Does the Top or Bottom String Break First? You Can Decide With Physics.

Rhett Allain
6 min readFeb 12, 2024
Photo: Rhett Allain. Pulling down on a mass with two strings

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.

So, if you said the top string would break — you win. But wait! Let me pull the string again and see what happens.

Here you can see if you pull down VERY FAST on the bottom string, then the bottom string breaks and not the top one. Isn’t that fun?

But why? If you pull down very fast then the top mass would try to accelerate. However, this mass requires a large force to cause an acceleration — that force has to come from the bottom string (since the top string pulls up). If the tension exceeds the breaking strength — boom, it breaks.

Free Body Diagram

Let’s create a force diagram to help us understand what makes the string breaks. There’s really just 3 forces to consider: the tension in the top string (T_t), the bottom string (T_b) and the downward gravitational…

--

--

Rhett Allain

Physics faculty, science blogger of all things geek. Technical Consultant for CBS MacGyver and MythBusters. WIRED blogger.