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LPH 105 W15 6:intro

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Richard Datwyler

on 17 May 2016

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Transcript of LPH 105 W15 6:intro

Work and Energy
constant force
variable force (graph only)
Energy (mechanical)
Kinetic (moving energy)
potential (energy of location)

Changes in energy
conservative (mechanical energy trading places)
dissipation (mechanical goes into heat - 'loss')
How long (time it takes to change the energy)
OR from a good picture
Work is done on an object with a force acts on it for a distance.
If W = Fd

[J] = [N][m]

Joule = [J]

How much work is done carrying a box across a room?

how about up the stairs?
Lowering it from a building?
Every force can do work.
There will be a net work on an object.
If I pull a 10 Kg block 5 m across the floor, with force of 9.8 N, against friction having a coefficient of .1, what is the net work done?
What if I pulled at 19.6 N?
How would you describe the motion of this first example?

Where did the 'extra' work go in the second example?
Newton would say, lets look at
the forces, the pull was bigger than
the friction, thus there was a net
Force, and thus an acceleration.

How much energy is required to speed up a 1000kg car up to 20 m/s ?
As a ball is thrown into the air, it starts out with kinetic energy. As it gets higher, it eventually stops, but it is at a new location 'above' the earth, with the 'potential' to fall back down.
It seems that we have related two types of energy

We now define
Potential energy due to gravity
Also as before we can talk about
how Work is associated with PE.
Note: we choose 'y', in with our coordinates!
Also need the potential of a spring
If I compress a spring, what happens?
Force changes
If we want the Work we can't just W=Fd
Rather it is the area under a graph F vs d
Area = .5 (kx) x
Sum up Spring


F = -kx
F = -kx
Conservative Force make conservative energy
Air resistance
All conserved then:
This brings us to the point of solving

There will be two positions of interest.
At both positions we will look at the
Kinetic and potential energies
and set them equal

This is the conservation of Energy
1. Identify initial position
2. Find final position
3. Where is y=0
4. Find initial energy
a) is it moving
b) is there a spring
c) y=?
5. Find final energy
a) is it moving
b) is there a spring
c) y=?
d) - ( is there dissipation)
Dissipation takes the form of friction or air resistance, and it comes in the equation as the work done by that force.
Power is the rate at which work is done, or the
rate at which energy is transformed
This can be read both ways
'power is work done(energy transfered) in a period of time'
'power is what is needed to maintain an average velocity against
dissipative forces'
The units for Power is a Watt
Examples to illustrate.

I may have the energy to walk up to the Kimball from here
this is a work to be done.
I may not be able to run up to the Kimball in 20 seconds.
This would require a different amount of power.

A car may have the energy to go a distance, but lack the power
to do it in a given time.
think 0 to 60 in 5 seconds vs. 30 seconds.
"I don't yet understand conservative and non-conservative forces."
"How can work be both positive and negative?"
"Where does the cosine of theta in the work equation come from?"
"Total energy is never increased or decreased, it is just changed."
"I want to learn more about the concept of increasing and decreasing in energy. What makes a change rather than an increase or decrease."
Number 3 from the quiz can you explain more of that how does that work.
Full transcript