Falling Ring Catch

Engage your friends with this puzzling trick as you combine gravitation force and friction

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Is it possible to catch a falling ring with a loop of string? Sure it is! It’s going to take a bit of experimentation and trial and error, but you’ll get the hang of it in no time! You’ll see how motion and force can sometimes create unexpected results.


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Here's What You'll Need

  • Thin string or yarn
  • Roll of masking tape
  • Metal washer

Let's Try It

  1. Cut approximately 1 meter of string (or yarn).

  2. Run the string through the hole in the middle of the washer and then loop it through the washer two or three more times.

  3. Knot the ends of the string, forming a loop, with the washer at the bottom of the string and the knot at the top of the string.

  4. Use your non-dominant hand to hold the string, with the washer at the bottom of the loop. Spread your fingers so that the top of the loop has a slightly larger width than the roll of masking tape.

  5. Use your dominant hand to hold the roll of masking tape parallel to the floor. Lift the string and guide your washer string loop down through the middle of the roll of masking tape.

  6. Keeping the tape parallel to the floor, bring it all the way up to the top of the string loop.

  7. Let go of the ring. Does it fall to the floor?

  8. If it fell to the floor, repeat the steps and try it again. But this time, as you let go, flick one side of the tape downward. Bet it doesn’t fall to the floor this time!

How Does It Work

The combination of gravitational force and friction caused by the tape roll falling toward the ground is to blame for the washer/string loop tying itself to the tape roll. Giving the tape roll just the right flick at the top of its fall will cause the washer to loop around both the roll of tape and the string loop, forming what’s called a lark’s head knot.

Take It Further

You may be thinking, “How will I apply this experiment?” This activity is a great tool for teaching observation, trial and error, and experimentation. Encourage kids to keep trying to get the tape roll caught by the string and washer loop until they succeed. Then discuss the process.

Science Fair Connection

Performing the Falling Ring Catch is pretty cool, but it isn’t a science fair project. You can create a science fair project by identifying a variable, or something that changes, in this experiment. Let’s take a look at some of the variable options that might work:

Try testing different lengths of string. How long or how short can the string be in order to still successfully catch the falling ring?

That’s just one idea, but you aren’t limited to that! Try coming up with different ideas of variables and give them a try. Remember, you can only change one thing at a time. If you are testing a different lengths of string, make sure that the other factors are remaining the same!

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