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The Physics Problem That Fooled You: Why a Submerged Ball Changes a Scale’s Reading
There are some physics questions that keep popping up on social media channels. Here’s one that I see quite a bit. The great thing about this problem is that you can basically just show the picture — no prompt needed. Of course, I’m going to describe the situation anyway. It goes like this.
Imagine you have two identical cups with the same volume of water in them. The cup on the left has a submerged ping pong ball (connected to the bottom of the glass so that it stays underwater). The right one has a steel ball (with the same size as the ping pong ball) submerged in water and suspended from a string. Which way will the balance tip? Or maybe it will stay balanced.
What do you think?
OK, I’m going to fully explain this question (with actual cups of water and balls). But if you want to really understand this problem, there are three big physics concepts you need to grasp. Let me go over these ideas.
Newton’s Second Law
What is the nature of a force and motion? We say that a net force changes the motion of an object. The change in motion is described with the acceleration (the rate of change of velocity). As an equation, Newton’s second law looks like this.
