# Day 53: Equal vs. Balanced

My students are struggling on the difference between equal and balanced. This is what I wrote on the board, but not sure I like it. Thoughts?

##BFPM

### 4 Comments on “Day 53: Equal vs. Balanced”

1. John Burk says:

I like the definition of balanced, but I’ve started to wonder if saying Fnet = 0, is a better approach, and having students understand that Fnet is the one force that could replace all the others. We definitely went through a period where students were saying that N3 pairs were balanced on 2 different FBDs, and so we then decided to add a caveat about N3L pairs that they can’t be added (and thus can never balance). This seemed to help a bit.

• Yeah, this was in the context of students trying to say N3L pairs balanced. I think they just can’t get that in one sitting…we’ve gone back to it a couple of times and recent quiz shows a pretty good understanding now, but I don’t know that they really understand the difference between equal and balanced yet. Good suggestion to add Fnet=0 to balanced!

2. Andy Fitz says:

This is for 3rd Law? I would not say that 3rd Law pairs balance or cancel out, because they are on different objects. We would never draw a 3rd law pair on the same force diagram and add them together.

For net force, I like to say “add to zero” rather that “cancel out” because the “cancel out” gives the impression that there are no forces on the object, when in fact if you pull my arms in opposite directions with 1 million newtons of force, sure, they will “cancel out,” but those forces will still certainty still have an effect on me and it won’t be like they don’t exist at all, which is what many students seems to think.

Do you use the “equal in magnitude, opposite in direction” phrasing at all?

• Right, they were trying to say balance and I was trying to emphasize that they were equal but not balanced. Good call on zero rather than cancel!