Mechanics - Energy, Work and Power Flashcards
Name active energies
Forms of energy are ACTIVE if their effects are easily seen or detected.
Kinetic energy is the ability of an object to do work due to its movement
- Kinetic energy - energy of an object when it is moving
- Light energy
- Heat energy
- Sound energy
Name potential energies
Energy is potential when energy is stored and only has an effect when they are changed to active energy.
The ability of an object to do work due to a change in position or shape.
- CPE: changed to active energy by chemical reactions. eg food, fuels and explosives
- EPE: energy stored when a material is stretched or compressed. This energy is changed to active energy when the TENSION is released.
- GPE: the energy an object gains when it is lifted up. Changed to active energy when the object falls down.
- Electrical energy: energy that is stored in a battery source which is changed to active energy when the circuit is switched on.
- Nuclear energy: energy that is stored in a nucleus. This energy is changed to active energy in nuclear reactors or atomic bomb explosions. Solar energy (light, heat and other forms) is produced when nuclear reactions happen within the Sun.
What is energy measured in?
J, joules.
1kJ = 1000J
1J is the amount of energy needed to apply a force of 1N through 1metre
What is gravitational potential energy?
The ability of an object to do work due to its height.
Potential energy an object has due to its height.
When the height of an object changes, its gravitational potential energy changes.
Formula for gpe?
E_p = mg△h
E_p = J m = kg of object lifted △h = m
VERTICAL change in height must be used.
An example of energy transformations in a dam
Water in the dam has a huge amount of gpe. When the water falls the gpe is transformed into kinetic energy. The moving water pushing on the turbines, making them spin generates electrical energy.
What is kinetic energy?
An object’s ability to work due to its movement. The energy an object has because of its movement (because it’s moving).
What is the formula for kinetic energy?
E_k = 1/2 x m x v^2
mass = kg v^2 = ms^-1 E_k = J
E_k is proportional to mass and velocity because as mass and/or velocity increases, E_k increases as well.
If the speed of an object doubles (mass remaining same), then the E_k is FOUR TIMES the original amount. This is because E_k relates to the SQUARE OF THE SPEED.
So basically:
- If 2v^2, then 2^2 = 4, so 4 x original E_k
This is important for road safety, because if a car doubles its speed it will have four times the Ek meaning that the car travelling twice as fast will take four times the distance to stop/brake.
Example of transformation of energy in a garage door
pg 42 of textbook
A garage door often have a spring to make it easier to raise the garage door.
When the door is raised it gains gpe. The spring is not stretched (no tension) so no elastic potential energy
When the door is shut it loses its gpe. The spring is now stretched (has tension) and gains elastic potential energy which makes it easier to raise the door again.
Example of transformation of energy in a battery / torch
A battery contains chemicals (CPE) which react together to give the battery its voltage (electrical potential energy). Ina torch, the electrical potential energy is transformed into light energy so the torch shines.
What is the law of conservation of energy?
Energy cannot be created or destroyed, merely transformed from one form into another.
What is wasted energy?
Where kinetic energy is one of the energies being transformed, friction will be acting in some way and will cause part of the energy to be transformed into heat. This heat will be lost into the surroundings, so is regarded as WASTED ENERGY. or non-useful energy.
What is work?
Work is done when a force makes an object MOVE from one place to another. When work is done, energy is transformed from one type into another.
The energy transformed = work done
Therefore, both are measured in J.
The change in energy = work done.
Formula of work done
Wd = force x distance
F = N, newtons d = distance in metres wd = work done in J, joules
The distance must be the distance in the direction of the force.
How is work done?
Work is done whenever an object is lifted. The work done is the ENERGY CHANGED.
When an object is lifted, the energy change is the gravitational potential energy gained by the object.
Wd = E_p, fd = mgh, where d = h
so, f = mg. ie force needed to lift object is equal to the weight of the object.
