AQA A Level Physics复习笔记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

5.2.4-Kinetic-energy-equation

5.2.4-Kinetic-energy-diagram

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?

WE-kinetic-energy-answer-image

 

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:

5.2.2-GPE-equation

5.2.2-GPE-diagram

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

5.2.2-GPE-graphs

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

 

WE-GPE-answer-image

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.

 

转载自savemyexams

 

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