Forms of Energy

Can you identify a push or pull or some other way that work is being done for each of these kinds of energy?

Brought Newton's cradle and pendulum to class to talk about exchange of grav and KE.

Showed students roller coaster problems.

Kinetic energy

Moving objects do work in the process of coming to rest.

In the case of crashes, most of the kinetic energy is used to push metal and other material into new shapes.

Imagine two identical cars going 30mph and 60mph.

Which one has more kinetic energy?

$$K.E. = \frac{1}{2}mv^2$$

This means energy has units of kg*m/sec^2?? Try Wolfram alpha...

What if two cars have the same velocity (speed and direction). If they 'crash'=touch will there be any damage?

So energy is in some sense relative.

Smart rocks & brilliant pebbles

You don't need a bomb or fancy lasers to protect yourself from incoming missiles. It turns out that a handful of rocks will do the job quite nicely. This is an ABM - anti ballistic missile - system.

Warheads are moving at around 7 km / sec, that is $s=$7,000 m/sec. From the point of view of the missiles, they see the rocks coming at them with a speed of 7 km/s, and with a kinetic energy (for each gram of rock) of

KineticE = $\frac{1}{2}(0.001)$ kg $\cdot (7000{\rm \ m/sec})^2=25000$ J$\approx 6 $ Watt*hrs

This is about 6 times the energy in a gram of TNT!

One improvement on just throwing up rocks is to give them a computer guidance system to stay in the path of the missiles if they re-adjust.

Mention that this is why the force of air resistance is proportional to speed squared

Gravitational energy

Water drops down from the sluice and pushes the wheel.

Elastic energy


Objects that have been stretched, deformed, or compressed can do work when they snap back to their resting shape.


microscopic motionWhen we talked about the atomic picture, we said:

"When atoms go faster on average, the temperature increases."

Now we can be more precise:

Temperature measures the average kinetic energy of a collection of molecules.

As we shall see later, we're talking about the disorganized motion of molecules.

This simulation (Java). suggests the microscopic picture: when temperature goes up when the average speed of a collection of molecules increases.



How might we extract work from disordered molecular motion, a.k.a.....

Thermal energy (heat)

The hot air inside this balloon is rising as a bubble within the cooler air around it, and lifting weight against gravity.


Electric energy

A van de graaf generator shoves electric charges onto the surface of your body.

Magnetic energy

An (electro-)magnet can interact with iron or other magnets.

magnet lifting roll

Chemical energy

Rearranging chemical bonds...


Nuclear energy

Mushroom cloud over NagasakiThis one resulted from the bombing of Nagasaki.


Radiant energy

sunlight through redwoods


Energy is forever

The remarkable thing about energy is that we never seem to lose any of it.

Law of Conservation of Energy

The total energy of all the participants in any process remains unchanged through that process. That is, energy cannot be created or destroyed.

Energy can be transformed (changed from one form to another).

Energy can be transferred (moved),

...but the total amount always remains the same.

Work-energy principle

Where does work (= Force `times` distance) fit in?

Work is an energy transfer. Work reduces the energy of the system doing the work and increases the energy of the system on which work is done, both by an amount equal to the work done.

The discovery of the neutrino

alphaErnest Rutherford describes alpha radiation (~1899) given off when Uranium decays to Thorium.

An alpha is a small nucleus with two protons and two neutrons.

The alphas emitted in this decay have a unique kinetic energy. So, we conclude that there's a unique "binding energy" when the alpha is bound to the Uranium nucleus.

[Hey wait, what about the kinetic energy of the big nucleus??]

Lise Meitner

Lise MeitnerLise Meitner was one persistent woman.

  • 2nd woman to ever obtain a doctoral degree (1905) from the U. of Vienna.
  • Moves to Berlin where Max Planck makes an exception to allow a woman to attend his lectures. (The next year, she would become his assistant.)
  • Works for a time without a salary with Otto Hahn who would later receive a Nobel prize for the discovery of nuclear fission.

Beta decay

beta decayOther nuclei give off beta particles (later found to be electrons or positrons)

Meitner and Hahn measured in 1911 that, unlike alpha decay, the beta particles had a continuous distribution,like this:

Was the binding energy not unique? Or maybe energy wasn't being conserved in these new nuclear processes?

Pauli's suggestion

Wolfgang Pauli suggested ~1930, that maybe there was a new particle that no one had ever heard of before that was also being emitted.

KE `beta` KE ??
10.0 J 5.0 J
11.5 3.5
4.0 11.0

When you added up the kinetic energies of the beta particle and ?? the sum would always be the same = binding energy.

He had no idea what this particle was, and no one had any other evidence of it.

Observation of the neutrino

In 1956 Clyde Cowan, Frederick Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire reported the detection of this particle, since dubbed the neutrino.

The energy of the neutrino complemented the kinetic energy of the beta in beta decay in precisely the way that Pauli had suspected.

The picture at left shows a rare direct observation (1970) of a bubble chamber event in which a neutrino (not seen) strikes a proton (seen rebounding, top-most track) converts to a muon, and also creates a pi-meson (lower track).

Suggested exercises

Conceptual Exercises from Chapter 6: 1, 3, 4, 6, 7, 10, 11, 16