Study Kinetic Energy with a Rube Goldberg Machine

Background Information for Teachers

Kinetic energy, also called the energy of movement, is the amount of energy an object has due to its mass and its speed. In science, energy refers to the ability to create change (e.g. lift an object, deform an object, warm up a material, etc). The change a moving object can create is a measure of its kinetic energy. In this lesson, students will roll marbles down a ramp and use the distance over which a cup placed in the marble's path can be pushed as a measure of the marble's kinetic energy. Increasing the number of marbles allows students to study the impact of mass. Students will see that kinetic energy is proportional to mass. Students know this from experience, as they know that a heavy brick falling onto a foot can create more damage compared to a light plastic brick falling onto a foot from the same height.

Letting the marble(s) roll from different heights along the ramp, together with a graph showing the correlation between starting position and speed, allows students to explore how kinetic energy correlates with the speed of an object. A detailed analysis of the data will reveal that kinetic energy is proportional to the square of the speed of the moving object, or, for less mathematically inclined classes, that that kinetic energy quadruples when the speed doubles. Students also know this from experience, as they know that objects that move faster can create a lot more damage and thus, have a lot more kinetic energy. A shopping cart crashing at a slightly higher speed into a car can do a lot more damage than a shopping car that slowly rolls into a car. Or, a small increase in a car's speed quickly creates a much more severe car crash.

Once students have explored the concept of kinetic energy, they will use it to design a Rube Goldberg machine. Rube Goldberg machines are named after the American cartoonist, author, engineer, and inventor Rube Goldberg (1883-1979). According to Webster's New World Dictionary, a Rube Goldberg machine is "a comically involved, complicated invention, laboriously contrived to perform a simple operation." Rube Goldberg drew many of these machines. Usually, they are a series of simple contraptions, each of which triggers the initiation of the next, achieving the completion of a simple task in a convoluted and complicated way.

Many concepts studied in physics can be explored using Rube Goldberg machines; this lesson highlights kinetic energy. Several of the simple contraptions will have moving parts, and kinetic energy is often an accessible way to understand why they can trigger the next contraption, and if needed, how to make adjustments. For example, a sickle will need to swing fast enough, or be heavy enough, to have the energy it takes to cut a rope. A marble will need to roll fast enough or be heavy enough to have enough energy to knock over a domino. Students will use the concept of kinetic energy in the planning phase, the iterative improvement phase, and while communicating their results.

Additional Background Links

• Kinetic Energy, Ducksters
• Kinetic Energy, the Physics Classroom
• Who was Rube Goldberg?, Rube Goldberg
• Object of interest: Rube Goldberg Machines, The New Yorker
• 12 ways to teach about potential & kinetic energy!, Science Buddies

Prep Work (10 minutes)

• Create a ramp by applying insulation tape to the edges of a meterstick or ruler. Have a few books ready to prop up the ramp about 5 cm.
• Cut a hole in the mouth of a small disposable cup, at the top edge, so a marble can roll into the cup when the cup is placed upside down (Figure 1). Place two pennies centered on top of the cup and use tape to keep the pennies in place.

Image Credit: Sabine De Brabandere, Science Buddies / Science Buddies
Figure 1. Illustration showing how to prepare a disposable cup for the experiment.

• See what happens if you roll a marble down the ramp into the hole in the cup. The setup is shown in Figure 2. The cup should shift back a few centimeters. You will use this demonstration in class to show that the marble has kinetic energy; it can make the cup move.

Image Credit: Sabine De Brabandere, Science Buddies / Science Buddies
Figure 2. Illustration of the demonstration setup.

Engage (20 minutes)

1. Introduce the concept of Rube Goldberg machines.

   Does anyone know about Rube Goldberg machines?

   Listen to students' responses.

   Explain that Rube Goldberg was an American cartoonist, author, engineer, and inventor who drew machines that perform simple tasks in an indirect and complicated way. [Show the example shown on slide 1 of the slideshow, or Figure 3.] This machine is a self-operating napkin. It starts with the person moving the spoon.
   
   
   Image Credit: Originally published in Collier's, September 26 1931 / Public domain, Wikimedia
   Figure 3. Figure illustrating one of Rube Goldberg's machines. By Rube Goldberg - Originally published in Collier's, September 26 1931, Public Domain, Wikimedia
   
   Walk the students through the series of steps in the machine depicted in this cartoon image.

   Can someone see the next step in this machine?

   Here are the steps: Person moves the spoon. Wire attached to the spoon pulls a trigger. Wire pulls down the handle of a second spoon. The second spoon launches a cookie. The bird jumps up to catch the cookie. The lever on which the bird was sitting tilts to one side. The tilting lever spills seeds in a bucket. The bucket gets filled. The heavier bucket falls and pulls on a wire in the process. The wire ignites a lighter. The lighter sets off a rocket attached to a sickle. The lever with the sickle swings down, cutting a rope attached to a pendulum, The pendulum with a napkin swings back and forth.

2. Refresh students on the concept of energy.
   1. Tell your students that these machines use a lot of physics principles. In this lesson, they will make a Rube Goldberg machine themselves and use the concept of energy to explain why their machine works.
   2. Explain that in general, "energy" in science is the ability to change something (lift an object, push an object, etc). Energy comes in many different forms and this lesson will focus on kinetic energy or the energy due to motion.
3. Either introduce or refresh the term kinetic energy.

   Give a quick demonstration of how kinetic energy—the energy of motion—can create change by rolling a marble from a ramp into the disposable cup. The marble will push the disposable cup over a distance, as illustrated in Figure 4.

   
   
   
   Image Credit: Sabine De Brabandere, Science Buddies / Science Buddies
   Figure 4. A demonstration of how the kinetic energy of a rolling marble can be used to shift a disposable cup. The distance over which the cup shifts (indicated in red) is a measure of the kinetic energy of the marble just before it rolled into the cup.

   How could the marble move the cup? What energy did it use to do so?

   The marble used its kinetic energy to make the cup move. If students ask where the kinetic energy of the marble comes from, explain how the marble gained kinetic energy while rolling down the ramp as gravitational potential energy is converted to kinetic energy. In this lesson, you will help them to discover what the marble can do with this energy.

   Do you think the cup will move when I start the marble from here? (Show a position not far up from the bottom of the ramp so the marble will not have enough kinetic energy to move the cup.)

   Listen to the students' predictions, and then show what happens. The cup will barely move or not move at all because the marble did not have enough energy to make it move. Tell students that this lesson will teach them how to predict how much kinetic energy the marble has.

4. Make students aware that the concept of kinetic energy is key to building a Rube Goldberg machine.

   What are some examples of kinetic energy that create change in the Rube Goldberg machine in the picture (show Slide 1 or Figure 3)? Would the machine work if that object had less kinetic energy? What would happen if it had much more kinetic energy?

   Some examples are: The spoon lever uses kinetic energy to launch the cookie (step D), It needs enough kinetic energy to launch the cookie. The cookie uses kinetic energy to move up. It needs enough kinetic energy to move high enough. The bucket falls (step I) gaining kinetic energy. It needs enough kinetic energy to ignite the lighter. The sickle falls (step L) gaining kinetic energy. The sickle needs enough kinetic energy to hit the wire hard enough and cut it.

5. Introduce the need to plan when creating Rube Goldberg machines.

   Thinking in terms of kinetic energy can be useful when planning out a Rube Goldberg machine. Why do you think this is the case?

   Let students answer this question. Do not correct; you will come back to this question at the end of the lesson.

Part 1: Explore Kinetic Energy

Explore (45 minutes)

This part explores how kinetic energy varies linearly with mass, and increases quadratically with speed. If students have explored these relationships before, briefly refresh this relationship and move on to Part 2.

1. Explain the experiment.

   Tell students that to make planning their own Rube Goldberg machines easier, they will first do an experiment that explores the kinetic energy of a marble. Inform students that the experiment is similar to the marble demonstration and point them to the Student Worksheet for details.

2. Let students perform the experiment.

   Create groups of 3–4 students, provide the materials, and ask groups to get to work. Walk around while students perform their measurements and guide the students where needed. Have a look at the graphical representation of their data to see if it looks correct.

   Figure 5 and 6 show a graphical representation of data taken with the experimental procedure explained on the Student Worksheet. The distance the cup shifts is used as a measure of the kinetic energy of the marble(s) causing the shift.

   Image Credit: Sabine De Brabandere, Science Buddies / Science Buddies
   Figure 5. Graphical representation of the distance the cup shifts versus number of marbles used for different starting positions along the ramp.
   
   
   
   Image Credit: Sabine De Brabandere, Science Buddies / Science Buddies
   Figure 6. Graphical representation of the distance the cup shifts versus the starting position of the marble(s) for 1, 2, and 3 marbles.

   From their version of Figure 5, students should conclude that the kinetic energy increases with an increasing number of marbles. If the marbles are identical, one can see that the number of marbles—and thus, the mass of the rolling object causing the shift— is directly proportional to its ability to move the cup, or to its kinetic energy.

   To study how kinetic energy correlates to the speed of the moving object, students will first use the graph shown in Figure 7 to find the speed of the marble(s) at the end of the ramp. With this data in hand, students can make a graphical representation of the distance the cup shifts versus speed of the marble(s) (Figure 8, left) and optionally, the distance the cup shifts versus the square of the speed of the marble(s) (Figure 8, right). From their version of Figure 8, students should conclude that the kinetic energy increases with increasing speed. All students should be able to confidently conclude that the relationship is not a direct proportionality, nor an indirect proportionality. Groups who make the optional graph will see how the distance the cup shifts is proportional to the speed of the marble, squared.

   Image Credit: Sabine De Brabandere, Science Buddies / Science Buddies
   Figure 7. Speed of a marble at the end of a 1-m ramp versus the starting position on the ramp. The measurements are for a 1-m ramp with an elevation difference of 5 cm between the ends of the ramp. Using a ramp with a different slope will change the values along the y-axis, it will not change the shape of the curve.
   
   
   
   Image Credit: Sabine De Brabandere, Science Buddies / Science Buddies

   The left graph shows that the distance the cup shifts is not directly proportional to the speed but grows faster as the speed increases. The right graph shows that for each of the series, the data points fall on a line that goes through the point (0,0).

   
   Figure 8. Graphical representation of the distance the cup shifts versus the speed of the marble (left) and the speed of the marble squared (right) for 1, 2, and 3 marbles.
3. Discuss the outcome of the experiment.

   After students have done their experiment and completed the first part of the worksheet, guide the class through a discussion.

   1. The first goal of the discussion is to come to the following conclusion:

      The kinetic energy of an object is proportional to the mass of the moving object.

      Some questions that might help lead the discussion in that direction:

      The ability to move an object was a measure of the kinetic energy of the object rolling into it. So, what did the data show about the relationship between the number of marbles and their kinetic energy? How can we see that from the graph showing the distance that the cup shifts (vertical y-axis) versus the number of marbles (horizontal x-axis) (the graph made in question 9 of the worksheet)?

      We are looking for a rule for how the mass of a rolling object influences its kinetic energy. Can this graph help us find that out? If we see several rolling marbles as one moving object, and we look at the mass of that object, how would that mass relate to the number of marbles? How can we replace the x-axis of our graph to represent the mass of the moving object? What can we conclude about the relationship between the mass of a moving object and its kinetic energy?

   2. The second goal of the discussion is to come to the following conclusion:

      The kinetic energy of an object increases unproportionally quickly with increasing speed of the moving object.
      and
      The kinetic energy of an object is proportional to the square of the speed of the moving object.

      Some questions that might help lead the discussion in that direction:

      What does the graph displaying the distance that the cup shifts (vertical axis) versus the speed of the marble(s) look like (the graph made in question 11 of the worksheet)? What does this graph show about the relationship between the speed of the marble(s) and its ability to move an object, or its kinetic energy?

   3. Finish the discussion by asking if these conclusions make sense.

      A question that might help lead the discussion in that direction:

      Is this consistent with what we see in real life?

      Shopping carts, cars, and a person on roller blades are all objects that can move, acquire kinetic energy, and can create change. A heavier cart, car, or person can create more change; similarly, a fast-moving cart, car, or person can create more change compared to a slow-moving one, and a small increase in speed can create an unproportionally big increase in ability to create change.

Part 2: Design a Rube Goldberg Machine

Explore (70 minutes)

This part allows the students to be creative and build a Rube Goldberg machine. They will use their knowledge about kinetic energy to plan and create their machine.

1. Explain the design challenge.

   Tell students that to make their own Rube Goldberg machines, they will first plan their machines, keeping their knowledge of kinetic energy in mind. Then they will build and improve their machines, and at the end, they will showcase what they built to the class.

   To get students excited, consider showing this video.

   

   https://www.youtube.com/watch?v=qybUFnY7Y8w

   Before you let students watch the video, ask them to pay attention to:

   • Clues that show that this was not the first attempt to run the machine.
   • How contraptions use kinetic energy to get to and initiate the next step.
   
   
2. As a class, come up with a goal for the Rube Goldberg machine.

   Decide on a goal for the Rube Goldberg machine. An example of a task a Rube Goldberg machine can do is popping a balloon, especially one that is not fully blown up. Filling a bowl with animal food is another example that can be done with cardboard and recycled materials.

   Their machine should consist of at least 3 different actions before it performs its final task. Their machine should be freestanding and once set in motion, it should operate on its own.

3. Let students plan their Rube Goldberg machines.

   Show the students the materials they will be able to use before they start working together in their groups to plan a Rube Goldberg machine. They should document their plan and use their knowledge of kinetic energy to predict if the moving objects in their design will have enough energy to initiate the next step or perform the final task. The Student Worksheet can guide them through the process.

4. Let students build and test their machines.

   Their machine will probably not work in the first trial, or not work smoothly. Encourage students to make detailed observations of where their machine fails. Let them brainstorm solutions with their group and make changes.

   Groups who are done early can expand on their machine or make a second machine.

5. Let students prepare to demonstrate their machines.

   Give students 5 minutes to clean up and prepare to demonstrate and explain their machine to the class.

   Groups that have a machine that does not work or partially works should still present what they have and what they had planned.

Reflect (20 minutes)

1. Let groups present their machines to the class.

   Let each group present their machine and explain what it does or was supposed to do, and where and how kinetic energy is used in their design.

   Let the other students comment on design choices that they find clever or creative, and different design choices they might make, including the reason why they would do it differently. Remind students to think about how kinetic energy can create change.

   Some questions that might help students think about kinetic energy:

   Do you think your machine would work if we replaced the heavy ball with a lighter ball like a ping pong ball? Why do you think this? Can you explain your reasoning using the concept of kinetic energy?

   Why does this lever start from so high? How would having a shorter, lower lever relate to its kinetic energy when it rotates down?

   Did you run into challenges where an object could not trigger the next step? Was it because it did not have enough energy? How did you solve it? Could making it heavier or giving it more speed have solved the problem?

2. Come back to the question "Thinking in terms of kinetic energy can be useful in planning a Rube Goldberg machine."

   Did your knowledge of kinetic energy help you design and make your machine? Can you see how thinking about a problem in terms of scientific principles could help engineers design products and machines?

   Listen to students' answers.

   If students felt that thinking in terms of kinetic energy did not help but hindered them, explain that in this case, they can often predict what would happen because they often experience kinetic energy in real life. In this case, thinking about it in terms of kinetic energy gave them practice in explaining daily experiences in scientific terms. When the tasks are no longer that intuitive, knowing the scientific background and being able to think in these terms is a tremendous help as it gives testing direction, it allows one to predict what might work and what will not work.

Assess

You can use the worksheet to assess student engagement and understanding.

A quiz to assess student learning after the activity is available as well:

• Online quiz, assignable in any LMS
• Quiz (pdf) and Answer key (pdf)

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Mechanical Engineer

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Career Profile

Did you like building and testing the machine? Mechanical engineers build and test many types of machines, from safety equipment for fun roller coaster rides, to robots that can help with medical procedures, to spaceships. They use the engineering design process to improve these machines and make them safer for people. Read more

Precision Instrument & Equipment Repairer

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Career Profile

Your Rube Goldberg machine probably had several parts that had to work together. Did you enjoy fine-tuning your machine, making sure it all lined up perfectly? Precision instrument or equipment technicians need good fine motor skills and a love for working with their hands. They often combine a love of music, medicine, electronics, or antiques with delicate mechanical repair work. Read more

Lesson Plan Variations

• In this lesson, students did not measure the speed of the marble because accurate speed measurements are not easy. Manually measuring the time the marble takes to roll 1 m does not provide accurate enough data to see the relationship between kinetic energy and speed. A more-accurate measurement can be obtained using slow motion video. One can count the number of frames it takes for the marble to roll 1 m (or ½ m for lower starting positions) after it rolls off the ramp. Using video and a video analysis tool like Tracker is another great option.
• If making a Rube Goldberg machine seems daunting, other activities can be used to let students apply what they learned about kinetic energy. The Balloon Car Lesson Plan lesson can be adapted to mainly focus on kinetic energy; a heavier car will not move as fast as a lighter one when powered by the same balloon.
• Instead of kinetic energy, use the six simple machines (inclined plane, lever, wedge, wheel and axle, screw, pulley) as scientific concepts to plan and improve their machine. Can they build a machine that uses all these simple machines?