Energy Flashcards
What is the formula for work done
work done = force x distance
w = f x d
what is the formula for gravitational potential energy and what is it
gravitational potential = mass x gravitational field x height
Ep = m x g x h
GPE is the energy of an object that has by its vertical position in a gravitation field. When the object is raised work is done because it goes against the force of gravity
what is the formula for kinetic energy and what is it
kinetic energy= 0.5 x mass x speed^2
kinetic energy of something depends on mass and speed, anything that’s moving has kinetic energy
Explain that energy transferred is work done (for the example of a car)
when a car is moving it has kinetic energy
- moving car has a lot of kinetic energy and to slow it down, the kinetic energy needs to be converted into other types of energy
- to stop a car the kinetic energy needs to be converted to heat energy as friction between the wheels and the brake pads, causes temperature of brakes to increase
kinetic energy to transferred=work done
explain this statement
‘when a force moves an object through a distance energy is transferred and work is done’
- when something moves, something is putting ‘effort’ to make it move
- the thing putting in effort needs a supply of energy
- it then does ‘work’ by moving the object - it transfers the fuel it receives into other forms
- whether the energy is transferred usefully or wasted-work is done (J)
what energy is transferred when objects fall
- when something falls it’s gravitational potential energy (Ep) converts into kinetic energy (Ek).
- so further it falls the faster it goes
- some of this kinetic energy is then transferred to heat and sounds eg meteors and space shuttles (with a very high kinetic energy). friction due to collisions with particles in the atmosphere transfers kinetic energy to heat energy.
- these temperatures can get so extreme that most meteors burn up completely and never hit the earth - only big meteorites make it to the earths surface.
- space shuttles have heat shields made from special materials which lose heat quickly, allowing the shuttles to re-enter the atmosphere without burning up
explained the work done to an elastic object
- work is done to an spastic object to change its shape. the energy is not lost but STORED by the object as ELASTIC POTENTIAL ENERGY
- the elastic potential energy is then converted to kinetic energy when the force is removed (and the object returns to its original state)
what is the formula for the extension of an elastic object
Force applied = spring constant x extension
F= k x e
the extension of a stretched spring (metres) is directly proportional to the force applied (newtons).
the spring constant (N/m, newtons per metre) depends on the material you are stretching
what’s the issue with saying extension of an elastic object is directly proportional to the force
this stops working when the force is great enough (there is a limit)
for small forces, force and extension are the proportional
BUT
there is a maximum force that the elastic object can take and still extend proportionally - known as the LIMIT OF PROPORTIONALITY
what is power
the rate of doing work
power= work done E
———– p= —
time taken t
what is the unit power is measured in
Power is measured in Watts or J/s
One watt= 1 joule of energy transferred per second
how can you calculate your Power Output
- timed run upstairs= energy transferred is the potential energy you gain ( mass x g x height)
so Power= mgh/t - timed acceleration = energy transferred is the kinetic energy you gain ( 1/2mv^2)
so Power= 1/2mv^2/t
what is momentum
momentum is a property of moving objects
the greater the mass of an object
the greater it’s velocity
the more momentum it has
momentum is a vector quality as it has size and direction
what is the formula to calculate momentum and what units is it measured in
momentum = mass x velocity p= m x v
momentum is measured in kgm/s
what is the conservation of momentum for collisions
total momentum before the collision= the total momentum after
so you need to work out the total momentum before the collision and then work out the total momentum after and use algebra
what’s the conservation of momentum in explosions
total momentum before the explosion =the total momentum after the explosion
so you need to work out the total momentum before the collision and then work out the total momentum after and use algebra
how do forces cause changes in momentum
- when i force acts on an object it causes a change in momentum
- larger force= larger change of momentum
- momentum changes quickly= large forces, more likely to cause an injury
this is why cars are designed with safety features that slow people down over a longer time when they have a crash- the longer it takes for a change in momentum the smaller the force
how do brakes work against the kinetic energy of a car
Apply the brakes to slow a car down, work is done.
Brakes reduce kinetic energy by transferring it to heat and sound
Regenerative breaking systems make use of the energy instead of converting it to heat by putting the vehicles motor in reverse, slowing the car down. The motor acts as an electric generator and converts the kinetic energy into electrical which is stored as chemical in the vehicles battery
(stores energy instead of wasting it)
How are cars designed to convert kinetic energy safely in a crash
Car will slow down very quickly when it crashes.
There will be a big change in momentum and a lot of energy will be converted in a short amount of time - DANGEROUS
So cars are designed to increase the time over which momentum changes happen which lessens the force on passengers
- Crumple Zones
- Side impact bars
- Seat belts
- Air bags
How do cumple zones increase the time over which the momentum changes happen
Crumple Zones at the front and back of the car crumple up on impact
Kinetic energy converted into other forms of energy as the car changes shape
Crumple zones increase the impact time,which decreases the force produced by the change in momentum
How do side impact bars increase the time over which the momentum changes happen
side impact bars are strong metal tubes fitted into car door panels.
help direct the kinetic energy of the crash away from passengers to other areas of the car (eg crumple zones)
So increase impact time so decreases force produced by change in momentum
How do seat belts increase the time over which the momentum changes happen
Seat belts stretch slightly, this increases the time taken for the wearer to stop.
this reduces the force acting in the chest and some of the kinetic energy of the wearer absorbed by the seatbelt stretching
what is an elastic object
- when you apply a force to an object it may stretch or change shape
- any object that returns to its original shape after the force has been removed is an elastic object
