Work-energy principal

An object can gain or lose energy when something else (another object) does work on it (or has work done on it).

Work = Force $\times$ distance

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.

Energy conservation


We've seen how to analyze the change of energy for a single object: from GravE to KE, assuming that the overall amount of energy is conserved for a single object.

Bringing in the work/energy principal, that object could also transfer energy to another object.

So a broader way of stating this principal of conservation of energy would be...

The 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) from one object to another,

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

Energy is forever

Back to dropping books...

Where does the energy go when the book hits the floor?

Kinetic energy of atoms

Friction is a transfer of energy into the kinetic energy of atoms

Temperature is proportional to the average kinetic energy of moving atoms

Other forms of energy

Elastic energy

Electrical, chemical, nuclear...

Exercises

Read Hobson, Chapter 6.4-6.6

Conceptual Exercises in Chapter 6: 3, 4, 7, 11, 15, 16, 19, 20, 21, 22, 33, 36

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