# Unit 2

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define scalar
they have no direction - it's an amount
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define vector
they have a magnitude and direction
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force and velocity are both
vectors
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give some examples of scalars
mass, temperature, time, length, speed and energy
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give some examples of vectors
displacement, force, velocity, acceleration and momentum
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what is the quantity of 'g'
9.81ms-2
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the greater the mass
the greater the resistance to a change in velocity
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define center of mass
is a single point that you can consider its whole weight to act through
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how to make an object more stable
bigger base and lower the center of mass
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what is a moment?
is the turning effect of a force
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state the principle of moments
for a body to be in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about the same point
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what is a couple?
is a pair of forces of equal size which act parallel to each other, but in opposite directions
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state newtons 1st law
velocity of an object will not change unless a resultant force acts on it
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state newtons 2nd law
acceleration is proportional to the force - the more force you have acting on a certain mass, the more acceleration you get - the more mass you have the less acceleration you get
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state newtons 3rd law
if an object (A) experts a force on another (B), then (B) exerts and equal but opposite force on object (A)
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they always act in the opposite direction to the motion of the object, they can never speed things up or start something moving, they convert kinetic energy to heay
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principle of conservation of energy
energy cannot be created or destroyed. energy can be transferred from one form to another but the total amount of energy in a closed system will not change
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state hookes law
extension is proportional to force
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define elastic stretch
the material returns to it's original shape once the forces are removed
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define plastic stretch
the material will be permanently deformed
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a stress cause
a strain
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how do you find the elastic strain energy from a graph
area under the graph
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youngs modulus is
stress / strain
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limit of proportionality
where the graph stops being straight
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elastic limit
the point the material starts to behave plastically
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yield point
the material starts to stretch without any extra load
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the graph for brittle materials
don't cure
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brittle material break suddenly
without deforming
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electromagnetic waves
causes things to heat up
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x-rays and gamma rays
knock electrons out of their orbits, causing ionisation
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what is a progressive wave?
they carry energy fro one place to another without transferring any material
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what is the displacement of a wave?
how far a point on the wave has moved from its undisturbed position
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what is the amplitude of a wave?
its maximum displacement
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what is a waves wavelength?
teh length of one whole wave, from crest to crest or trough to trough
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what is a waves period?
the time taken for a whole vibration
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what is a waves frequency?
the number of vibrations per second passing a given point
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what is a waves phase difference?
the amount by which one wave lags behind another wave - measured in radians or degrees
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define reflection
the wave is bounced when it hits a boundary
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define refraction
the wave changes direction as it enters a different medium
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frequency =
.
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in transverse waves
the vibration is a t a right angle to the direction of travel
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in longitudinal waves
the vibrations are along the direction of travel
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give examples of transverse waves
ripples on water, electromagnetic waves and waves on rope
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give examples of longitudinal waves
sounds and earthquake shock waves
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if you have two polarising filters at right angles of each other
then no light will get through
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what speed do electromagnetic waves travel at?
2.998*10-8
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they are transverse waves consisting of
vibrating electric and magnetic field - they are at right angles of each other
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all EM can be
reflected, refracted, diffracted, polarised and can undergo interference
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all EM obey
v=fh
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all EM carry
energy
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in transverse waves energy
is directionally proportional to the frequency
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the higher the energy in wave
the more dangerous the wave
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passing through matter -- radio transmissions
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microwaves
mostly pass through matter, but cause some heating -- radar, microwave cooking, tv transmissions
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infrared
mostly absorbed by matter, causing it to heat up --heat detectors, night-vision cameras, remote controls, optical fibres
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visible light
absorbed by matter, causing some heating effect -- human sight, optical fibres
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ultraviolet
absorbed by matter, slight ionising -- sunbeds, security markings show up in UV light
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xrays
mostly pass through matter, but cause ionisation as they pass -- to see damage to bones and teeth, airport security scanners, to kill cancer cells
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gamma rays
mostly pass through matter, but cause ionisation as they pass -- irradiation of food, sterilisation of medical instruments, to kill cancer cells
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radio waves production and effect on the body
oscillation electrons in an aerial -- no effect
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microwaves production and effect on the body
electron tube oscillators, masers -- absorbed by water - danger of cooking human body
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infrared production and effect on the body
natural and artificial heat sources --heating, excess heat can harm the body's system
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visible light production and effect on the body
the sun --tans the skin, can cause skin cancer and eye damage
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xrays production and effect on the body
bombarding metal with electrons -- cancer due to cell damage, eye damage
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gamma rays production and effect on the body
radioactive decay of the nucleus -- cancer due to cell damage, eye damage
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is the least damaging, although it's thought to be a major cause of skin aging
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is more damaging than UVA, it's responsible for sunburn and can lead to cancer
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is ionising, it can cause cell mutation or destruction, and cancer - it's almost entirely blocked by the ozone layer
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what is the speed of light?
.
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what is the refractive index of air?
1
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light leaving an optically denser material is
refracted away from the normal
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what is an optical fibre?
a very thin flexible tube of glass or plastic fibre that can carry light signals over long distances and round corners
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in an optical fibre the fibres themselves
have a high refractive index but are surrounded by cladding with a lower refractive index
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what happens when light in shone through one side of the fibre
the fibre is so narrow that the light always hits the boundary between fibre and cladding at an angle bigger than the critical angle so all the light is totally internally reflected
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why is using optical fibres better than using electricity
the signal can carry more information because light has a high frequency, the light doesn't heat up the fibre (so almost no energy is lost), there is no electrical interference
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when does superposition happen?
when two or more waves pass through each other at the moment the waves cross, the displacements due to each wave combine
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what is the principle of superposition?
when two or more waves cross, the resultant displacements equals the vector sum of the individual displacements
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to get an interference pattern
the two sources must be coherent
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two sources are coherent if
they have the same wavelength, frequency and a fixed phase difference between them
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constructive interference occurs when
.
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destructive interference occurs when
.
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you get a standing wave when
a progressive wave is reflected at a boundary
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what is a standing wave?
is a superposition of two progressive waves with me wavelength, moving in opposite directions
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what properties must the string have to produce a low note?
longer, heavier and looser
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when the gap a lot bigger than the wavelength
diffraction is unnoticable
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when the gap is several wavelengths wide
you get noticeable diffraction
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when hte gap is the same size as the wavelength
you get the most diffraction
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if the wavelength of a light wave is about the same size as the aperture
you get a diffraction pattern of light and dark fringes
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what do the light and dark fringes look like? and what kind of light source is needed?
what does electron diffraction look like and what does this mean?
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what happens in young's double slit experiment?
you get a pattern of light and dark fringes, depending on whether constructive or destructive interference is taking place
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interference patterns get sharper
when you diffract through more slits
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shining white light through a diffraction grating
produces spectra
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what does the diffraction of white light look like?
.
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how does the diffraction of white light help scientists?
astronomers and chemists often need to study spectra to help identify elements, they use diffraction gratings rather than prisms because it's more accurate
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## Other cards in this set

### Card 2

define vector

#### Back

they have a magnitude and direction

### Card 3

#### Front

force and velocity are both

### Card 4

#### Front

give some examples of scalars

### Card 5

#### Front

give some examples of vectors