Inertia Example #406 – Which String Breaks?
Posted June 20, 2008on:
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.