not assessed paper 2 Flashcards
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
- at 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 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
what do all magnets produce?
a magnetic field
what is a magnetic field?
a region where other magnets or magnetic materials experience a non-contact force
how can you show a magnetic field?
by drawing magnetic field lines
what are the two poles that all magnets have?
north and south poles
in which direction do magnetic field lines always go?
from north to south
what do magnetic field lines show?
which way a force would act on a north pole if it was put at that point in the field
what do magnetic field lines that are closer together show?
a stronger magnetic field
what happens to a magnetic field the further away from a magnet it gets?
it gets weaker
where is a magnetic field the strongest? Where are the magnetic forces strongest?
at the poles of a magnet
what do compasses show?
the directions of magnetic fields
how can you use a compass to build up an image of what a magnetic field looks like?
you can move a compass around a magnet and trace its position on some paper to build up a picture of what the magnetic field looks like
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
