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Objects in Water as an Example of Newton’s Third Law
This is maybe one of my favorite physics demonstrations showing both the buoyancy force and Newton’s third law. You can start it off as the following question:
Imagine I have a beaker of water on a scale that reads 4.06 Newtons (most scales you can switch to display units of Newtons instead of grams). What will happen to the scale reading if I lower a metal cylinder into the water (but still hanging from a support)? Will the scale reading go down, stay the same, or go up?
Go ahead and consider this question. I’ll wait. Here’s a picture of a dog and a flower (while you think).
I’m going to show you the answer (of course), but let’s review some physics first. I want to go over two things: Newton’s third law and buoyancy.
Newton’s Third Law
I really think that we focus on the “three laws” too much. But no one listens to me. They are essentially a way to describe interactions as a force and the consequence of having a net force on an object. The third law should go something like this:
Forces are an interaction between two objects. If object A pushes on B, then B also pushes on A with the same magnitude force but in the opposite direction.
Here’s a diagram that might be useful.
It’s just the way forces work. Just like the distance from New York to Los Angeles is 2,800 miles and the distance from LA to NY is ALSO 2,800 miles (but in the opposite direction).
OK, I just want to say something that I don’t like. Often I will see this force law (Newton’s third law) as: “for every action there is an equal and opposite reaction”. That’s not wrong, but it’s sort of misleading. It sort of implies that the rule only works when things are moving (action and reaction). There, I said it.
Here’s one other common mistake with this two object nature of forces. Suppose I have two…
