# What is energy

## Main Question or Discussion Point

I am new to physics and having trouble understanding what energy is. Is energy something physical? I read that energy and matter are essentially the same thing. To me it seems that energy is an invisible force acting on matter. Thank You.

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I am new to physics and having trouble understanding what energy is. Is energy something physical? I read that energy and matter are essentially the same thing. To me it seems that energy is an invisible force acting on matter. Thank You.
It is best at your level (and thus mine) to only (for now) view energy as a property that is conserved within a system, without fully "grokking" it. I know that doesn't sound like a concrete answer, but unfortunately energy isn't a concrete concept. It can take on many forms, (kinetic, potential etc...) and that is a huge contributor to energy not easily being clearly defined. Also, I think it is best not to dwell too long on energy and matter being the same thing for now. Viewing energy from an accounting standpoint when analyzing interactions within a system would be most beneficial to you. The insights you will draw on your own from your exploration of the conservation of energy will be more valuable to you than any description we might attempt on this board. I of course don't mean to discourage others from chiming in (or even disagreeing with me)!

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I am new to physics and having trouble understanding what energy is. Is energy something physical? I read that energy and matter are essentially the same thing. To me it seems that energy is an invisible force acting on matter. Thank You.
Energy is the ability to do work. Work is the application of a force (push or pull) over a certain distance, so an equivalent definition for energy would say that energy is the ability to apply a force over a distance. Energy is primarily measured in joules, which is kilograms times (meters squared divided by seconds squared). In that sense, yes, energy is very physical. What does all of that mean? Well, you use energy every single moment of your life. For example, when you lift an apple (1 kg) up about one meter, you've just used 1 joule of energy.

The two primary forms of energy are kinetic and potential. Kinetic energy is the energy associated with motion while potential energy is typically associated with position (although you can also have velocity-dependent potentials). The faster something moves, the greater its kinetic energy, which is what you expect from every day life. From our definition above, an object with lots of kinetic energy has the capacity to do a whole lot of work (like to lift or to pull). And what about potential energy? Thinking about gravitation is the easiest way to see what's going on here. If you hold up a ball in the air with your hands, it's got some potential energy. The ball is not moving, but you know that it falls under Earth's gravitational field when you let go. The ball's potential energy gets converted into kinetic energy and the ball hits the ground. Potential energy is very important because it determines how things move. Different potentials will produce different kinds of motion.

It's also important to keep in mind that the total energy in the universe is always conserved. Energy is never created or destroyed. Energy merely changes form, by going from kinetic to potential, potential to kinetic, or some other variation.

Hope that helps. If not, just tell us what you need help with.

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Dale
Mentor
Is energy something physical?
Energy is most definitely physical since "physical" means "of or relating to physics". As Cryxic says, energy is simply the capacity to do work, and work is a force applied over a distance.

You should search through older threads. I believe there were very similar questions.

I would argue energy is not something physical in the way you mean by "physical" (I suppose).

Rather on the classical level energy can be seen as a mathematical trick that can be deduced from knowing that all forces are
$$F_{ij}=\frac{\alpha\vec{r}^0}{r^2}+\vec{b}\times\vec{v}$$
You can show that with this assumption and some auxiliary term called potential energy, there will be a scalar whose total sum is conserved with time.

Most of these posts have already nailed it, but if you want to view energy as what it truly is, then energy is simply a photon(s) that's being transferred from one atomic system (atom; molecule) to another. This is the quantum view, and it's what's actually being done in nature. Kinetic energy, therefore, is the transfer of many photons from one object to the object it's reacting with.
Striking the golf ball with the golf club is the simple act of transferring a great number of photons from the golf club to the golf ball.

The photons can never be observed; they are called, "virtual photons."

Thank you all for your replies! you helped me to understand some uncertainty
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