mock specific Flashcards
is force a vector or a scalar?
vector
what’s the difference between a vector and a scalar?
vectors have a magnitude and a direction, scalars only have a direction
name 5 vector quantities
force, velocity, displacement, acceleration, momentum, etc.
name five scalar quantities
speed, distance, mass, temperature, time
what are vectors usually represented by?
an arrow - the length of the arrow shows the magnitude, and the direction of the arrow shows the direction of the quantity
what is the equation for weight? what are the units for all of the measurements?
weight(N) = mass (kg) x gravitational field strength (N/kg)
what is the relationship between weight and mass?
they are directly proportional
when a chair is sat on the ground, what is the force of the ground on the chair called?
normal contact force
what is a force?
a push or pull on an object that is caused by it interacting with something
what is gravitational force?
the force of attraction between masses
what is weight?
the force acting on an object due to gravity (the pull of the gravitational force on the object)
what is force measured in?
newtons
where does a force act from on an object?
a single point, called it’s centre of mass (a point at which you assume the whole mass is concentrated)`
what is weight measured with?
a calibrated spring balance (or newtonmeter)
is mass a force?
no
what is mass measured in?
kilograms
what is mass measured with?
a mass balance
what do you need to know to calculate the weight of an object?
its mass and gravitational field strength
what do free body diagrams show?
all the forces acting on an object
what do the sizes of the arrows show in a free body diagram?
the relative magnitudes of the forces
what do the directions of the arrows show in a free body diagram?
the directions of the forces acting on the object
what is a resultant force?
the overall force on a point or object
what can you do if you have a number of forces acting at a single point?
you can replace them with the resultant force - a single force that has the same effect as all the original forces together
how do you find the resultant force when multiple forces all act along the same line (they’re all parallel)?
you add together those going in the same direction and subtracting any going in the opposite direction
what happens if a resultant force moves an object?
work is done
what happens when a force moves an object through a distance?
energy is transferred and work is done on the object
what must be done to make something move (or keep it moving if there are frictional forces)?
a force must be applied
when a force is applied, what does the thing applying the force need?
a source of energy (like fuel or food)
what happens when you push something along a rough surface (like a carpet)?
you are doing work against frictional forces. Energy is being transferred to the kinetic energy store of the object because it starts moving, but some is also being transferred to thermal energy stores due to the friction. This causes the overall temperature of the object to increase
what can you use scale drawings for?
to find the resultant force
how do you use scale drawings to find the resultant force?
- draw all the forces acting on an object ‘tip-to-tale’ (start drawing the arrow of the second force from the point (end) of the first arrow). Make sure it’s to scale (make sure you choose a sensible scale, e.g. 1 cm = 1 N)
- draw a straight line from the start of the first force to the end of the last force (making a triangle). This line is the resultant force
- measure the length of the resultant force on the diagram to find the magnitude of the force (using your scale to convert it back into newtons)
- the direction of the resultant force is measured as a bearing (clockwise from north)
what is a scale diagram?
a diagram of all the forces acting on an object, drawn so that one force begins where the previous one ends (the arrow of one force starts at the tip of the arrow of the previous force)
why might you want to split a force into components?
not all forces act horizontally or vertically - some act at awkward angles. To make these easier to deal with, they can be split into two components at right angles to each other (usually horizontal and vertical). Acting together, these components have the same effect as the single force
what does it mean to resolve a force?
to split it into components
how do you resolve a force?
by drawing it on a scale grid. Draw the force to scale, and then add the horizontal and vertical components along the grid lines. Then you can just measure them
what does it mean if a force causes an object to stretch, compress or bend?
there must be more than one force acting on the object (otherwise the object would simply move in the direction of the applied force, instead of changing shape)
what does it mean if an object has been elastically deformed?
it can go back to its original shape and length after the force has been removed
what are objects that can be elastically deformed called?
elastic objects
what does it mean if an object has been inelastically deformed?
it doesn’t return to its original shape and length after the force has been removed
what happens when a force stretches or compresses an object?
work is done - energy is transferred to the elastic potential energy store of the object
where is energy transferred to if an object is elastically deformed?
the object’s elastic potential energy store
what is the relationship between the extension of a spring and the force applied to the spring?
they are directly proportional
what is the equation that links the Spring constant, the force applied and the extension/compression? (include units)
Force (N) = spring constant (N/m) x extension/compression (m)
F = ke
what does the spring constant depend on?
the material that you are stretching - a stiffer spring has a greater spring constant
will the extension of a spring carry on increasing proportionally to the force applied forever?
no
what is the limit of proportionality?
the point where extension stops being proportional to force - the spring has stretched too far. On an extension-force graph, the graph starts to curve after the limit of proportionality
describe a practical to investigate the link between force and extension
- set up a clamp stand so that a spring is hanging next to a fixed ruler
- measure the natural length of the spring (when no load is applied) with a millimetre ruler clamped to the stand. Make sure you take the reading at eye level and add a marker (e.g. a thin strip of tape) to the bottom of the spring to make the reading more accurate
- add a mass to the spring and allow it to come to rest. Record the mass and measure the new length of the spring. The extension is the change in length
- remove the mass and check that the spring returns to the same length as before (that the limit of proportionality hasn’t been reached)
- repeat this process, adding more mass each time, until you have at least 6 measurements
- plot a force-extension graph of your results. It will only start to curve if you exceed the limit of proportionality.
what is the gradient of a force-extension graph (with force on the y-axis)?
the spring constant
what formula should be used to find the work done in stretching (or compressing) a spring (so long as the spring is not stretched past its limit of proportionality)? (give units)
Elastic potential energy (J) = 1/2 x spring constant (N/m) x extension (m)
Ee = (1/2)ke^2
how do you use a force-extension graph to find the energy in the elastic potential energy store of a stretched spring?
you find the area under the line of the graph up to that point
what can the formula Ee = (1/2)ke^2 be used for?
- finding the work done by stretching or compressing a spring
- calculating the energy stored in a spring’s elastic potential energy store
- calculating the energy transferred to the spring as it’s deformed (or transferred by the spring as it returns to it’s original shape)
what is displacement?
a vector quantity that measure the distance and direction in a straight line from an objects starting point to its finishing point
how could an object travel at a constant speed with a changing velocity?
if it is changing direction whilst staying the same speed
what is the formula that links speed, time, and distance travelled?
distance travelled (m) = speed (m/s) x time (s) s = vt
what is the average speed of a person walking?
1.5 m/s
what is the average speed of a person running?
3 m/s
what is the average speed of a person cycling?
6 m/s
what is the average speed of a car?
25 m/s
what is the average speed of a train?
30 m/s
what is the average speed of a plane?
250 m/s
what factors affect the speed a person can walk, run, or cycle?
their fitness, their age, the distance travelled, the terrain, etc.
what is the speed of sound?
330 m/s in air
what affects the speed of sound?
what the sound waves are travelling through
what is wind speed affected by?
things like temperature, atmospheric pressure, and if there are any large buildings or structures nearby (e.g. forests reduce the speed of the air travelling through them)
what is uniform acceleration? give an example.
constant acceleration, e.g. acceleration due to gravity for objects in free-fall
what is acceleration?
the change in velocity in a certain amount of time
what equation can you use to find the average acceleration of an object? (include units)
acceleration (m/s^2) = change in velocity (m/s) / time (s)
a = Δv/t
what is deceleration?
negative acceleration (if something slows down, the change in velocity is negative)
what is the approximate acceleration for objects in free fall?
9.8 m/s^2 - the same value as gravitational field strength
what equation can you use for uniform acceleration?
v^2 - u^2 = 2as v = final velocity (m/s) u = initial velocity (m/s) a = acceleration (m/s^2) s = distance
what does a non-zero resultant force result in?
a non-zero resultant force will always produce acceleration (or deceleration) in the direction of the force. This “acceleration” can take 5 different forms: starting, stopping, speeding up, slowing down and changing direction.
On a free-body diagram, the arrows will be unequal.
what is the relationship between acceleration and the resultant force?
they are directly proportional
what is the relationship between the acceleration and mass of an object?
they are inversely proportional
what is the formula that describes Newton’s second law (linking acceleration, mass, and resultant force)?
(include units)
resultant force (N) = mass (kg) x acceleration (m/s^2) F = ma
what is an estimate for the mass of a car?
1000 kg
how long approximately would it take for a car to accelerate from rest to a typical speed?
typical speed = ~25 m/s
time it takes = ~10 seconds
describe an experiment to investigate how mass and force affect acceleration (testing Newton’s second law)
- set up the apparatus so that a trolley of known mass is connected to a piece of string that goes over a pulley and off the side of the bench. The other end of the string is connected to a hook (that you know the mass of and can add more masses to). a light gate (connected to a data logger or computer) is suspended above the string.
- set up the trolley so that it holds a piece of card with a gap in the middle that will interrupt the signal on the light gate twice. If you measure the length of each bit of card that will pass through the light gate and input this into the software, the light gate can measure the velocity for each bit of card. it can use this to work out the acceleration of the trolley.
- The weight of the hook and any masses attached to it will provide the accelerating force, equal to the mass of the hook x acceleration due to gravity
- the weight of the hook and masses accelerates both the trolley and the masses, so you are investigating the acceleration of the system
- mark a starting line on the table the trolley is on, so that the trolley always travels the same distance to the light gate
- place the trolley on the starting line, holding the hook so the string is taut, and release it
- record the acceleration measured by the light gate as the trolley passes through it. This is the acceleration of the whole system.
- repeat this twice to get an average acceleration
- to investigate the effect of mass, add masses to the trolley one at a time to increase the mass of the system. Don’t add masses to the hook, or you’ll change the force. Record the average acceleration for each mass,
- to investigate the effect of force, you need to keep the total mass of the system the same, but change the mass on the hook. To do this, start with all the masses loaded onto the trolley, and transfer all the masses to the hook one at a time, to increase the accelerating force. Record the average acceleration for each force
what is the equation for stopping distance?
stopping distance = thinking distance + braking distance
what is thinking distance?
how far the car travels during the driver’s reaction time (the time between the driver seeing a hazard and applying the brakes)
what is the braking distance?
the distance taken to stop under the braking force (once the brakes are applied)
what is the typical car braking distance at 30 mph?
14m
what is the typical car braking distance at 60 mph?
55m
what is the typical car braking distance at 70 mph?
75 m
what two things is your thinking distance affected by?
- your speed - the faster you’re going the further you’ll travel during the time you take to react
- your reaction time - the longer your reaction time, the longer your thinking distance
what four things is braking distance affected by?
- your speed - for a given braking force, the faster a vehicle travels, the longer it takes to stop
- the weather or road surface - if it is wet or icy, or there are leaves of oil on the road, there is less grip (and so less friction) between a vehicles tires and the road, which can cause tyres to skid
- the condition of your tyres - if the tyres of a vehicle are bald (they don’t have any tread left) then they cannot get rid of water in wet conditions. This leads to them skidding on top of the water
- how good your brakes are - if brakes are worn or faulty, they won’t be able to apply as much force as well-maintained brakes, which could be dangerous when you need to brake hard
why are speed limits really important?
speed affects the stopping distance so much
what does braking rely on?
friction between the brakes and the wheels
how do brakes work?
when the brake pedal is pushed, this causes brake pads to be pressed onto the wheels. This contact causes friction, which causes work to be done. The work done between the brakes and the wheels transfers energy from the kinetic energy stores of the wheels to the thermal energy stores of the brakes. The brakes increase in temperature
why is it harder to stop vehicles that are going faster?
the faster a vehicle is going, the more energy it has in its kinetic energy stores, so the more work needs to be done to stop it. This means that a greater breaking force is needed to make it stop within a certain distance. A larger braking force means a larger deceleration. Very large decelerations can be dangerous because the may cause brakes to overheat, or could cause the vehicle to skid
how can you estimate the braking force needed to stop a car?
use the equation v^2 - u^2 = 2as to find the deceleration, and then use the equation F = ma to find the force
what is a typical reaction time?
between 0.2 and 0.9 s
what can reaction time be affected by?
tiredness, drugs, alcohol, or distractions
why can’t you measure reaction times with a stopwatch?
because they’re so short
name two ways of measuring reaction time
- using a computer-based test (e.g. clicking a mouse when the screen changes colour)
- using the ruler-drop test
what are the 8 steps to the ruler drop test?
- sit with your arm resting on the edge of a table (this should stop you moving your arm up or down during the test). Get someone else to hold a ruler so it hangs between your thumb and forefinger, lined up with zero. You may need a third person to be at eye level with the ruler to check it’s lined up.
- without giving any warning, the person holding the ruler should drop it. Close your thumb and finger to try to catch the ruler as quickly as possible
- the measurement on the ruler at the point where it is caught is how far the ruler dropped in the time it takes you to react. the longer the distance, the longer the reaction time
- you can calculate how long the ruler falls for (the reaction time) because acceleration due to gravity is constant (roughly 9.8 m/s)
- you can use the equation v^2 - u^2 = 2as to find the final velocity, and then the equation a = Δv/t to find the time, which is the total reaction time
- make sure you repeat this experiment lots of times, as it is difficult to do it accurately.
- make sure it’s a fair test - use the same ruler for each repeat, and have the same person dropping it
- you could try to investigate some factors affecting reaction time, e.g. you could introduce distractions by having some music playing or by having someone talk to you while the test takes place
is momentum a scalar or a vector?
a vector
what is the equation for momentum? (include units)
moment (kg m/s) = mass (kg) x velocity (m/s)
ρ = mv
what are the units for momentum?
kg m/s
what is the conservation of momentum?
in a closed system, the total momentum before an event (e.g. a collision) is the same as after the event. (momentum before = momentum after)
how does an explosion adhere to the conservation of momentum?
in an explosion, the momentum before is zero. after the explosion, the pieces fly of in different directions, so the total momentum cancels out to zero
describe how momentum is conserved by a gun recoiling as it shoots a bullet (4 marks)
before the gun fires the bullet, the total momentum is zero (neither the gun nor the bullet are moving) [1 mark]
when the bullet leaves the gun, it has a momentum in one direction [1 mark]
the gun moves backwards so it has momentum in the opposite direction [1 mark]
this means that the total momentum after the bullet has been fired is zero. momentum has been conserved [1 mark]
what are radio waves mainly used for?
communication
what are radio waves?
electromagnetic radiation with wavelengths longer than about 10 cm
what are the wavelengths of long-wave radio waves?
1 - 10 km
why can long-wave radio signals be received at long distances from the transmitter?
long wavelengths diffract (bend) around the curved surface of the earth. Long-wave radio wavelengths can also diffract around hills, into tunnels, and all sorts. This makes it possible for radio signals to be received even if the receiver isn’t in line of sight of the transmitter
what are the wavelengths of short-wave radio signals?
10m - 100m
why can short-wave radio signals be received at long distances from the transmitter?
they are reflected from the ionosphere
what is the ionosphere?
an electrically charged layer in the earth’s upper atmosphere
how does bluetooth work?
bluetooth uses short-wave radio waves to send data over short distances between devices without wires
how long are the wavelengths of the radio waves used for TV and FM radio transmission?
they have very short wavelengths - to get reception, you must be in direct sight of the transmitter - the signal doesn’t travel far through buildings
what type of waves are used by satellites?
microwaves
why do satellites use microwaves?
they can pass easily through the earth’s watery atmosphere
how do satellite TVs work?
the signal from a transmitter is transmitted into space, where it is picked up by the satellite receiver dish orbiting thousands of kilometres above the Earth. The satellite transmits the signal back to earth in a different direction, where it’s received by a satellite dish on the ground. There is a slight time delay between the signal being sent and received because of the long distance the signal has to travel
name two uses of microwaves
satellites and microwaves
how do microwave ovens work?
- the microwaves are absorbed by water molecules in food
- the microwaves penetrate up to a few centimetres into the food before being absorbed and transferring the energy they are carrying to the water molecules in the food, causing the water to heat up
- the water molecules then transfer this energy to the rest of the molecules in the food by heating - which quickly cooks the food
what can infrared radiation be used for?
to increase or monitor temperature
what can infrared cameras be used for?
to detect infrared radiation and monitor temperature
how do infrared cameras work?
the camera detects the IR radiation and turns it into an electrical signal, which is displayed on a screen as a picture. The hotter an object is, the brighter it appears. E.g. energy transfer from a houses thermal energy store can be detected using infrared cameras
what happens to objects that absorb IR radiation?
they get hotter - food can be cooked using IR radiation, e.g. in a toaster
when does an object give out lots of infrared radiation?
when it’s really hot
how do electric heaters work?
they contain a long piece of wire that heats up when a current flows through it. This wire then emits lots of infrared radiation (and a little visible light - the wire glows). The emitted Infrared (IR) radiation is absorbed by objects and the air in the room - energy is transferred by the IR waves to the thermal energy stores of the objects, causing their temperatures to increase
what are optical fibres? how do they work?
thin glass or plastic fibres that can carry data (e.g. from telephones of computers) over long distances as pulses of visible light.
They work because of reflection - the light rages are bounces back and forth until they reach the end of the fibre
why is visible light used in optical fibres?
because it is easy to refract light enough so that it remains in a narrow fibre, and light is also not easily absorbed or scattered as it travels along a fibre
what is fluorescence?
a property of certain chemicals, where ultra-violet (UV) radiation is absorbed and then visible light is emitted. That’s why fluorescent colours look so bright - they actually emit light
how do fluorescent lights work?
they generate UV radiation, which is absorbed and re-emitted as visible light by a layer of a compound called phosphor on the inside of the bulb. They’re energy-efficient so they’re good to use when light is needed for long periods (like in a classroom)
what can security pens be used for?
they can be used to mark property with your name - under UV light the ink will glow (fluoresce), but it’s invisible otherwise. This can help the police identify your property if it’s stolen
why can tanning salons be dangerous?
overexposure to UV radiation can be dangerous
what are three uses of ultra-violet radiation?
- fluorescent lights
- security pens
3 sun tans - naturally emitted from the sun, and also used in UV lamps at tanning salons
which two EM waves are used in medicine?
X-rays and gamma rays
how do X-rays work?
X-rays pass easily through flesh but not so easily through denser material like bones of metal. So it’s the amount of radiation that’s absorbed (or not absorbed) that gives you an X-ray image
which EM waves do radiographers use to treat people with cancer? (in radiotherapy)
X-rays and gamma rays
why are X-rays and gamma rays used in radiotherapy (treating cancer)?
high doses of these rays kill all living cells, so they are carefully directed towards cancer cells, to avoid killing too many normal, healthy cells
how can gamma radiation be used as a medical tracer?
a gamma-emitting source is injected into the patient, and its progress is followed around the body. Gamma radiation is well suited to this because it can pass out through the body to be detected.
how and why do radiographers keep there exposure to radiation at a minimum?
both X-rays and gamma rays can be harmful to people, so radiographers wear lead aprons and stand behind a lead screen or leave the room to keep their exposure to them to a minimum
what does the danger/harmlessness of EM radiation depend on?
how much energy the wave transfers
is EM radiation harmful to people?
some of it is
how harmful are low-frequency EM waves? Why? Give an example of a low-frequency EM wave
low frequency waves, like radio waves, don’t transfer much energy and so mostly pass through soft tissue without being absorbed
how harmful are high frequency EM waves (give 3 examples)? why?
high frequency waves like UV, X-rays and gamma rays all transfer lots of energy and so can cause lots of damage.
how is UV radiation damaging?
UV radiation damages surface cells, which can lead to sunburn and cause skin to age prematurely. Some more serious effects are blindness and an increased risk of skin cancer.
how are X-rays and gamma rays harmful?
they are types of ionising radiation (they carry enough energy to knock electrons off atoms). This can cause gene mutation or cell destruction, and cancer
what is considered before any type of EM radiation is used?
whether the benefits outweigh the health risks
what is radiation dose? what is it measured in?
radiation dose (measured in sieverts) is a measure of the risk of harm from the body being exposed to radiation. It is NOT a measure of the total amount of radiation that has been absorbed.
what does the risk of radiation depend on?
the total amount of radiation absorbed and how harmful the type of radiation is.
what is a milisievert? why are they used?
a sievert is pretty big, so you’ll often see doses in millisieverts (mSv) where 1000 mSv = 1 Sv
is the radiation dose higher for a patient when having a CT scan on their head or their chest?
their chest - they are four times more likely to suffer damage to their genes when having a chest scan
what are the two types of magnet?
permanent and induced
which type of magnet produces its own magnetic field?
permanent magnets
what are induced magnets?
magnetic materials that turn into a magnet when they’re put into a magnetic field
the force between permanent and induced magnets is always…
attractive
what happens when you take an induced magnet out of a magnetic field?
they quickly lose their magnetism (or most of it) and stop producing a magnetic field
what does a moving charge create?
a magnetic field
what happens when a current flows through a wire?
a magnetic field is created around the wire
what is the magnetic field of a wire with current flowing through it made up of?
concentric circles (sharing the same centre) perpendicular to the wire, with the wire in the centre
does changing the direction of the current change the direction of the magnetic field?
yes
how do you work out the direction of a magnetic field from the direction of the current?
using the Right-Hand Thumb Rule:
using your right hand, point your thumb in the direction of current, and curl your fingers. The direction of your fingers is the direction of the field.
what affects the strength of a magnetic field caused by a moving charge?
the current and the distance from the wire - the larger the current through the wire, or the closer to the wire you are, the stronger the field is.
when can the motor effect happen?
when you put a current-carrying wire in a magnetic field
what is the motor effect?
when a current-carrying wire (or any other conductor) is put between magnetic poles, the magnetic field around the wire interacts with the magnetic field it has been placed in. This causes the magnet and the conductor to exert a force on each other. This is called the motor effect and can cause the wire to move
in order to get the full force of the motor effect, where does the wire have to be in relation to the magnetic field?
at 90 degrees to it
how much force will a wire experience if it runs parallel to the magnetic field?
no force at all
what two things does the magnitude of the force of the motor effect increase with?
- the strength of the magnetic field
2. the amount of current passing through the conductor
how can you find the size of the force of the motor effect when the current is at 90 degrees to the magnetic field it is in?
using the equation force (N) = Magnetic flux density (T,telsa) x current (A) x length (m)
what three things does the force acting on a conductor in a magnetic field depend on?
- the magnetic flux density - how many field (flux) lines there are in a region. This shows the strength of the magnetic field
- the size of the current through the conductor
- the length of the conductor that’s in the magnetic field
how can you find the direction of the force on a current-carrying wire in a magnetic field?
using Fleming’s left-hand rule
what does your first finger represent in Fleming’s left-hand rule?
the magnetic field (F irst finger = magnetic F ield)
what does your second finger represent in Fleming’s left-hand rule?
the direction of the current (seCond finger = Current)
what does your thumb represent in Fleming’s left-hand rule?
the direction of the force (thuMb = Motion)
what does Fleming’s left-hand rule show?
that if either the current or the magnetic field is reversed, then the direction of the force will also be reversed.
show Fleming’s left-hand rule on your hand and name what all the fingers mean?
did you do it on your left hand?
first finger = magnetic field
second finger = current
thumb = force
what happens to a current-carrying coil of wire in a magnetic field?
it rotates
why does a current-carrying coil of wire rotate in a magnetic field?
the forces that act on it are just the usual forces which act on any current in a magnetic field. Because the coil is on a spindle and the forces act one up and one down, it rotates
what is a split-ring communicator?
a clever way of swapping the contacts every half turn to keep the motor rotating in the same direction
how can the direction of a basic dc motor be reversed?
either by swapping the polarity of the dc supply (reversing the current) or by swapping the magnetic poles over (reversing the firld)
how can the speed of a basic dc motor be increased?
either by increasing the current, adding more turns to the coil or increasing the magnetic flux density
what direction does current flow in?
from positive to negative
