Does the Top or Bottom String Break First? You Can Decide With Physics.
There are a couple of features that make a physics demo great. First, you need something that presents a question to give students an opportunity to make a prediction. Second, you need an event that looks difficult, but is actually easy.
This demo is one of my favorites. There’s a large mass (I’m using a 1 kilogram mass) that is hanging vertically from a string. There’s a second string on the bottom of the mass. It’s just like the image above. So, here’s the question:
If I pull down on the bottom string, which string will break first? The top string or the bottom string. Why?
See. This demo starts with a question and most students will be able to pick an answer and give a reasonable explanation for it. A very common response is that the top string will break first. As you pull down on the bottom string, the top string has to compensate for the bottom tension AND it has to pull up to support the gravitational force on the mass. That makes sense.
Here’s what it looks like when I pull down on the string.
