gcse science physics unit P5
- 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
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
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
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
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
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
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.
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
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
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
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
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
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
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...
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
ray diagrams
draw ray diagram to show image from convex lens :
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
Related discussions on The Student Room
- Hannah's way of making sure she doesn't have a breakdown »
- BTEC H&S Care Unit 12: Supporting Individuals with Additional Needs Work Explained »
- Getting into uni with Btecs »
- Pharmacy/a levels »
- Access course for Pharmacy »
- aiming for all 9s »
- GCSE Exam Discussions 2023 »
- Bangor University GCSE Revision guides? »
- Natural science advice?? »
- Regret taking single award science »
Comments
No comments have yet been made