Mechanical Energy

1
Mechanical Energy

• Kinetic vs Potential

2
Potential Energy

• A boulder about to fall off a cliff has potential
  energy due to its position. It has the potential
  to fall on the ground below, squashing you. This
  is called gravitational potential energy.
• When an archer pulls the string on a bow, he does
  work on the bow. This creates elastic potential
  energy.

3
Potential Energy

• Potential energy depends on
• The force acting on the object.
• The displacement of the object

4
Conservative Forces

• The force of gravity and the elastic force of an
  ideal spring or bow are conservative forces. When
  work is done moving an object against a
  conservative force, the work may be stored as
  potential energy and can be recovered.

5
Conservative Forces cont

• Examples
• Work is done against gravity to lift a pile
  driver the pile driver now has potential
  energy. When it is dropped, it does work on the
  pile
• If a force is exerted on a spring, work is done
  on the spring and it gains potential energy. It
  can now propel an object (do work on it).

6
Friction

• Friction is not a conservative force. Work done
  against friction cannot be recovered. It is
  converted into another form of energy, usually
  thermal.

7
Potential Energy in a nutshell

• Symbol Ep
• Definition Energy due to position, or
  stored energy.
• Formulae Ep Fd
• Ep mgh
• Units Same as work, joule (J)

8
Example problems

• Look at the example problems on the first page of
  the handout...
• try them and see if your answers match up.

9
Kinetic Energy

• When an object is thrown, a force is applied and
  this force acts through a distance therefore,
  work was done.
• W Fd
• However, when the object is released, it has
  speed and therefore, kinetic energy (the energy
  of motion).

10
Kinetic Energy

• Kinetic Energy is calculated using
• Ek 0.5mv2
• The word kinetic comes from the greek word for
  motion.

11
Potential Energy in a nutshell

• Symbol Ek
• Definition Energy of motion
• Formula Ek 0.5mv2
• Units joule (J)

12
Kinetic Energy Examples

• Look at the example problems on the fourth page
  of the handout...
• try them and see if your answers match
  up.

13

• Take the rest of class to work on the handout.
  It will be due on Wednesday.