[yor_on]

It's a pleasure reading you all.
Thank God (or someone else? All depending..) That TNS exist

[RubyPerkins]

Thanks sharing the concept of work and energy.

[yor_on]

Is energy time?
Time expended?

No

If not, what came first?

[PmbPhy]

I can't get the actual concept or meaning of various forms of energies . Actually what is energy ?

See - http://www.newenglandphysics.org/physics_world/cm/what_is_energy.htm

My textbook defines energy as capacity to do work.

In my opinion that's a poor definition of energy. It omits the defining property that energy is always conserved.

I also want to know what is work.

If the force is constant and the object on what you're pushing moves in a straight line then the work is the product of force times distance. In general its the integral of force times dl where l is the infinitesimal distance moved.

Actually, why do we need these quantities?

Because it makes things simpler.

[PmbPhy]

For example, a given mass of fuel can be converted into thermal energy (heat) by burning it, but it won't do any work.

Heat can be used to do work. One needs only to transfer the thermal energy to a gas which will increase its pressure which can them be used to cause an expansion which can do work.

Work and energy are expressed in the same units, so you might prefer to think of work as mechanical energy or useful energy.

EM radiation is not mechanical energy but can be used to do work.

[Bill S]

In my opinion that's a poor definition of energy. It omits the defining property that energy is always conserved.

Is energy conserved in an expanding universe?

[jeffreyH]

An expanding universe changes the value of the energy of photons travelling between distant objects. I would say that this requires a dark energy field that contains it's own energy. In that respect you could argue that energy is conserved. There is a transfer of energy from the photon to the field. It could also be viewed as a simple Doppler shift due to the recessional acceleration of the sources. Acceleration does require a force and this does suggest a field. Is this the same as the vacuum? I have no idea.

[alancalverd]

Momentum of a photon = hf/c. Momentum is conserved, so every photon source exhibits recoil.

[yor_on]

you're right Alan. And that is the weirdest part, 'momentum is conserved'. So a 'photon recoil' is needed for this to be correct, and that makes the recoil a result of conservation laws. So different from the idea of 'action and reaction' in where something 'shoots out' a photon, accelerating it to the limit 'c'. There is no acceleration existing here.