Physics & Physical Science Demos, Labs, & Projects for High School Teachers

Inertia Example #406 – Which String Breaks?

Posted on: June 20, 2008

Inertia with a clay blobI do so many demonstrations of inertia, you’d think it would sink in. Anyway, this is a nice little one I usually start a class with soon after I discuss inertia.

The string at “A” is tied to a solid surface. In my room, it’s the TV arm on the wall. String “B” is the same type of string, I usually set it up with two “B” strings hanging down. In the middle is a piece of wood with two eyescrews attached. Around the wood is a pound of dollar store clay. (I really hate dollar store clay, the dyes come out and stain everything it touches. I just don’t touch the clay and this setup goes back in a bag to be used one day a year.)

First, ask the students what happens if you yank quickly on one of the “B” strings. Allow some time for discussion. Inertia of the blob of clay is going to resist the motion of the yanking and the “B” string is going to break. Do it for them.

Part two, tell them you are now going to gently pull on string “B” and ask what is going to happen. In this case, you are slowly applying a force downward. Let’s say you pull down with a force of 10 pounds. String “B” feels 10 lbs of force, but due to the weight of the clay blob, string “A” should feel 11 pounds of force. Yeah, I know, kg, not pounds, but since clay comes by the pound, I wrote in pounds. Sue me. Anyway, string “A” is going to break first because it always has one more pound of force on it.

For the very first time this year, the “B” string broke for both of them in one of my classes. That sure took the wind out of my sails. I cut another piece of string and did it again, although I lost the wow factor. Can’t win them all.

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3 Responses to "Inertia Example #406 – Which String Breaks?"

No, you were right to measure your force in pounds. The way you wrote it, you seem to be measuring force, which cannot be measured in kg.

Yeah, I know, I could say Newtons and convert the pound of clay, but for the write-up, it just flowed to stick with pounds.

No, I’m saying you were right the first time, and you don’t need to apologize for not measuring force in pounds. Force IS measured in pounds. It is never measured in kilograms.

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