying satellites need to be low in orbit

• geostationary satellites are too high and too stationary to take good photos
• for this you need low polar orbits
• the satellite sweeps over the poles while the earth rotates beneath it
• the time taken for a full orbit is just a few hours

GPS satellites and space telescopes are in other stable orbits

• GPS (global positioning system) satellites and the hubble space telescope

Gravity attracts all masses

• it makes everything accelerate towards the ground with the same acceleration
• it gives everything a weight
• it keeps planets, moons and other satellites in their orbits

provides centripetal force that cause orbits

• if an object is travelling in a circle it is constantly changing direction
• the orbit is a balance between the forward motion of the object and the force pulling it inwards (centripetal force)
• the planets move around the sun in almost circular orbits, the centripetal forces that make this happen are provided by the gravity between each planet and the sun

Gravity decreases as you get furthur away:

• with very large masses like stars and planets gravity is very big
• the closer you get to a star or planet the stronger the attraction
• because of the stronger force planets nearer the sun move faster
• moons, artificial satellites and space stations are held in orbit by gravity, further they are from earth they orbit the slower they move
• the size and force due to gravity follows the 'inverse square' relationship- the mean effect is that the force decreases very quickly with distance
  • double the distance from a plant the size of the force will decrease by factor of 4
  • if you treble the distance the force of gravity will decrease by factor of 9
  • on the other hand if you are twice as close the gravity becomes four times stronger

• comits orbit the sun in eliptical orbits
• the sun isn't at the centre of the orbit but near one end, so their orbits take them a long way out from the sun, then back close again
• the comet travels much faster when it is nearer the sun than it does in more distant parts this is because there is an increased pull of gravity which makes it speed up

Speed is just a number but velocity has direction too

• to measure speed of an object you just measure how fast its going, it is only a number so is a scalar quantity
• to get velocity you must measure both speed and direction so it is a vector quantity

you can calculate average speed to get the overall picture

Relative speed compares the speed of two different objects

• if going the same way you subtract
• if going opposite ways add

1. with or against the current

• draw the vectors end to end

2. across the current

• can draw a scale diagram (triangle)
• speed you need Pythagorus 
• direction- trigonometry

it is the same with forces and any vectors with right angles

u= initial velocity

v= final velocity

s= distance/displacement

t= time

a= acceleration

The path of a projectile is always a parabola

• a projectile is something that is projected or dropped and has things like gravity acting upon it

deal with horizontal and vertical motion separately 

• they are totally separate, one doesn't affect the other

for something that starts off horizontally the initial velocity= 0

constant horizontal velocity- no horizontal forces

vertical velocity increases steadily

vertical velocity calculations use equations of motion

horizontal velocity calculation = distance= speed * time

Momentum=mass*velocity

momentum before = momentum after

when no external forces act, momentum is conserved

forces cause changes in momentum

when a force acts on an object it causes a change in momentum

force acting (N)= change in momentum (kgm/s) / time taken for change to happen

a larger force means a faster change of momentum

if someones momentum changes very quickly the forces on the body will be large

• this is why cars are designed to slow people down in a crash

A long wave radio transmitter sends out continuous high frequency radio carrier waves

the signal is superimposed or encoded on the carrier wave using amplitude modulation

AM= amplitude modulation- sound waves from music 'modulates' or changes the carrier wave by changing its amplitude

Ground waves: travel in close contact with the ground as they spread out from the transmitter, used by LM/MW radio bands- up to 3MHz

Sky waves: frequencies of up to about 30MHz and can reflect off the ionosphere- allows waves to travel longer distances and deals with the curvature of the earth- fm radio and tv

Space waves: microwave signals have a very high carrier frequency- over 3000 MHz, they are easily passed through the atmosphere and reflect off satellites orbiting the earth enabling them to reach distant parts of the planet , some satellites are passive, simply reflecting signal waves that hit them, others are active, receiving the signal and retransmitting it

the highest frequency that can be used to carry satellite transmissions is about 30GHz, above that rain and dust in atmosphere absorb and scatter the radio waves- this reduces the strength of the signal

All waves tend to spread out when they pass through a narrow gap or past an object

a 'narrow' gap is one which is about the same size as the wavelength

the longer the wavelength of the wave the more it will diffact, you get maximum diffraction when the size of the gap is equal to the wavelength

this means that long wavelength radio waves can have a really long range, they spread out in all directions so are good for broadcasting and can diffract over hllls, through tunnels and over the horizon

When waves meet you get disturbance:

• when two waves meet at a point they both try to cause their own disturbance
• constructive interference- disturb in the same direction
• destructive interference- opposite directions
• draw possible outcomes

• the total amplitude of the waves at a point is the sum of the displacements at that point

two speakers both play the same note, depending on where you stand in front of them, you'll hear a loud sound or almost nothing

• at certain points the waves will be in phase- constructive interference, amplitude doubles so hear a loud sound
  • these points occur where the distance travelled by the waves from both speakers is either the same or different by a whole number
• at certain points the waves will be exactly out of phase- destructive interference and the waves cancel out so you get almost no sound
  • these point occur where the difference in the distance is travelled is 1/2 wavelengths or 1 1/2 wavelengths, 2 1/2 wavelengths etc

Observing the interference of light is difficult because the wavelengths are so small

path difference with light have therefore got to be really tiny

man called young found this out by:

• pair of narrow slits that are a fraction of a millimetre apart, this light hits the screen and there is a pattern, light bands (constructive) and dark bands (destructive)

When light diffracts you get patterns of light and dark

• get interference patterns when waves of equal frequency or wavelength overlap
• when a wavefront passes through a gap, light from each point along the gap diffracts. it is at every point along the wavefront is a light source in its own right
• diffracted light from each of these points interferes with light diffracted from all the other points, so you get an interference pattern from 1 slit
• the pattern has bright fringes with alternating dark and bright fringes each side

most waves are transverse- light (most EM waves) , ripples on water

Transverse waves can be plane polarised

• ordinary light waves are a mixture of vibrations in different directions
• passing light through a polarising filter is (like passing a rope through a fence) the filter only transmits vibrations in 1 particular direction
• that means if you have 2 polarising filters at right angles to eachother then no light can get through

Polaroid sunglasses act like a polarising filter

• when light is reflected from horizontal surface it is partly horizontally polarised
• so a vertical polariser can filter out reflected glare from the sea or the snow specially well

Refraction is when waves change direction when they enter a different medium, caused by change in speed, speed change causes wavelength to change- no change in frequency

refraction of light:

• ray of light through glass block
• the ray bends towards the normal as it enters the denser medium and away from the normal as it emerges from the less dense medium

refraction is always caused by the waves changing speed

• when waves slow down they bend towards normal
• when light enters light it slows down to about 2/3 of its normal speed
• the ratio of light in a vacuum to the speed= the refractive index, the higher the refractive index the more light bends
• when waves hit a boundary along the normal there will be no change in direction but there will still be a change in speed and wavelength

Different colours of light are refracted by different amounts

This is because they travel at slightly different speeds in any given medium

A prism can be used to make the different colours of white light appear at different angles

This produces a spectrum showing all the colours of the rainbow- effect= dispersion

red light is the least refracted, violet is the most refracted

order of colours- red orange yellow green blue indigo violet (richard of york gave battle in vain)

Infra-red would appear above red if it was being detected

ultraviolet would appear below violet if it was being detected

Angle of incidence less than the critical angle- most of the light passes through into the air but a little bit of it is internally reflected

Angle of incidence is equal to the critical angle- the emerging ray runs along the surface and there's quite abit of internal reflection

Angle of incidence greater than the critical angle means no light comes out- totally internally reflected

glass- critical angle= 42'

• very handy because it means 45' angles can be used to get total internal reflection as in the prisms in binoculars and periscopes

Diamond- critical angle= about 24'

• thats why diamonds sparkle so much because there are lots of internal reflections

refractive index, n=speed of light in a vacuum (c) / speed of light in that material (v)

light slows down a lot in glass so the refractive index is high (1.5) , the refractive index of water is much lower (1.33)- light doesn't slow as much in water as it does glass

speed of light in air is about the same as in a vacuum so the refractive index of air is 1

according to snells law the angle of incidence, angle of refraction and refractive index are all linked:

n= Sin i / Sin r

refractive index of a medium is the ratio of speed of light in a vacuum to speed of light in that medium

so any material has a different refractive index for each different speed of light

red light slows down least when it travels from air into glass, so it is refracted least and has slowest refractive index

violet light has the highest refractive index

When light leaves a material with higher refractive index and enters a material with a lower refractive index it speeds up so bends away from the normal

if you keep increasing the angle of incidence the angle of refraction gets closer and closer to 90', eventually it reaches a critical angle for which r=90', the light is refracted along the boundary.

you can find the critical angle using the equation:

sinC= nr (refractive index of stuff travelling towards) / ni (refractive index of the material moving away)

A real image is actually there, a virtual image is not

A real image is where light from an object comes together to form an image on a 'screen' like the retina

a virtual image is when the rays are diverging, so the light from the object appears to be coming from a completely different place

when you look in the mirror you see a virtual image

when looking at an object through magnifying lens the virtual image makes the object look bigger and further away than it actually is

to describe an image:

• how big it is compared to the object,
• whether it is inverted or upright,
• whether its real or virtual,
• where it in in relation to the lens and focal point

• A converging lens is convex- it bulges outwards
• it causes a ray of light to converge (move together to focus)
• if the rays entering the lens are parallel to each other to the axis, it focuses them at a point called the focal point
• the distance between the centre of the lens and the focal point= focal length
• converging lenses can make real or vitual images depending on how close they are

1. pick a point on the top of the object to the lens. Draw a ray going from the object to the lens parallel to the axis of the lens

2.Draw another ray from the top of the object going right through the middle of the lens

3. the incident ray that's parallel to the axis is refracted through the focal point, draw a refracted ray passing through the focal point

4. the ray passing through the middle doesn't bend

5. mark where the rays meet. that's the top of the image

• An object at 2F will produce a real, upside down image the same size as the object
• Between F and 2F it'll make a real, upside down image bigger than the object beyond 2F
• An object nearer than F will make a virtual image the right way up, bigger than the object on the same side of the lens

Magnifying glasses work by creating a magnified virtual image

• the object being magnified must be closer to the lens than the focal length
• the image produced is a virtual image, the light rays don't actually come from the place where the image appears to be
• you can't project a virtual image onto a screen

the magnification formula is used to calculate the magnification produced by a lens

magnification= image height/ object height