Energy Flashcards
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
a pendulum setup
a fixed point, string and a mass
what is a closed system
in a closed system no energy or matter can enter or leave
pendulum energy transfers
at the top the mass has the maximum store of gravitational potential energy. as the pendulum swings, this is transferred to the kinetic energy store. the mass has the maximum kinetic energy store at the bottom of the spring. why does the mass have the maximum kinetic energy store at the bottom- because thats where its moving at the fastest speed. as the mass swings back up, the kinetic energy store is transferred into the gravitational potential energy store again.
what is the problem with the pendulum model
friction has not been considered. as the pendulum swings there is friction in the fixed point. there is also friction as the pendulum passes through air particles. friction causes energy to be transferred to thermal energy stores. the fixed point and the air around the pendulum gradually get warmer. these stores of thermal energy are less useful- energy has been dissipated. this causes pendulum to gradually swing with less energy and it will eventually stop
how can we reduce unwanted energy transfers?
-reduce friction by using lubricants on moving parts. in the case of the pendulum we could use oil around the fixed point to reduce friction around it. or we could also remove the air particles from around the pendulum
Energy transfers- bungee jumping
at the start of the jump all of the energy in the system is the store of gravitational potential energy. As the jumper falls, energy is transferred from the gravitational potential energy store to the kinetic energy store. when the bungee rope starts to tighten, the kinetic energy store is now at its maximum. when the rope is fully extended the kinetic energy store is zero because the jumper is not moving. at this point, all of the energy has been transferred to the elastic potential energy store. when the bungee jump recoils, energy is transferred from the elastic potential energy store back to the kinetic energy store. during the ascent, energy is transferred from the kinetic energy store back to the gravitational potential energy store. at the top of the ascent all of the energy is now in the gravitational potential energy store.
why does the jumper never return to their original position
jumper never returns back to original position because energy is dissipated as thermal energy this is due to friction with air particles as jumper swings through the air. also due to stretching effects in the bungee rope which is not fully elastic. bungee ropes are not fully elastic which is why some thermal energy is always dissipated during stretching and recoil.
Work done by force what is the definition
-work is done when energy is transferred from one store to another