On Wednesday afternoon (8/7/13) in E1, a switch was made
from energy to forces. The class was
taken to square one: considering the forces acting on a person sitting in a
chair. Surprisingly, this led to a
nearly one hour conversation, both in small groups and the whole class. In this video segment, Table 3 is presenting
their whiteboard, which shows a representation of the forces acting on the
chair.
During a previous whiteboard, Lane (the class instructor)
brought forth an interesting example.
First, he shook hands with another teacher in the class and asked about
the interactions. The class responded
that each hand exerts a force on the other.
Then he posed: what if the other person no longer existed? There would no longer be a force of one hand
on the other. This example was then
shifted to the earth/chair interaction.
What would happen if the earth is gone?
Rob poses this very question at the beginning of the video.
I am not so much interested in the exact physics details
here. Trevor makes an astute observation
that in order for the chair and floor to interact, something would need to “push”
them together now that gravity is no longer doing so. He comments that part of the issue here is
that “we haven’t talked about what causes the normal force.”
What interests me the most, and is directly related to the
comment above, is the question posed at the very end. Mikes asks: “We’re talking about the forces,
we’re not necessarily talking about why they’re there, right?”
In the moment, I wanted to jump out of my seat and say, “Yes! We DO care about why the forces are there!” I think we NEED to know the origin of any
given force, or else we cannot possibly include it in our diagram
correctly. For the most rudimentary
force diagram of the chair on the ground, we could certainly draw an
unidentified force vector pointing down which is balanced by an unidentified
force vector pointing up. However, this picture
is unsatisfying. It shows no insight
into “what is really going on.”
This requires that we have a class consensus definition for
each different force. We must agree upon
the definition of a normal force. To do
that, we should be looking at the CAUSE of the normal force, down to
compression at the molecular level.
Once the class is on the same page, then the word normal force can be
used with comfort. Similarly, we must agree upon the definition of the
gravitational force. We must agree that
the CAUSE is two (or more) objects interacting with each other via an
attractive force at a distance.
The moral of my story: knowing WHY the forces are there is
crucial in order to draw an accurate force diagram. And the WHY can be determined simply by coming to a class consensus early on.
Full video title: E1 130807 1256 T3-1
Very interesting to me Brad. We are having a lot of conversations about "mystery interactions" in E2--including metabolism (how does the body change chemical energy to kinetic energy) and the thermoelectric effect (a temperature gradient can generate a current, apparently.) I don't know about force diagrams, but the energy diagrams definitely seem to be these infinite accordions (I just made up this phrase)--there are so many transformations that you end up having to simplify some of them and just "this happens". But I agree with you--I always want to know the cause.
ReplyDeleteInfinite accordions...I like it! Oh yes, simplification seems to be quite important in order to get a diagram down to a manageable few elements. My belief is that understanding the CAUSE or origin of each possible type of force or energy is what allows us to deem whether or not we should include it in our diagram for a given scenario. Maybe I can start tracking down specific evidence of that in other videos.
DeleteYes, very interesting! Two things:
ReplyDelete(1) You say, "And the WHY can be determined simply by coming to a class consensus early on." I think coming to consensus as a class on a definition/causal explanation is anything BUT simple. I suspect that the confluence of events is much more complicated than that.
(2) Have you ever heard the expression, "It's turtles all the way down?" I tend to be most satisfied when an explanation includes micro-level, causal stuff, but that may be cultural, rather than absolute. (I'm sure it is, actually.) A force diagram with two opposing, equal-length arrows _does_ explain why something accelerates or doesn't -- it's one turtle down from constant speed, it seems. So I guess I'm saying whether or not we need to know _why_ the forces are there depends on what our question is. Can you help me understand why you disagree?
1) I agree that the original coming to consensus is not necessarily an easy task. But I think it's vital. E1, to me, appears to take this idea of consensus seriously, and the instructors have dedicated much time to letting the teachers explore what they know and don't know, and eventually come to an agreement. With energy at least, they are developing a macro and micro story simultaneously, showing that the two go hand-in-hand. That's why I want to tell Mike in this episode that we DO care about the why question with forces, just like we do with energy.
Delete2) I haven't heard the turtle expression before. I like it! You're right that the complexity or depth of a force diagram really depends on what you are trying to answer. But I feel the example in this episode suggests that a deeper understanding of the normal force would help Rob. I will make a guess that his idea of the normal force might be as simple as "the force applied by two objects that are next to each other." A full treatment, however, identifies the normal force as the equal and opposite force that an object exerts when another force is applied to it. At the micro-level, a compressed spring is a good analogy. At an even deeper level, you talk about electrostatic interactions.
Let's go back to the example in the episode. With no earth, there is no force to push the chair and floor together, and therefore no normal force is experienced. But since Rob might be working from a mental model that says "objects next to each other experience a normal force," he cannot make the leap to the imagined case of "no earth." A strong understanding of this particular force, down to the micro-level, is crucial for correctly discussing and even diagramming this rather simple scenario.