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Physics of Media Arts: Conceptual Physics for Students of the Media Arts

Aaron Wong

Lab Activity 6.5.1 Tablecloth Pull Trick

Introduction.

You’ve probably seen it. The entertainer walks up to a table setting and pulls the tablecloth out from underneath it without knocking anything over. Of course, it’s all physics. At the top level, this is a story about Newton’s First and Second Laws of Motion. Objects that are at rest will remain at rest unless acted on by an external force, and \(F = ma\text{.}\) It’s impossible to move the cloth away from the dishes without exerting any force, so there will always be some movement. This means that the challenge is to try to control that force as much as possible to minimize the acceleration of the dishes.
There will always be at least two forces acting on the dishes. There will always be gravity pulling the dishes downward, and there will be a friction force that opposes the cloth from sliding out from under the dishes. In some cases, there may be an additional force between the fabric and the dishes, depending on the angle that the fabric is making with the table.

Equipment List.

  • Table cloth with no hem at least 60 cm square (about 2 feet)
  • Table with a straight edge
  • Plastic cups and plates (unless you have a large budget, in which case you might opt for something breakable)
  • Small Weights (paper clips, coins, marbles, small bags of rice, etc.)
  • Slow motion camera (cellphones are acceptable)
  • Fabric chalk or tape for making small markings

Procedure.

The first part of this lab is going to be learning about all the ways this trick can go wrong. Then you will have a chance to practice the technique before we start adjusting the parameters.
  1. Mark a spot 20 cm from one of the edges with the fabric chalk or a small piece of tape. Be sure that the edge does not have a hem, otherwise the trick will become far more difficult to complete successfully. Make another mark about 40 cm from the same edge. Each time you do the experiment, you will be placing an object at the 10 cm mark and have the tablecloth so that the edge of the table is at the 40 cm mark. This will help with the consistency of the observations.
  2. One of the techniques that people try to use for this trick is to pull the cloth towards their body. This will almost always fail. Record yourself doing this (using slow motion), making sure to focus your attention on the glass and the tablecloth near the cup. Watch the footage of the instant the tablecloth starts to move near the cup. Make some observations about what you see and why this is not a good technique.
  3. The correct technique is to pull the cloth downward, against the edge of the table. But even then, you need to be careful. Some people hold their hands up above the height of the table before pulling down. Record yourself doing this a couple times (in slow motion). Watch the footage of the instant the table cloth starts to move near the cup. Watch the footage of the instant the tablecloth starts to move near the cup. Make some observations about what you see and why this is not a good technique.
  4. Set up the dishes again. Record yourself pulling downward slowly on the cloth. You should be going slowly enough that the cup does not slide. Think about what is happening with the friction when the cloth is being pulled slowly, and why it is necessary to do this very quickly.
  5. Attempt the trick several times. Make notes of your successes and failures. (It is possible to be successful with some of the dishes while failing with others.)
  6. Do the trick again, but this time add some weights to the cup and/or dishes. Try this experiment again a few times. Does this seem easier or harder to do? Record yourself and make observations about the movement of the cup that can help explain what’s happening.

Discussion.

  1. Explain why pulling the tablecloth towards your body does not work. You may find it helpful to draw a schematic diagram of the tablecloth and the dishes at the moment the tablecloth near the dishes starts to move.
  2. Explain the observations that you made when you held your hands above the table before pulling downward. Specifically, discuss the interaction between the fabric and the cup, and how this interaction leads to problems.
  3. Which objects were the most difficult to do the trick with? What reasons do you think this is the case?
  4. Explain the result of adding weights to the dishes. What physical principle is in play that changes the result?
  5. Watch the following clip: The Grinch Who Stole Christmas Table Scene
     1 
    www.youtube.com/watch?v=dO-LeKbsa6U
    . In the original script, the dishes were supposed to fly everywhere when the Grinch pulled the cloth. Instead, we got an improvised scene of the Grinch pulling off the tablecloth pull trick, and then going back to knock everything over anyway. Based on what you observe, explain the mistakes that the prop team and the Grinch made in the original plan.

Conclusion.

This parlor trick has some interesting physics embedded in it. Hopefully, you were able to appreciate and understand that better based on your experiences with it.

Lab Write-Up Guidance.

Use the following outline to help you write up your lab report completely and correctly.
  • Title and Header.
    What is the title of the lab and who was in the lab group?
  • Introduction.
    In your own words, what was the purpose of the lab activity?
  • Procedure.
    Describe the steps of the lab.
  • Results and Data.
    Describe the observations you made in the various failed attempts to perform the trick. Also, describe what happened when you changed the amount of weight in the cup.
  • Discussion.
    Answer the discussion questions. Be sure that you state the original question and organize it in a way that is easy to follow.
  • Conclusion.
    Write a couple paragraphs about the lab. Did you find anything interesting or surprising?
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