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PH 121 10.1-10.2

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Richard Datwyler

on 15 June 2018

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Transcript of PH 121 10.1-10.2

We use and abuse the term energy everywhere

What is your favorite form of energy?
The conservation of energy is one of the great fundamentals
of physics.

Conservation of momentum was cool, but it seemed to
be a bit contrived. We said it was conserved so long as we
squinted our eyes, and looked the other way.

Energy is much, much more elegant than that.
We are going to spend nearly the rest of the semester
talking about energy.

As we go, we will add to our understanding of Energy and
how it is transferred, and ultimately conserved.
In following the book in previous editions, I give the parable of the penny.

Story time.
Key points.
money in and out
Two types to be accounted for
conservation when sick at home
internal conversion didn't change wealth
This edition lost it
the first edition was 10 dollars being used
to buy different things.
Yet the net value was still 10 dollars.
The analogy to money is pretty good,
In energy we have
energy going into system and out
Internal changes
two main types
As noted by the author, although the great
significance of conservation of energy,
it wasn't recognized until less than 200 years ago.
Note this equation for objects in free fall, a=g.
Now times by m and divide by 2
This has been a fun quick and dirty way to get energy conservation
but it is not general, it was only for free fall.
try this instead:
Lets take a closer look at the derivative term.
And make the substitution.
Now because our differential equation is divided
into the separate parts, we can integrate both
sides separately.
Low and behold, they are the same.
With this very nice, and cryptic equation
we specify the parts parts.

Here are both kinetic and gravitation potential energy.
Just like our money analogy
we can change our type of
form one way to another.
Also note that the units are
that we can choose our zero
point for problem.
Lastly the author uses bar charts
to show conservation of energy
With what minimum speed must you toss a 100 g ball straight up to just touch 10-m-high roof of these gymnasium if you release the ball 1.5 m above the ground? (solve using energy)


energy conservation.
"Energy as I understand it can leave the atmosphere through heat so what happens to that energy once its in space?"
"So if energy can only be changed, so would the heat from a fire turn to cold energy once the fire died or how would that work?"
" How is that an object at one position can have two different values for its potential energy?"
" Can we go over potential gravitational energy?"
"Can you go over example 10.1?"
"I don't understand what Energy Bar Charts are supposed to be used for. Could you explain?"
"Could you explain equations 10.10 and 10.2?"
"could explain example 10.2 "
What is the speed when it hits the ground? (if you didn't catch it)
You're driving at 35 km/h when the road suddenly descends 15 m into a valley. You take your foot of the accelerator and coast down the hill. Just as you reach the bottom you see the policeman hiding behind the speed limit sign that reads '70 km/h." Are you going to get a speeding ticket?
12.9 m/s
14 m/s
Full transcript