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### Course: High school physics—NGSS (DEPRECATED)>Unit 3

Lesson 1: Introduction to energy

# What is energy?

Energy is a quantitative property of a system that depends on the motion and interactions of matter and radiation within that system. That there is a single quantity called energy is due to the fact that a system’s total energy is conserved even as, within the system, energy is continually transferred from one object to another and between its various possible forms. Created by Khan Academy.

## Want to join the conversation?

• If we are pushing a wall for a while, we will feel tired and maybe hungry after a while, although the work is 0, how come this happens? Where did the energy go?
• The way I'd see that is that the displacement distance is multiplied based off of the initial distance.
So for an object that hasn't moved, the displacement is simply 1 as that is the same starting position.
The work at that point would just be the force applied to the wall.

You're still applying force/energy to an object, but not enough to displace it in any way.
It would simply be converted into heat or some kind of absorbed vibration as you push on the wall.
• what is energy composed of
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• At does it depend on what you eat the amount of energy you get from different types of food
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• that is it
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• Does the E in E = mc^2 refer to energy as a general thing? Since there're different equations to finding out different types of energy, like Kinetic energy = mv^2 / 2 .
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• The E in that equation refers to ALL of the energy in that object. That being said, it refers to the vibration of quarks inside protons/neutrons, each molecule's thermal energy, the chemical potential energy between the molecule's bonds, the object's kinetic energy, etc. It's the sum of all those energies combined. This is why in classical mechanics (when dealing with macroscopic, everyday objects), we use other formulas since it is quite difficult to measure and pinpoint how much of each different type of energy the object has. I've only seen E=mc^2 used in nuclear physics or in relativity (although it may have more applications than that).
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