Mechanics and Materials (5): Work, Energy and Power Flashcards
What are the 4 types of energy stores that you need to know about?
- Gravitational potential - the position of objects in the gravitational field
- Chemical - In bonds/batteries
- Kinetic store - moving objects
- Thermal store - hot objects
- Elastic store - objects compressed or stretched
What are the 3 types of energy transfers that you need to know about?
- Radiation (light, EM radiation etc.)
- Electrically
- Heat (hot object to a cold object vie conduction, convection or radiation)
- Mechanically (work done by one object to make another move)
What is the unit of energy?
Joule, J
State the principle of conservation of energy
Energy cannot be created or destroyed, when energy is transferred the total amount of energy before is equal to the total amount of energy after
Work done on an object = …?
energy transferred to that object
What is the equation for the work done on an object that has a force, F applied to it for a distance, d?
W = Fd
where d is the distance moved in the direction of the force
What is the equation for the work done on an object when a force is applied to it at an angle?
W = Fscos(theta)
Where Fcos(theta) is the component of the force applied in the direction that the object is moving
What does the area under a force-distance graph when a constant force is applied represent?
The total work done on that object
- As W = Fs
How do you calculate the work done from a force-distance graph where the force applied to the object is varied?
- Split the area under the graph into rough strips
- Determine the work done for each of the strips using W = F(delta)s
- Add the individual work done’s to obtain total work done on the object by the force
State Hooke’s Law
The force needed to stretch a spring is proportional to the extension of that spring
What is the extension of a spring?
The difference in from the initial length of the spring to its length after the force has been applied
What is a force-extension graph the same as?
force-distance graph
What is the area under a force-extension graph equal to?
The work done applied to the spring
For a spring that obeys Hooke’s Law, how to you calculate the work done on the spring?
W = 1/2F(delta)L
What is the equation for the kinetic energy of an object?
Ek = 1/2mv^2
When does the equation for kinetic energy of an object not stand?
When the speed of the object is close to the speed of light
What is the definition of potential energy?
The energy of an object due to its position
What is the equation for GPE?
mg(delta)h
When does the equation for GPE not stand?
When the change in height, h is much smaller that the radius of the body that produces the gravitational field strength -> otherwise the value of g will vary across the height that the object travels
What is the relationship between GPE and KE?
KE = loss of potential energy/GPE
What is the equation for the work done to overcome friction and air resistance for an object that goes from a higher height to a lower height?
W = GPE - KE
- the difference between the two values is the energy lost to friction and air resistance
What is the definition of power?
The rate of energy transfer per unit time
What is the unit of power?
Watts, W
What is the equation for power (energy transfer)?
P = (delta)E/t
What is the equation for power (force doing work)?
P = (delta)W/t
What is the equation for the power output of an object driven by a constant force, F at a velocity, v? How do you derive it?
P = Fv
P = W/t
W = Fs
P = Fs/t
P = Fv
What is the equation for the motive power,P for a car?
Motive power = energy per second wasted due to resistive forces + the gain of kinetic energy per second
What is the equation for the output power of a machine?
P = Fv
F = output force of the machine
v = speed of the object
What is the definition of useful energy?
Energy transferred for the desired purpose
What is the equation for the efficiency of a machine?
Efficiency = useful energy transferred by the machine OR work done by the machine/energy supplied to the machine
What is a more efficient bulb than a filament light bulb?
Fluorescent lamp
What are three sources of renewable energy?
- Wind power - wind turbines
- Water power - tidal power station generates hydroelectricity
- Solar power - solar panels
