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 ½.
- If a question asks about the ‘loss of kinetic energy’, remember not to include a negative sign since energy is a scalar quantity.
