gcse science physics unit P5

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  • 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 
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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 
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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 
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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 
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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 
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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 
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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. 
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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 
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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
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radio waves + microwaves

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 

long wavelegth radio waves diffract 

  • 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 

  • regualr tv + radio signals transmitted via radiowaves received using aerial 
  • 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 
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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 
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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 
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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
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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...
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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 
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ray diagrams

draw ray diagram to show image from convex lens : 

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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 
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