Download L29-Energy

Class Notes
31 October
• Pick up HW / Turn in new HW
 #1, 2, 3: Use Kepler’s third law.
 #2b: T needs to be in seconds!
 #4a: Ratio is compared to total dist
 Today : Don’t leave out the cosq!
• Today:
 Kinetic and Potential Energy
 Non/Conservative Forces
If 1 Joule of work is required to stretch
a spring 1 cm, how many Joules of
work are required to stretch the spring
another cm?
.
5
a.
b.
c.
d.
e.
f.
0
1
2
3
4
other
3
3
1
1
0
a.
b.
c.
d.
e.
f.
Could you give an example where the
work done by a force is positive,
negative, and zero?
a.
b.
c.
d.
Yes, all three, many
examples.
I could think of a few.
Maybe one if I were
awake.
No I’m completely
baffled.
7
4
2
0
a.
b.
c.
d.
Energy
• Definition: Loosely, the capacity
or ability to do work
• Various types of energy
 Chemical, Electrical, Thermal,
Solar, Nuclear, …
• Mechanical Energy (ME)
 Kinetic Energy (KE)
 Potential Energy (PE)
Kinetic Energy
• Define (translational) Kinetic
Energy as the energy associated
with motion
KE  ½ (mv2)
• Work–Energy Principle
 The net work done on an object is
equal to the CHANGE in its kinetic
energy.
Wnet = DKE
Example
• A 10.0 N force pulls a 2.00-kg
object at a 60.0° angle across a
frictionless surface. Find the
work required to go from 0 to
5.00 m and final speed if it starts
with a speed of 3.32 m/s.
W = 25.0 J, vf = 6.00 m/s
Which has more energy?
Recall that energy is the
ability to do work!
The higher one, of
course. External work
was required to move it
there. How much?
PEg = mgy
Potential Energy
• Rather than talk about the
amount of work done on an
object (that may have occurred
previously) let us realize that this
object has the ability to do work.
That is, it has stored energy! 
PEg = mgy
Potential Energy
• Potential energy is stored energy
associated with position or
configuration.
• This implies that you must
choose a reference, a zero point!
• Only a DPE matters (that is has a
physical significance)!
• By the same logic, for a spring
PEspring = ½kx2
Homework
• Read chapter 6, sections 6-7
• Do problems on WebAssign #26
• Do problems using the workenergy principle where
appropriate.
• HW#27: Test Rework due
Thursday, 5:00 PM