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Unit 1•Lesson 2 3 min read

The Law of Conservation of Energy

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Learning Objectives

State the Law of Conservation of Energy.
Explain that energy cannot be created or destroyed, only transformed from one form to another.
Trace the energy transformations in a simple system, like a falling object or a pendulum.

The First Rule of Energy

There is a fundamental rule in the universe that governs all energy: the Law of Conservation of Energy. This law states that:

Energy cannot be created or destroyed; it can only be changed from one form to another.

This means that the total amount of energy in an isolated system (a system that doesn't exchange energy with its surroundings) always remains constant. You can't make energy from nothing, and you can't make it disappear. You can only convert it.

Energy Transformations in Action

Let's look at some examples of this law at work.

A Bouncing Ball

1.At the Peak: When you hold a ball high in the air, before you drop it, its energy is almost entirely gravitational potential energy (GPE). It has maximum stored energy and zero kinetic energy.
2.Falling: As you release the ball, it starts to move faster. Its height decreases, so its GPE is converted into kinetic energy (KE). Halfway down, it has half GPE and half KE.
3.Just Before Impact: Right before it hits the ground, it's at its lowest point and moving its fastest. Almost all the potential energy has been transformed into kinetic energy.
4.The Bounce: When the ball hits the ground, it deforms (squishes). The kinetic energy is temporarily converted into elastic potential energy (like a compressed spring) and thermal energy (the ball and ground get slightly warmer) and sound energy.
5.Moving Up: The elastic potential energy is then converted back into kinetic energy, pushing the ball back up. As it moves up, the kinetic energy is converted back into gravitational potential energy.

Why doesn't the ball bounce back to its original height?

Because at each bounce, some of the mechanical energy (GPE + KE) is 'lost' from the system when it is transformed into non-mechanical forms like heat (thermal energy) and sound. The total energy is still conserved, but the energy useful for bouncing is reduced.

A Pendulum

A swinging pendulum is another perfect example.

At the highest points of its swing, the pendulum bob stops for an instant. Here, it has maximum potential energy and zero kinetic energy.
As it swings down towards the middle, its potential energy is converted into kinetic energy.
At the very bottom of its swing, it is moving fastest. Here, it has maximum kinetic energy and minimum potential energy.
As it swings up the other side, the kinetic energy is converted back into potential energy.

This process repeats, with a constant transformation between potential and kinetic energy.

In any system, if you add up all the forms of energy at the beginning and all the forms at the end, the total amount will always be the same.

Key Terms

**Law of Conservation of Energy
A fundamental principle of physics stating that energy cannot be created or destroyed, but can be altered from one form to another.~|~Energy Transformation: The change of energy from one form to another (e.g., chemical to thermal).~|~System: A group of interacting, interrelated, or interdependent elements forming a complex whole. In physics, it's the part of the universe being studied.~|~Gravitational Potential Energy (GPE): Energy an object possesses because of its position in a gravitational field.~|~Kinetic Energy (KE): The energy an object possesses due to its motion.

Check Your Understanding

1

According to the Law of Conservation of Energy, what happens to energy in a closed system?

2

A roller coaster car is at the top of its first, highest hill. At this point, it has its maximum ________ energy.

3

When you burn a piece of wood, the chemical energy stored in the wood is transformed into what two main forms of energy?

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Renewable vs. Non-Renewable Energy Sources