November 18th - Kinetic & Potential Energy; The Law of Conservation of Energy

We started out this week discussing different types of energy. I asked the students to name any types of energy they could think of. They did a great job and named almost all the ones I had listed! We also talked about where energy can be found - everywhere.

Energy: The ability to do work or the ability to move something with a force.
We spent some time comparing the two categories of energy: kinetic (working energy) and potential (stored energy). We then moved on to a quick discussion of renewable and non-renewable energy sources.
To provide some examples of kinetic and potential energy, I used the image of a flowerpot sitting on a windowsill. If something, a cat for example, knocked the pot off the windowsill, gravity will cause it to fall toward the ground. As it falls, its potential energy changes to kinetic energy.
We each stretched rubber bands and discussed what type of energy the band will have if we were to let it go. Again, the energy changes from potential to kinetic if you let the rubber band go. The stored energy in the stretched rubber band, or any other stretched or compressed elastic object (a spring or bungee cord, for example), is known as elastic potential energy.
We placed a textbook on the edge of a classroom table and talked about the energy in the book when it is just sitting on the edge and the energy in it if we pushed it off the table. Since gravity will work on the book to push it to the ground, the stored energy in the book is known as gravitational potential energy.

"Loser" Lab - This lab determined the effect mass has on kinetic energy. We attached a string to the handle of a bucket and then taped the other end of the string to a table. A wooden block was placed on the floor in front of the bucket. A student pulled the bucket back and let it swing into the block. We observed how far the block moved. The students then added several large pieces of modeling clay to the bucket and repeated the exercise, again measuring how far the block moved.
Kinetic energy is increased as the mass of a moving object (in this case the bucket) is increased. This allowed the heavier bucket to push the block further than the empty bucket.

"Bump!" Lab - This lab showed the transfer of kinetic energy. Each student was given a textbook and 6 marbles. The students opened the book and placed 5 of their marbles in the book's center groove and pushed them close together. The students then positioned the last marble in the groove about an inch from the other marbles. They gave the marble a push to make it bump the others. The students noticed that the 6th marble stopped once it hit the first marble in the row. They also saw the last marble in the row move away from the group.
This happens due to a transfer of kinetic energy from the marble that was pushed to the first stationary marble which then transferred energy to the next marble, and so on. The last marble eventually receives this kinetic energy and is able to move forward as a result.

"Energy Change" Lab - This lab demonstrated the effect height has on the energy of a moving object. The students were each given a paper cup, a pencil, a ruler (one with a center groove), a pair of scissors, and a marble. The students cut a "door" in the paper cup and then placed the cup over one end of the ruler. They used the pencil to raise the other end of the ruler and then rolled the marble down the ruler. The students observed how far the cup moved after the marble hit it.
The students were then each given a textbook and replaced the pencil with the book. This raised the ruler a little higher. They repeated the experiment and discussed the results.
The students discovered the cup moved farther when the marble started from a higher point. Objects that are higher off the ground (or table, in our case) have higher potential energy than those closer to the ground. As we learned earlier in this lesson, when the object falls or rolls, the potential energy changes to kinetic energy. The height increase also served to increase the kinetic energy, allowing the marble to hit the cup with more force.

At this point in the lesson, we took a break from labs to discuss the law of conservation of energy. This law states that energy cannot be created or destroyed. The students learned today that energy can change; it is just always there in some form.

We completed two labs to demonstrate the law of conservation of energy.
"Bonk!" Lab - The students were given a ruler, a 2-foot piece of string, some tape, a book, and 2 small bouncy balls (equal size). One end of the ruler was placed inside the book. The students then tied the center of the string around the other end of the ruler. They used small pieces of tape to attach the two balls to the two hanging ends of string, making sure they were at the same length. The balls were then pulled away from each other and released.

The students saw the balls continue to hit and bounce off each other until they eventually stop moving. This is another lab showing the change from potential to kinetic energy. After the students used their energy to pull the balls away from each other, the balls had potential energy. When the balls were let go, this energy changed to kinetic energy. The balls will then change their kinetic energy to heat (thermal) and sound energy.

"Shifter" Lab - For this lab, each student used two classroom chairs, some string, and two washers. The students placed the chairs about 1 yard apart and tied a piece of string between them so they had a tight light between the chairs. They then cut two 24-inch pieces of string and tied a washer to the end of each piece. The free ends of these strings were tied to the string between the chairs. One of the strings with an attached washer (String A) was lifted and pulled away so it was level with the top of the chairs. The students then let their String A go and watched it swing back and forth. As this happened, they noticed String B start to move.
Again, this shows the transfer of energy from one object to another. The strings form a pendulum and the energy is constantly transferred back and forth between String A and String B until friction eventually causes them both to stop swinging.

The labs "Bonk!" "Loser," and "Energy Change" are from Physics for Every Kid.
VanCleave, J. (1991). Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound. San Francisco: Jossey-Bass.

The labs "Bump!" and "Shifter" are from 200 Gooey, Slippery, Slimy, Weird, and Fun Experiments.
VanCleave, J. (1993). 200 Gooey, Slippery, Slimy, Weird, and Fun Experiments. New York: John Wiley and Sons, Inc.