Kinetic Energy

Kinetic energy is the energy of movement. If you see something moving, like an airplane or a baseball, for example, you easily can tell that the object has kinetic energy.

However, many times kinetic energy refers to the motion of molecules that we cannot see. When the molecules of an object move more rapidly, they have greater kinetic energy. We can detect this molecular movement as heat. Therefore, heat is a form of kinetic energy.

On the left of this slide are images and words that should help you get the “feel” that kinetic energy implies movement, action, heat, light, and so on.

Note: Another two examples of kinetic energy that might not be obvious to you are electrical and light energy. Both of these forms of energy are kinetic energy because of movement. In the case of electricity, it is the movement of electrons through a wire or a bolt of lightning. In the case of light, it is the movement of very small particles called photons that travel faster than anything else in the Universe!

Potential Energy

Potential energy is the energy that is stored in an object or substance. This is often more difficult to imagine than the concept of kinetic energy. Whereas it is easy to visualize the kinetic energy (the energy of movement) of a falling ball, the potential energy of the ball before it falls is much harder to visualize.

Some examples of potential energy are shown on the right side of this slide. The runner is at the starting block, ready to go. He has stored chemical potential energy in his muscles which will be converted into kinetic energy when the gun sounds. In a similar manner, gasoline contains stored chemical potential energy that will be released as heat, sound, and mechanical kinetic energy when it is ignited in an engine.

The battery stores electrons that will begin moving (become kinetic) the moment it is inserted into a complete electrical circuit.

Finally, the archer in the back has drawn his bow. At this point, all of the energy in the bow and arrow system is in a potential form, stored in the fibers of the bent bow. Upon release, the bow will spring back to its original shape and all of the stored potential energy will be transferred to the arrow, which will be propelled toward its target. We will discuss the potential/kinetic energy conversion of the drawn bow in greater detail in a later slide.

Gravitational potential energy is a form of potential energy that children will be most familiar with. It is the energy of position and is related to an object’s height. Before discussing this slide with your child, do a simple experiment to quickly demonstrate gravitational potential energy. Hold any small object in your outstretched hand, a pencil for example. At first, the pencil is not moving and doesn’t appear to have any energy at all. But now, release (don’t throw) it from your hand. It immediately begins to move toward the floor. You can see its kinetic energy easily now because of its movement. But that kinetic energy had to come from somewhere (remember, energy can not be created from nothing). The kinetic energy of the pencil’s downward movement came from the original gravitational potential energy that it contained while it still was held above the floor in your hand.

Now let’s begin applying some math to the kinetic energy/potential energy conversion. The basic mathematical formula for potential energy (PE) is PE = mgh. The mass (m) of an object is expressed in kilograms (kg). The height (h) of an object above a surface (water in this case) is expressed in meters. The final term (g) is a number that never changes and is therefore called a “constant”. In this case, we are referring to the gravitational constant (g), which is 9.81 m/s² (that is 9.81 meters per second squared).

If we multiply the units for m x g x h, the final unit for potential energy (PE) is kg x m²/s². This is referred to as a unit of energy called a Joule, which itself is abbreviated as J. Let’s really understand this concept by further applying the PE (potential energy) equation on the next two slides.

Note: While the concept of m² is not difficult to think about, children are likely to have more of a problem with the concept of a second squared (s²). The s² concept comes from a speed (which would be some distance per second, m/s, for example) that changes over time and thus is a property of something called acceleration. We will not discuss acceleration further at this time.

This slide provides practice on working through several gravitational potential energy problems.

Discuss the solution to the problem when the diver is at 25 m.

Ask your child to calculate the PE at 20 m.

Ask your child to calculate the PE at 12.5 m.

Ask your child to calculate the PE at 6.5 m.

Note: A second important aspect of this slide is that it provides us with a perfect platform to introduce the important concept of the conversion of potential to kinetic energy.

As the diver loses potential energy during the fall, he gains kinetic energy. At the moment he hits the water, all of the initial potential energy will have been converted to kinetic energy, the energy of motion. Thus, when he hits the surface of the water, he will have 19,865.25J of kinetic energy!

This slide provides the correct answers for the calculations on the previous slide. Note again that all of the gravitational potential energy at any position is converted to kinetic energy. Therefore, when gravitational potential energy reaches zero (at the water’s surface), all of the original gravitational potential energy has been converted to kinetic energy and the diver hits the water with 19,865.25 J of energy.

You may also discuss that, when hitting the surface, the diver will continue through the water for some distance before his buoyancy stops his further progress and he floats back up. To simplify the situation, we could have used an example of diving onto a solid concrete surface from a height of 25m. This tremendous amount of energy (19,865.25 J) hitting an immoveable surface and abruptly stopping would, of course, likely be lethal.  

This slide shows various positions of a pendulum. Starting from the left, before the ball is released, 100% of the energy is in the form of potential energy. If we knew the height and mass of the ball we could easily calculate the potential energy and therefore the total energy.

When the ball is released, the potential energy decreases as it is driven by gravity toward the perpendicular (straight down). At exactly the same time, the kinetic energy of the system increases. Halfway down, for example, 50% of the total energy is in the form of kinetic energy and 50% is still in the form of potential energy.

At the perpendicular, 100% of the total energy is in the form of kinetic energy. There is no more stored, potential energy at this point.

Unlike the situation with the cliff diver, the pendulum ball continues on. As it does, it gains height due to the push of its kinetic energy, and therefore gains potential energy. As it gains more and more potential energy, it loses kinetic energy.

Finally, when the pendulum ball reaches the top of its arc to the right, all of the energy in the system is once again 100% potential energy. There is momentarily a pause in movement as the pendulum changes direction from up and to the right to down and back to the left. At precisely that moment, there is no kinetic energy. All of the energy of the pendulum at that moment is in the form of gravitational potential energy.

This is similar to when you throw a ball straight up into the air and it momentarily pauses at its peak before falling back to Earth. After this momentary pause, the ball will be forced down again by gravity. Imagine what would happen if you threw the ball in outer space, where there is no gravity and therefore no gravitational potential energy!

This slide follows directly from the previous slide in which the pendulum was introduced. If possible, go outdoors with your child and let them call out how much potential and kinetic energy they have at various swing positions. In other words (see the illustration for numbers):

1. “I’m all potential energy!”

2. “I’m getting more kinetic energy.” or “I’m losing potential energy.”

3. “I’m all kinetic energy!!”

4. “Help! I’m losing kinetic energy.”

5. “I’ve stopped. I’m all potential energy again.”

Of course, in performing this demonstration, your child can’t “pump” to go faster. Pull them way up from behind and tell them to not pump their legs when you let go. As they swing to and fro, discuss how they are slowing down. Discuss how their height gets smaller and smaller with each cycle. Since energy can not be destroyed, where could this energy be going? [The answer complicates the demonstration but is nonetheless interesting. The energy is lost due to friction. Friction of your child’s body pushing against the air molecules and the rubbing of metal at the swings attachment point. We make up for this loss of gravitational potential energy to friction by pumping our legs, adding energy from our muscles.]

Note: This slide provides students with another opportunity to consider the potential/kinetic energy conversion.

When a bow is drawn, energy is stored in the structure of the deformed bow. There is no movement; there is no kinetic energy. All of the energy is potential energy.

Note: In the example in this slide, the amount of stored potential energy is shown to be 35 J. This is a reasonable amount of energy for draw on a bow.

Once the string is released and the bow snaps back to its original shape, the potential energy of the bow quickly collapses to zero. All of the stored potential energy is immediately converted to kinetic energy.

Notice the important equation shown in this slide:

PE + KE = Total Energy

This is a good time to return to the idea of the Law of Conservation of Energy (energy cannot be created or destroyed, but can change forms). In the systems we have discussed thus far (cliff-diver, pendulum, swing, and archer) the total energy is always the sum of the total potential energy and the total kinetic energy. The Law of Conservation of Energy is one of the most fundamental Laws of science.

This and the following slide should induce discussion regarding potential and kinetic energy. For example, before releasing his hand, the child on the slide is all gravitational potential energy. He will lose this potential energy as he goes down the slide and his kinetic energy increases.

The child climbing the ladder (assume he is going up the ladder) is increasing his gravitational potential energy with each step up. To climb the ladder and increase his height and potential energy, he must add kinetic energy from the muscles in his legs and arms. 

The three swinging children are all in various combinations of kinetic and potential energy depending on their position. Notice that, without knowing if a child is swinging forward or backward, it is impossible to say whether they are increasing or decreasing in potential energy.

Finally, the little girl sitting under the slide appears to be totally at rest. However, she has gravitational potential energy nonetheless. She would demonstrate this if she were to fall off her seat!

This final slide shows three positions of a car on a rollercoaster and asks three questions:

-Which car has the most kinetic energy? [Answer: Car#2 has the most kinetic energy.]

-Which car has the least potential energy? [Answer: A bit tricky. Even though it is moving and therefore has the most kinetic energy of the three cars, it also has the least height and therefore has the least potential energy.]

-Which car has the most potential energy? [Answer: Car#3 has the most potential energy because it has the most height.]

In terms of potential energy, Car#3 > Car#1 > Car#2.