Transcript Work Energy Theorem & KE & PE

Energy
WORK ENERGY THEOREM
• For an object that is accelerated by a
constant force & moves in the same
direction…
• Lets derive this…
Work-Energy Theorem
The work done by the sum of the
forces acting on a body is equal to the
change in the kinetic energy of the
body.
Kinetic energy exists whenever an object which has mass is
in motion with some velocity. Everything you see moving
about has kinetic energy. The kinetic energy of an object in
this case is given by the relation:
KE = (1/2)mv2
m=mass of the object
V=velocity of the object
The greater the mass or velocity of a moving object, the
more kinetic energy it has.
The greater the mass or velocity of a moving object, the more kinetic energy it
has.
A fire truck of mass 16000 kg,
travelling at some initial speed,
has -2.9 MJ of work
done on it, causing its speed to
become 11 m/s. Determine the
initial speed of the fire
truck.
Text Book:
pg 186 # 1,2,4-8
pg 188 #1-8
pg 226 # 5,6,12,13
Potential Energy
• What do we mean by
your potential?
• It is what you are capable
of doing, not what you are
actually doing.
• Maybe you are getting a
C+ in Physics, but you
have the potential to get a
B or even an A
Potential Energy
F
• What is the formula for
Work?
Work = F * d
• So if we are going to lift
the object at a constant
velocity, what is the work
done?
W = FG * h
h
FG
Potential Energy
F
W = FG * h
• But wait, what does FG
equal?
FG = m * g
• So…
W=m*g*h
h
FG
Potential Energy
W=m*g*h
Mass = m
• Work is equal to the
change in energy
• Potential energy due to
gravity equals:
E g  mgh
h
g = 9.80m/s²
• Measured in Joules
Need a volunteer to lift an object
• Gravity acts vertically, we will use ∆y for
the magnitude of the displacement.
• The force applied to the object to raise it is
in the same direction as the displacement
& has a magnitude equal to mg therefore,
W= (F Cos θ)∆h
W = mg(Cos θ)∆h …where θ is 0
• Gravitational Potential Energy (G.P.E) is a
relative quantity in which the height of an
object above some reference level must
be known.
• The work done in increasing the elevation
of an object is equal to the change in the
G.P.E
∆Eg = mg∆h
Conservative & Non- Conservative
Forces
• A force is conservative if the kinetic energy of a
particle returns to its initial value after a round
trip (during the trip the Ek may vary). A force is
non-conservative if the kinetic energy of the
particle changes after the round trip (Assume
only one force does work on the object).
Gravitational, electrostatic and spring forces are
conservative forces.
• Friction is an example of a non-conservative
force. For a round trip the frictional force
generally opposes motion and only leads to a
decrease in kinetic energy.
Potential Energy
ΔEg = mgΔh
• This is only true for
distances close to Earth,
and where g is constant
• Eg is not dependent on the
path taken, only cares
about height!
• Does not care about
horizontal distance, cuz
gravity doesn’t care
Only care about height
Potential Energy
Potential energy doesn’t
only have to come from
Height.
• Electrical potential energy
• Elastic potential energy
• Chemical potential energy
Text Book:
pg.191 #1-6
pg. 194 # 1-5, 7