Energy
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What is kinetic energy
kinetic energy is the energy stored in moving objects
do stationary objects have kinetic energy
no stationary objects such as a parked car do not have kinetic energy
what is the formula for kinetic energy
Ek=1/2mv^2
what are the units for kinetic energy, mass, velocity
energy transferred= Joules Mass=kg velocity=m/s
car runs on petrol- store of chemical energy
a chemical reaction takes place in the engine and some of the energy is converted to kinetic energy store as the car moves
A car with a mass of 700kg is moving with a speed of 20m/s . calculate the kinetic energy of the car
Ek=1/2mv^2 1/2x700x20^2= 140,000J 140KJ
A cyclist and bike have a total mass of 100kg and a speed of 15m/s. Calculate the kinetic energy.
Ek=1/2mv^2 1/2x100x15^2= 11250J 11.25KJ
A tennis ball is travelling at 50m/s and has a kinetic energy of 75J. calculate the mass of the tennis ball.
Ek=1/2mv^2 75J=1/2m50^2 75J=1250m 75 divided by 1250=0.06kg
what is elastic potential energy
when we stretch a spring we are applying a force to change the length of the spring. we are putting energy in to stretch the spring and the stretched spring stores this energy. the energy that the stretched spring stores is called elastic potential energy.
if we apply a force, the spring will stretch, what do we call this stretch
the extension (e) the extension is directly proportional to the force up until the limit of proportionality.
what is the formula for elastic potential energy
Ee=1/2ke^2 energy=J spring constant=N/m e=metres value of spring constant depends on the spring.
A spring has an extension of 20cm.Calculate the elastic potential energy stored in a spring k=100n/m
Ee=1/2ke^2 1/2x100x0.02m^2 2J
What is gravitational potential energy?
-the energy stored in an object due to its position above the Earth’s surface it is due to the force of gravity acting on the object
lifting ball off ground example
a ball is sitting on a shelf, pete lifts the ball off the ground and puts it on a shelf. the chemical energy store in Pete’s muscles is being converted into gravitational potential energy in the ball. so the ball now has a store of gravitational potential energy. if the ball rolls off the shelf, the gravitational potential energy is changed to other forms of energy such as kinetic energy or thermal energy so the ball now has no gravitational potential energy.
what is the formula for gravitational potential energy
Ep=mgh mass=kg g=n/kg h=m Ep=Joules what is the gravitational field strength- measure of force of gravity
A crane lifts a 75kg mass from a height of 8m.Calculate the gravitational potential energy gained by the mass. g=9.8N/kg
Ep=mgh 75x9.8x8= 5880J
A ball with a mass of 500g is lifted onto a shelf which is 1.5m above the ground.calculate the gravitational potential energy gained by the ball. g=9.8N/kg
Ep=mgh 0.5kgx9.8x1.5= 7..35J
What is specific heat capacity?
the specific heat capacity of a substance is the amount of energy required to raise the temperature of 1kg of the substance by 1 degree celsius.
calculate energy required to increase the temperature of 2kg of water from 20 degrees celsius to 100 degrees celsius. the specific heat capacity of water is 4200j/kg
change in thermal energy= mxc change in temperature mass x specific heat capacity x change in temperature 2x4200x80=672000J 672Kj
An iron has an aluminium plate with a mass of 1.5 kg. calculate the thermal energy stored in the plate when the temperature rises from 20 degrees to 200 degrees. the specific heat capacity of aluminum is 913 j/kg
change in thermal energy= mass x specific heat capacity x change in temperature • 5 x 913 x 180= 246510J • 51KJ
A hot water bottle cools down from 80 degrees celsius to 20 degrees celsius releasing 756000J of thermal energy. Calculate the mass of the water in the hot water bottle. c=4200j/kg
change in thermal energy= mass x specific heat capacity x change in temperature 756000=massx4200x60 756000=252000m m=3 3kg
recap of different energy stores, kinetic energy, gravitational potential energy, elastic potential energy
kinetic energy: energy stored in moving objects gravitational potential energy: energy stored due to an object’s position above the Earth’s surface. It is due to the force of gravity acting on an object Elastic potential energy: elastic potential energy is the energy stored in a stretched spring thermal energy: energy stored due to object’s temperature
The law of conservation of energy:
Energy can be transferred fully, stored or disspated but cannot be created or destroyed.
What is a system
a system is an object or a group of objects