Newton's Pendulum - a Lesson Demonstrating Conservation of Energy

Newton's pendulum sometimes called Newton's cradle can be used to ignite the thinking process of students. Typically the pendulum is constructed using 5 steel balls, each suspended by a string attached to two "bars of wire". The device looks something like a miniature set of parallel bars. It looks something like this:
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|....|...|...|....|...|.....|
|....|...|...|....|...|.....|
|....|...|...|....|...|.....|
|....O.O.O.O.O....| The 5 steel balls represented by "O"s suspended by strings.
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This lesson could cover a variety of science standards, but in general could be under the general topics of "Force and Motion" and "Energy". You might first get everyone seated and facing front and then put the Newton's Pendulum on your desk. This is part of the "Gaining Attention" in a lesson plan. It should be placed high enough so everyone has a clear view. Some students will blurt out that they have one or have seen one before; some will not know what it is. You would ask them all to be a quiet telling them you will ask them a few questions about it in a few minutes. You could then ask them: "If you have seen one before raise your hand." and then say: "Can anyone who raised their hand tell me the name of this device? You would then pick up one of the steel balls in such a way, that when you release it it will swing into the other four that are stationary. You should ask them: "What would you predict will happen when you release the one steel ball?", Do not rush this - explain that as you lift the one steel ball up, you are giving it "potential" energy which will change into "kinetic" energy as it swings down after release.

You might relate this to a person on a swing. As the person gets to the high point in the swing all the kinetic energy of motion suddenly stops and only potential energy is available. You can explain that Newton was a scientist and mathematician that was interested in the motion of the planets and the changes of energy between kinetic and potential. After you release the one steel ball and one pops outward from the end you can explain how the energy has been conserved and moved through the steel ball masses. A small amount of energy is used in the generation of the "click" as the steel balls collide and there is also a little heat energy generated at the point of impact.

After doing this "experiment" you would take two steel balls and move them upward together. Hold them in place and ask the students what they would happen now that you have two steel balls in a potential energy position. Then you would let them go and you would see two steel balls pop out from the other side. This would reverse direction, but do exactly the same thing in the opposing directions until all of the energy has been used.

You would then do the same type of experiment with three steel balls, then with four, and yes finally with 5 steel balls. Each time you would hesitate and ask students to "predict" what would happen and then follow this with the experiment and observation. This concludes one typical set of experiments with the pendulum, but there is more possible.

Now you would take one steel ball from each end and ask what would happen if you dropped them both at the same time. Students would see the experiment conducted and make observations and discuss the options. You would then take 2 steel balls from each end and hold them upward and go through the whole process again. Now you have concluded this series of possibilities.

Now you would pick up three steel balls from one side and two from the other and lift these groups upward. Again you would ask students to "predict" the results, do the experiment, make the observations and discuss the process.

It is important that you realize that you have taken a very simple scientific device and used it to make many predictions, observations, and discussions. This device costs between $6.00 and $15.00, can be used year after year and creates interest. If students see this device on your desk as they enter the classroom, they will want to play with it immediately and this could cause the balls to mix together creating tangles in the strings which could prevent you from doing the experiments and observations. This is why keeping it out of view at first is helpful. The actual design of your lesson will depend on the age group you are working with at the time. At the high school level you can go into the mathematics of this device which can be found in some textbooks or through the internet.

For gifted students you could challenge them to do more research on this device or even consider other designs for the device. Although you might consider having students make these devices, the potential danger of students using the balls as projectiles is sufficient to remove this consideration, but other designs are possible and could be interesting.

If two large (about the size of a small orange) steel balls are used then lifted as the swing and strike each other, they can generate enough heat to burn holes in a piece of paper held between them where they impact. This however is the subject of a different scientific process (the change of kinetic energy into heat energy) and not the concervation of energy of Nwton's Pendulum. This lesson has always worked for me, but it generally lasts 20 to 40 minutes depending on how much other material you present and how long you take drawing out student predictions and discussions.