Page 69- What is Energy?

Quotes

"There is a fact, or if you wish, a law, governing ALL natural phenomena that are known to date. There is no known exception to this law -- it is exact as so far as we know. The law is called the conservation of energy."

"It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same.

Questions:

Can you picture ENERGY like $$?

Money doesn't, by itself, mean anything, but its power lies in what it can do. Money comes in different forms (cash, checks, credit cards, debit); you can have money or owe money, just like you can have excess energy stored, or be in a situation where your energy level is negative.

Pages 70-71- Blocks as an Analogy for Conservation of Energy

Quotes:

"Being extremely curious, and somewhat ingenious, [Mother] invents a scheme! ... She discovers the following [formula]."

"In the gradual increase in the complexity of her world, she finds a whole series of terms representing ways of calculation how many blocks are in places where she is not allowed to look. As a result, she finds a complex formula, a quantity which has to be computed, which always stays the same in her situation."

"The most remarkable aspect that must be abstracted from this picture is that there are no blocks."

Questions:

How many different storage locations do the blocks appear within?

What others might be possible in the room, and how could you discover them?

There are two analogies working here - one for the blocks as energy. What is the other?

Key terms:

Page 71-72 Energy (cont.)

Quotes

" The analogy has the following points:

"First, when we are calculating the energy, sometimes some if it leaves the system and goes away, or sometimes some comes in. In order to verify the conservation of energy, we must be careful that we have not put any in or taken any out.

" Second, the energy has a large number of different forms, and there is a formula for each one.

Questions:

Like the block analogy, sometimes these energy forms are obvious and visual, and other times they are hidden, and must be identified through other means.

How will we see examples of the energy types listed to the right?

Key terms:

gravitation energy

kinetic energy

heat energy

elastic energy

electrical energy

chemical energy

radiant energy

nuclear energy

mass energy

Pages 72-73 - Gravitational Potential Energy

Quotes

"...a great deal about nature can be extracted from a few facts and close reasoning."

"A machine that we actually use can be, in a sense, almost reversible; that is, if it will lift [one way], it will also lift [the other way].

Questions:

Why can't perpetual motion machines exist?

Examples of weight-lifting machines:

• levers
• pulleys
• elevators
• escalators
• people!

Key terms:

Sadi Carnot = developer of the theory in thermodynamics of the limit to the efficiency of any type of engine; the Carnot cycle is THE most efficient engine possible for any configuration.

Pages 74-79 - Reversible Lifting Machines

Quotes:

"This, then, is a very remarkable observation because it permits us to analyze the height to which different machines are going to lift something without looking at the interior mechanism. "

"We call the sum of the weights times the heights gravitational potential energy - the energy which an object has because of its relationship in space, relative to earth."

"Gravitational Potential Energy = weight x height"

"Change in Energy = force x distance (moved)"

"The only problem is that perhaps it is not true... Some of the assumptions may be wrong, or we may have made a mistake in reasoning, so it is always necessary to check. It turns out experimentally , in fact, to be true."

Questions:

What assumption does Feynman make when he supposes rolling the balls horizontally from the rack to the shelves takes no energy?

If you equate (4.3) and (4.4), what is weight?

How does using the screw jack (fig 4.5) follow the same idea as using the lever, or the inclined plane?

Key terms:

potential energy = energy "which has to do with location relative to something else." You can have gravitational potential energy, electrical potential energy, magnetic potential energy, even atomic and nuclear potential energy.

Simon Stevinus = Dutch Mathematician

Pages 80-81 Kinetic Energy

Quotes:

"[W]e can easily see that in the motion at the bottom must be a quantity of energy which permits [the pendulum mass] to rise to a certain height, and which has nothing to do with the machinery by which it comes up or the path by which it comes up."

"Kinetic Energy = [weight x (velocity)^2] / 2g"

Questions:

Why does Feynman say that the formulae for gravitational potential and kinetic energies are incorrect?

Key terms:

"g" in the formula for kinetic energy is the earth's gravitational constant of acceleration at the surface of the planet = 9.8 meters/s/s, about 32 feet per sec of velocity change/per sec

Pages 81-84 - Other Forms of Energy

Quotes:

"Elastic energy is the formula for a spring when it is stretched."

"Ordinarily when things roll, there is bumping and jiggling because of the irregularities of the material, and the atoms start to wiggle inside. So we lose track of that energy; we find the atoms are wiggling inside in a random and confused manner after the motion slows down. There is still kinetic energy, all right, but it is not associated with visible motion."

Pages 84-85 - Other Conservation Laws

Quotes:

"There are two other conservation laws which are analogous to the conservation of energy. One is called the conservation of linear momentum. The other is called the conservation of angular momentum."

"There are three other conservation laws, that are exact so far as we can tell today, which are much simpler to understand because they are in the nature of counting blocks."

Questions:

What common examples can you give to illustrate the other conservation laws (momentum and angular momentum)?

Key Terms:

invariance principle = a basic symmetry in physics that is always associated with a conservation law. Saying that the laws of physics are the same whether you are moving or stationary is one invariance. Saying that they are the same no matter when you do the experiment in another.

Page 85- Energy Today

Quotes:

" Our supplies of energy are from the sun, rain, coal, uranium, and hydrogen."

"Nature has conservation of energy, but does not really care; she spends a lot of it in all directions."

"Therefore it is up to the physicist to figure out how to liberate us from the need for having energy. It can be done." (1961)

Questions:

Key terms:

entropy = a measure of the "disorder" of a system of particles (or things.) In normal (real) nature, entropy of an isolated system always increases; at best it can stay the same in a perfect reversible process. You can clean your room (or office! :), and afterwards it will be more ordered, but including the energy you spend doing the cleaning, and the heat you generate, the entropy of the system (you + office) has increased.