Energy

Question 1

What is the formula for work done

Answer 1

work done = force x distance

w = f x d

Question 2

what is the formula for gravitational potential energy and what is it

Answer 2

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

Question 3

what is the formula for kinetic energy and what is it

Answer 3

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

Question 4

Explain that energy transferred is work done (for the example of a car)

Answer 4

when a car is moving it has kinetic energy

1. 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
2. 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

Question 5

explain this statement

‘when a force moves an object through a distance energy is transferred and work is done’

Answer 5

1. when something moves, something is putting ‘effort’ to make it move
2. the thing putting in effort needs a supply of energy
3. it then does ‘work’ by moving the object - it transfers the fuel it receives into other forms
4. whether the energy is transferred usefully or wasted-work is done (J)

Question 6

what energy is transferred when objects fall

Answer 6

1. when something falls it’s gravitational potential energy (Ep) converts into kinetic energy (Ek).
2. so further it falls the faster it goes
3. 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.
4. 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.
5. space shuttles have heat shields made from special materials which lose heat quickly, allowing the shuttles to re-enter the atmosphere without burning up

Question 7

explained the work done to an elastic object

Answer 7

1. 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
2. the elastic potential energy is then converted to kinetic energy when the force is removed (and the object returns to its original state)

Question 8

what is the formula for the extension of an elastic object

Answer 8

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

Question 9

what’s the issue with saying extension of an elastic object is directly proportional to the force

Answer 9

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

Question 10

what is power

Answer 10

the rate of doing work

power= work done E
———– p= —
time taken t

Question 11

what is the unit power is measured in

Answer 11

Power is measured in Watts or J/s

One watt= 1 joule of energy transferred per second

Question 12

how can you calculate your Power Output

Answer 12

1. timed run upstairs= energy transferred is the potential energy you gain ( mass x g x height)
   so Power= mgh/t
2. timed acceleration = energy transferred is the kinetic energy you gain ( 1/2mv^2)
   so Power= 1/2mv^2/t

Question 13

what is momentum

Answer 13

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

Question 14

what is the formula to calculate momentum and what units is it measured in

Answer 14

momentum = mass x velocity
p= m x v

momentum is measured in kgm/s

Question 15

what is the conservation of momentum for collisions

Answer 15

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

Question 16

what’s the conservation of momentum in explosions

Answer 16

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

Question 17

how do forces cause changes in momentum

Answer 17

1. when i force acts on an object it causes a change in momentum
2. larger force= larger change of momentum
3. 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

Question 18

how do brakes work against the kinetic energy of a car

Answer 18

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)

Question 19

How are cars designed to convert kinetic energy safely in a crash

Answer 19

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

1. Crumple Zones
2. Side impact bars
3. Seat belts
4. Air bags

Question 20

How do cumple zones increase the time over which the momentum changes happen

Answer 20

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

Question 21

How do side impact bars increase the time over which the momentum changes happen

Answer 21

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

Question 22

How do seat belts increase the time over which the momentum changes happen

Answer 22

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

Question 23

what is an elastic object

Answer 23

1. when you apply a force to an object it may stretch or change shape
2. any object that returns to its original shape after the force has been removed is an elastic object