What is energy or mass?

You probably won't find this very satisfying. Your question is some where between: Why? As asked by a young kid after every answer; and What is love?

Energy is the stuff that is used to make everything else. The Big Bang was pure energy. After the initial expansion of the universe, energy coalesced into matter. Current

"Theory of Everything" holds that the universe is made up of very tiny strands of energy called "Strings." Go to the PBS website> NOVA>Theory of Everything. They have a very good DVD of the subject. There is also a book of the same title. Your local library may have a copy of them.

http://www.pbs.org/wgbh/nova/elegant/

Energy is the capacity to do work. Water up on a hill has energy that you can harness to turn a flywheel as it falls. The flywheel has energy that you could use to drive electrical current. A battery has energy--you can release that energy to do something for you. There are many kinds of energy, but they all have to do with the ability to do work--to push something against a force over a distance.

Mass is inertia--the tendency of an object to resist changes to it's motion. Per Newton's 2nd law, it's the ratio of the force pushing on it to the acceleration it experiences.

One of the interesting things about relativity is that it turns out that mass and energy are the same thing (or they are proportional depending on your system of measurement). Energy has inherent inertia. And an object with mass has an inherent energy. They are exactly the same thing. Since we don't need two words for the same thing, though, we've adjusted the definition of mass a bit to mean just the energy of an object which is at rest. That way we can say that all electrons have a certain mass--their energy at rest--and not have to worry about every electron buzzing around having a different mass because of its kinetic energy.

The scientific definition of energy is the ability to exert a force through a distance.

"ability" does not have a scientific definition, it's the simple english definition, a form of "is able to"

If something has energy, then it is able to exert a force, and continue exerting that force even after the object begins to move.