by: Tami O’Connor
One of the units I enjoyed most as a middle school teacher was the section on energy. The many awesome hands-on experiments generated such a series of oohs and aahs that it made my already-enjoyable days even more enjoyable! One of my favorites was a lesson that dealt with the Law of Conservation of Energy. A consequence of this law is that energy cannot be created, nor can it be destroyed. (The students would have already explored potential and kinetic energy before the following activity.)
I initiated this lesson reviewing what happens with energy in a closed system. The students clearly remembered comparing the amount of potential energy to kinetic energy using the example that the height of a roller coaster’s first hill is always greater than the height of any of the remaining hills. It is, of course, possible to have a little hill followed by a higher hill as long as the roller coaster is going faster at the top of the little hill than the next higher one. The students were generally able to explain the transfer of energy including heat energy and sound energy in the overall system.
I would then take out a normal playground ball and a meter stick and ask the the students to predict the height the ball would bounce if dropped from a meter off the ground. Most students accurately predicted that the ball would not bounce as high as the height at which it was initially dropped. Of course, we would then test our hypothesis. A few students in each class would always insist that the ball could bounce higher than the height at which it was dropped, so I would invite them to show me how it could happen. Inevitably, the student would add energy to the system by throwing the ball down to the ground rather than simply dropping it. This was a great opportunity for discussion and was a topic that we would tap into later in the lesson.
I would then pull out my complete collection of balls that ranged from the hard, less bouncy baseballs to the rubber and highly bouncy super balls and have the students explore on their own. Though there were noticeable differences in the elasticity of the balls in my collection, none of them bounced higher than the height at which they were dropped.
My next demonstration utilized a racquetball that I had cut in half… well, actually a little less than half. I would again ask my students to predict how high the half-ball would bounce. The answers varied, but by this time, not one student predicted that it would bounce higher than its drop point. As before, we tested their hypotheses before moving on to the next step.
Because the racquetball is very flexible, I was able to turn the half-ball inside out thus storing elastic potential energy. Once again, I asked the students to predict what would happen when I dropped the ball. Based on their recent experience, they all answered that the half-ball would bounce lower than its drop point. Of course, because I stored elastic potential energy in this system, once the half-ball hit the ground, it popped right side out and was propelled significantly higher than the point at which it was dropped. Talk about a discrepant event!
Thank goodness Educational Innovations sells Dropper Poppers. This product eliminates the time and difficulty of cutting racquetballs in half, not to mention the expense of purchasing racquetballs really intended for use in the court!
Dropper Popper Activities
When this small, “half-ball” is turned inside out and then dropped onto a hard, flat surface, it releases the stored energy and “jumps” higher than the point from which it was released.
• Elastic potential energy is energy that is stored as a result of deformation of an elastic object such as a spring or a rubber band.
• Gravitational potential energy is energy that is stored as a result of an object’s position above the ground.
ACTIVITY #1 How High Will a Ball Bounce
Showing your students a regular ball such as a small super ball, basketball, or ping pong ball, survey the class to determine the height at which they predict the ball will bounce if dropped without additional energy. You may be surprised to learn that some students will predict that the ball will bounce higher than the point from which you drop it.
Drop the ball. Students will discover that the ball will never reach the height from which you dropped it. The Law of Conservation of Energy states that energy cannot be created nor destroyed; it can only be transferred as alternate forms of energy. The energy that initially went into the system was transferred out as sound energy and heat energy. The ball will never bounce higher than the initial drop point because the energy that comes out of a system can never exceed the energy that goes in.
Explain to your students that the ball’s energy was stored due to its position above the ground. Because of the force due to gravity, the ball falls down as it is attracted to the earth.
ACTIVITY #2 The Dropper Popper
Show your students the Dropper Popper (POP-100), and ask them to predict the height at which the popper will bounce if you drop it straight down. Drop the popper without turning it inside out and observe the height at which it returns.
Turn the Dropper Popper inside out and explain that by doing work on the popper you are storing energy in it. Have the class predict again the return height of the popper after it is dropped. Drop the popper with the “bulge” pointing upward. When the popper hits the ground the stored elastic energy will be released and will cause the popper to bounce higher than the point from which it was dropped.
Be sure all students wear protective eye wear.
This activity is truly best when each student has his/her own Dropper Popper and a Ping-Pong Ball, (PNG-100). Have the students store energy in the popper by turning it inside out. Then place the ping-pong ball in the “bowl” of the popper. Drop the popper onto a hard surface in such a way that the ping-pong ball remains above the popper and inside of its “bowl”. The bulge should be on the bottom of the popper so the ping-pong ball fits securely inside. The height your ping-pong ball will fly will be truly impressive!
• Have students estimate how high the ball travels.
• Change the height at which you drop the popper and determine if the height the ping-pong ball travels is based more on gravitational or elastic potential energy.
An additional demonstration of the Law of Conservation of Momentum and Energy can be shown using the AstroBlaster (SS-150). This device has several balls threaded on a plastic shaft. When dropped straight downward onto a hard surface, the top ball can rebound to a height equal to five times the original drop!