Energy

1
Energy
Points to take away

• The total energy of a closed system is
  conserved, or constant in time.
• Closed systems do not interact with any other
  parts of the universe
• systems can be made of two particles
• the entire universe itself is a closed system
• Energy can be converted, but never destroyed.

2
Types of Forces

• There are two kinds of forces
• conservative
• nonconservative
• The work done by conservative forces does not
  depend on the path taken, only the endpoints.
• gravity
• springs
• W ?U Uf - Ui

3
Types of Forces

• Friction is a perfect example of a
  nonconservative force.
• One way to think about this is that friction
  forces ALWAYS oppose motion.
• That means that the work done is always negative.

f
?x
4
Example

• The 10 kg block below performs two motions. It
  moves up the ramp from A to B. Then it moves
  down the ramp from B to A. What is the work done
  by gravity from A to B? From B to A? What is
  the total work by gravity?

B
A
T30o
T
5
2 Person Discussion

• For the same object and motions, include
  friction. µk 0.40.
• what is the work done by friction from A to B?
• what is the work done by friction from B to A?

B
A
T30o
T
6
Total Mechanical Energy

• In general, we will deal with conservative
  forces.
• In these cases, the sum of the kinetic and
  potential energies (the total mechanical energy)
  is conserved.
• Ei Ef
• ½ mvi2 Ui ½ mvf2 Uf

7
Energy Conservation Simulations

• Springs
• Skate Park

8
Energy Nonconservative Forces

• When nonconservative forces like friction are
  present, total mechanical energy is NOT
  conserved.
• The work done by nonconservative forces reflects
  the mechanical energy lost.
• The total energy of the system is the same, but
  the energy lost has been transformed into a form
  that is neither kinetic nor potential (think of
  the heat you get from rubbing your hands
  together).

9
Power

• Power is the rate at which work is done.
• How long does it take to change energy?
• If you climb a ladder to get from the ground to
  the second floor of a building while I take
  stairways, your power will be greater than mine.
• changes in energy are equal ?Umgyf mgyi
• time it takes for you to change your energy is
  shorter

10
Power

• The metric unit of power is the watt
• 1 W (watt) 1 J/(1 s)
• ltPgt W(work)/?t
• ltPgt (FcosT ?x)/?t
• ltPgt FcosT ltvgt
• What does a kilowatt-hour measure?

11
NEXT

• Sections 6.1 6.2 Momentum
• linear momentum
• impulse
• connecting force, momentum, and energy