Goal: To understand Energy

1
Goal To understand Energy

• Objectives
• To learn about What energy is
• To learn about Work
• To understand the relationships and differences
  between Potential and Kinetic energy
• To understand the relationships between Work and
  Kinetic energy
• To understand the transfer of energy

2
What is energy?

• Energy is what is needed to do stuff.
• Energy is required to heat.
• Energy is required to power an AC to cool.
• Energy is needed to move things
• Energy is needed to build things
• Energy lights our lights and powers our TVs.
• Energy drives our cars.

3
Potential and Kinetic Energy

• All energies are comprised of Potential and
  Kinetic Energies.
• Kinetic Energy energy of motion
• Potential Energy stored energy

4
Work

• One way to measure the use of energy is by
  measuring work (work is an energy).
• Work Force distance
• Net Work Net Force Net distance
• Units of Work/Energy
• Work Force distance Newton m
• Newton m Joule

5
Batman

• Batman slides across the lake at a constant
  velocity.
• If he travels 800 m horizontally and has weight
  of 900 N then what work has been done on Batman?

6
Pushing the house

• You push a icehouse across a frozen lake (assume
  you can neglect friction here).
• Your friend, who wants to do something else,
  pushes back.
• You push with a force of 300 N forward.
• Your friend pushes with a force of 200 N
  backwards.
• A) What is the work that you have done if you
  push the icehouse 30 m forward?
• B) What is the work that your friend has done if
  you push the icehouse 30 m forward (note
  direction, this will have an effect on the work
  even though work does not have direction)?
• C) What is the net work done on the ice boat?

7
Push start

• Where is this work going to go?

8
Work represents

• The work represents either the change in kinetic
  energy or the change in potential energy on an
  object.
• Positive work means that the object speeds up or
  that it goes up.
• Negative work means that you have stolen energy
  which means the object gets slower or goes down.
• Friction for example is an energy thief because
  its force is always negative.

9
Work vs. Kinetic energy

• Work Force distance
• Force mass acceleration
• Distance ½ acceleration time time
• So,
• Work mass acc ½ acc time time
• Acceleration time velocity
• Therefore,
• Work ½ mass velocity velocity
• Notice that the above is the equation for Kinetic
  energy!

10
Lets prove it!

• You push a box across a frictionless floor.
• You apply a 300 N force to the 20 kg box.
• You push the box for 5 m.
• A) What is the work you have done to the box?
• B) What is the acceleration on the box?
• C) How long does it take to push the box 5 m (we
  have distance and acceleration)?
• D) Using v at, what velocity is the box
  traveling when you get to the 5 m mark.
• E) Now, find the Kinetic energy of the box
• (KE ½ mass velocity velocity)

11
Energy transfers

• Energy is not created or destroyed, but it is
  always moving from one form to another.
• Lets examine gravitational potential transfers to
  kinetic energy and back.

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Gravitational Potential Energy

• Work Force distance
• What force do you need to overcome gravity?

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Gravitational Potential Energy

• Work Force distance
• What force do you need to overcome gravity?
• Work mass gravity distance
• The distance is the height,
• So, Work mass gravity height
• This is called Gravitational Potential Energy

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Example if we have time

• You are holding a 2 kg ball at a height of 0.6 m.
  What is the Gravitational potential energy of
  the ball?
• You drop the ball. What happens? By how much
  will the gravitational potential of the ball
  change?

15
Make up upwards example if time permitted
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Conclusion

• We have seen how energy is used, transferred
  between forms, and why it is useful.
• We have discovered how to find work, power,
  kinetic energy, and potential energy.