AQA A Level Physics復習筆記4.6.5 Kinetic & Gravitational Potential Energy
Kinetic & Gravitational Potential Energy
Kinetic Energy
- Kinetic energy is the energy an object has due to its?motion?(or velocity)
- The faster an object is moving, the greater its kinetic energy
- When an object is falling, it is?gaining?kinetic energy since it is gaining speed
- This energy transferred from the gravitational potential energy it is losing
- An object will maintain this kinetic energy unless its speed changes
Kinetic energy (KE): The energy an object has when its moving
Worked Example
A body travelling with a speed of 12 m s-1?has kinetic energy 1650 J.If the speed of the body is increased to 45 m s-1, what is its new kinetic energy?
Gravitational Potential Energy
- Gravitational potential energy (GPE) is energy stored in a mass due to its position in a gravitational field
- If a mass is?lifted?up, it will?gain?GPE (converted?from?other forms of energy)
- If a mass?falls, it will?lose?GPE (and be converted?to?other forms of energy)
- The equation for gravitational potential energy for energy changes in a?uniform gravitational field?is:
Gravitational potential energy (GPE): The energy an object has when lifted up
- The potential energy on the Earth’s surface at ground level is taken to be equal to 0
- This equation is only relevant for energy changes in a?uniform gravitational field?(such as near the Earth’s surface)
GPE v Height Graphs
- The two graphs below show how GPE changes with height for a ball being thrown up in the air and when falling down
Graphs showing the linear relationship between GPE and height
- Since the graphs are straight lines, GPE and height are said to have a?linear?relationship
- These graphs would be identical for GPE against time instead of height
Worked Example
To get to his apartment a man has to climb five flights of stairs.
The height of each flight is 3.7 m and the man has a mass of 74 kg.
What is the approximate gain in the man's gravitational potential energy during the climb?
A.? ? ?13 000 J? ? ? ? ? ? ???B.? ? ?2700 J? ? ? ? ? ? ? ?C.? ? ?1500 J? ? ? ? ? ? ? ?D.? ? ?12 500 J
Exam Tip
GPE:
- This equation only works for objects close to the Earth’s surface where we can consider the gravitational field to be uniform.
- At A level, you might have to consider examples where the gravitational field is not uniform, such as in space, where this equation for GPE will not be relevant.
KE:
- When using the kinetic energy equation, note that only the speed is squared, not the mass or the ?.
- If a question asks about the ‘loss of kinetic energy’, remember?not?to include a negative sign since energy is a scalar quantity.
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