# gcse science physics unit P5

HideShow resource information
• Created by: charlie
• Created on: 14-06-13 17:14

## speed + velocity

• scalar quantities : with only a number (speed, mass, temp,time,length...)
• vector quantities : has a number + direction (velocity,force,displacement,momentum,accel...)

relative speeds- compare speeds of two diff. objects

• cars going same way you minus the speeds
• cars going in diff. direction you add the speeds
1 of 17

## combining two vectors- add end to end

1) with or against the current (flying into the wind)

• draw vectors end to end - then add or subtract accordingly

2) across the current (side wind)

• draw vectors end to end making triangle
• working out resultant velocity use: PYTHAGORAS(speed) or TRIGONOMETRY(direction)
• same with forces + any vectors at right angles
2 of 17

## equations of motion- SUVAT

s= displacement

u= initial velocity

v= final velocity

a= acceleration

t= time

all 4 equations are given but need to know how to rearrange them

method:

• 1)write down which 3 things already know
• 2)write down which of the other things you want to find out
• 3)choose the equation that involves all of the things written down
• 4)stick in the numbers + do the maths
3 of 17

## projectile motion

path of projectile is always parabolic

• a projectile is something projected/dropped + only has earths gravity acting on it (ignore air resistance)

deal with horizontal + vertical motion spearately (one doesnt affect the other)

• gravity doesnt affect horizontal motion
• object projected horizontally accel. vertically due to gravity has no accel. horizontally (velocity stays the same)
• object projected at angle + horizontal + vertical split
• both horizontal velocity + vertical velocity are vectors + resultant at any point is the sum
4 of 17

## forces + newton's third law

forces occur when 2 objects interact

• object exerts force on another object it always experiences a force in return (interaction pair)
• newtons third law - if object A exerts a force on object B then object B exerts and equal + opposite force back
• same is true with collision- colliding objects exert equal + opposite forces on each other

objects exert downward force due to gravity

• e.g book on table pushes down equal to weight + table exerts equal + opposite force upwards
• this upwards force is called a 'REACTION FORCE'

things move because forces are applied to diff. objects

• recoil : gun fired- bullet exerts equal + opposite force on gun- bullet travels out of barrel- gun recoils back- bullet travels much faster forward as it is lighter
• rockets : hot gas collide with walls- wall exerts equal + opposite force- force on wall pushes gas particles out of exhaust- force from the gas on the walls pushes rocket forward- need large no. of particles at high speed to over come air resistance + gravity
5 of 17

## conservation of momentum

momentum (kg m/s) = mass (kg) x velocity (m/s) (equation is given)

momentum before = momentum after

• in collision where no other external forces act momentum is conserved
• e.g before explosion momentum=0 after particles chucked out is speeds of all diff. +ve and -ve directions resulting in momentum after=0
• if two opjects collide + join then total momentum of objects before=total combined momentum after

(LEARN THIS)

• (m1 x u1) + (m2 x u2) = (m1 + m2) x v
• m = mass of object     v= velocity of combined objects   u=velocity of object
6 of 17

## pressure

kinetic theory says gases are randomly moving particles

• contantly collide with each other, with walls of container + bounce off
• particles hardly take up any space, most of gas is empty space

decrease in vol. of container gives inc. in pressure

• gas particles when they collide exert a force + in sealed container creat outward pressure
• put same amount of gas in smaller container collision frequency against the walls inc. + therfore the pressure is inc.

inc. in temp. gives an inc. in pressure

• pressure depends on how fast particles are going + how often they hit the walls
• heat gas gives particles more kinetic energy- hitting walls harder + more often- inc. pressure

colliding particles change their momentum

• when collide with walls particles velocity changes which also means change in momentum
• (change in momentum = mass x change in velocity)  therefore  (force = change in momentum / time take)
• therefore when particles collide with container experience change in momentum + exert force creating pressure.
7 of 17

## gravity + orbits

gravity provides centripetal force that causes orbits

• object travelling in circle is always changing direction - force must be acting on it
• orbit is balance between forward motion of object + force pulling it back-(centripetal)
• centripetal force on planets is due to gravity provided by sun + the planet

gravity dec. quickly as get further away

• very large masses gravitational force is very big- closer the stronger force of attraction
• planets nearer the sun move faster + cover orbit quicker
• if you double distance from a planet size of gravitational force with dec. by factor of 4
• if you treble the distance from a planet gravitational force will dec. by factor of 9
• if you get 2x as close gravity becomes 4x as stronger
• comets change speed because of gravity
• periodic comets orbit sun with highly elliptical (elongated) orbits
• closer to the sun the greater gravitational force of attraction + much faster
8 of 17

## satellites

communication satellites (geostationary) stay over the same point

• high orbit, over equator, orbit once every 24 hrs- earth rotates with them
• ideal for telephone, tv, because stay above same point + can transfer signals in less than sec.

weather + spying satellites

• low polar orbits, sweeps over poles whilst earth rotates, closer - stronger gravity- faster orbits
• orbit in less than 2 hours- each time comes round can scan next part of globe
• so whole planets surface can be monitored each day

microwaves are used for satellite communication

• for satellite TV + phones signal from tansmitter is transmitted into space,
• picked up by satellite receiving dish + transmits signal back to earth in diff direction
• received by satellite dish on ground or satellites retransmit signal to other satellites

microwaves have higher frequency than radio waves

• microwaves over 3 GHz- pass easily through earths atmosphere so can reach far satellites
• satellites in low orbits use lower frequency - signal weaken further travelled- interference so use digital signal
9 of 17

different frequency waves travel by diff. routes

• EM waves diff. frequencies transmit diff. types of communication signals  because they behave diff in atmosphere
• below 30 MHz - radio wavesrefelcted off ionosphere - travel long distances due to earths curvature
• 30 MHz - 30GHz- radio+ microwaves pass straight through atmosphere - transmissions must be in line of sight
• above 30 GHz- rain+dust in atmosphere absorb + scatter microwaves - reducing strength + bad signal

• narrow gap (same size as wavelength) + waves diffract - longer wavelength more diffraction
• radiowaves have really large wavelength- long range + spread out in all directions- good for broadcasting

dishes used to receive microwave signals

• microwave wavelength too short for effect so received using dish
• dish many times larger so waves dont diffract much producing narrow beam
• transmitting + receving dishes need to be carefully aligned
10 of 17

## interference of waves

when waves meet they cause disturbance

• all waves cause distrubance in medium- sound waves + air particles
• two waves meet at point cause own disturbance - same direcction + reinforce- constructive or opposite directions + cancel which is called destructive interference
• total amplitude is either minus the waves or plus the two waves

get patterns of loud + quiet bits with sound

• two speakers, same note, same time- dependent on where standing here loads or nothing
• cetain points waves will be constructive (in phase) where distance travelled by wave is the same or diff. by whole wavelength
• certain points destructive (out of phase) where diff. in distance is 1.5, 2.5, 3.5 ect..
• pattern of load + quiet is called interference pattern- with all types of waves

interference patterns need coherent waves

• waves same frequency+wavelength, in phase (troughs+crests line up), same amplitude
• e.g for light- coherent source is monochromatic light needed for interference pattern
11 of 17

## diffraction patterns + polarisation

when light diffracts you get patterns of light + dark

• wavefront of light through gap- light from each point diffracts - every point along wavefront is an own light source
• gap must be same size as wavelength + diffracted light causes interference pattern
• bright solid centre with dark + bright fringes either side

light beahaves like a wave + stream of particles

• 17th century 2 theories to explain- particle theory (newton) + wave theory (huygens)
• particle theory explains: reflectiong, refraction not diffraction or inerferecence unique to waves
• young's double split experiment shows light could diffract + interfer

transverse waves can be plane polarised

• electromagnetic waves are transverse - polarisation filters out vibrations if diff. directions
• like passing rope through a fence
• ordinary light mixture of vibrations - passed through polarising filter- only lets one direction through
• light reflected from water- turns partially polarised+ sunglasses filter out reflected glare
12 of 17

## refraction

chaging speed of wave can chage its direction

• wave speed dec. bends to the normal + when increases bends away
• waves change speed going through diff. densities

the greater the change in speed the more waves bend

• light enters glass- slows down to 2/3 of normal speed (2x10^8 rather than 3x10^8)
• ratio of speed of light in vacuum to it in a medium is = refractive index
• refractive index is measure of amount of bending
• light changes speed but not direction at 90d + some reflected when it hits glass

every transparent material has refractive index  (equation given in exam)

• light slows a lot in glass so it has high refractive index but water has low r. index
• speed of light in air is same as in vacuum so r. index is 1
13 of 17

## refraction part 2

refractive index explains dispersion

• diff. colours of light refracted diff amounts - as they travel slightly diff. speeds through medium
• red light slows down  least+ is refracted least however blue light is opposite
• prism used to make diff. colours of light emerge at diff. angles
• produces spectrum showing all diff. colours of rainbow= DISPERSION

total internal reflection (tir) + critical angle

• tir only happens when light travels from more dense medium with higher r. index to less dense
• if angle of incidence large enough - ray will experience tir
• diff. mediums have diff. critical angles - higher r. index lower the critical angle
• for glass about 42d so 45d angles used for tir in optical firbres, bionculars, reflectors...
14 of 17

## images + converging lenses

real image is actually there- virtual image is not

• real image- light from object comes together to form image on screen- e.g image on eye retina
• real images can be projected onto screen
• virtual image is when rays diverging so light from object appears to be coming from diff place
• cant be projected onto screen - e.g when look on mirror you look further away
• to describe image properly you need to say:
• how big compared to object, real or virtual, upright or inverted, where in relation to lens + focal point

converging (convex) lenses focus light

• causes light to move together to a focus to focal point if rays are paralle to eachother + pricipal axis
• distance between centre of lens + focal point is called the focal length
• need to be able to desribe + explain light entering the lens + rarefraction of diff. rays
• convex lenses work other way up too - turn diverging light rays into parallel light
• convex lenses can make real or virtual images depending on how close to object lens is
15 of 17

## ray diagrams

draw ray diagram to show image from convex lens :

16 of 17

## magnification, cameras + projectors

convex lenses create magnified images - (maginifcation formula given = image size/object size mm)

• magnifiying glasses use convex lens to create larger images
• object magnified must be closer to lens then focal point + it produces virtual image

cameras make image smaller than object

• refracted by lens + forms image on light sensor + is real image as light rays actually there
• image smaller as object a lot further away than focal point  procing real inverted image

projections make image larger than object

• work in similar way to cameras but object is closer than focal length creating a larger image
• project the image the object needs to be close + upside down- light rarefracted by lens
• produces real, inverted + magnified image on screen

images focussed by moving then lens

• image in focus when light forming image converges on screen/sensor
• cameras + projectors image focused by moving lens closer/further from object- closer to object lens is further from lens image is
17 of 17