- Kinetic Energy Issue: We took out steps 3 and 4 (not just 3 as Lane mentioned in the previous post's comments) because we agreed that the ball would then be gaining kinetic energy. We did not put in an extra K in the ball and the hand because we believe that it is evident that the ball is moving because of the gain in gravitational energy.
- Friction btwn Ball and Air: We also felt better about our ideas surrounding the friction of the air with the ball/hand because we added in a K for the air. This K is the bulk movement of the air as it gets pushed out of the way by the ball/hand. We called this "macro" movement. As a secondary process, this "macro" movement dissipates into smaller "micro" movements, which we represented by transforming the K to a T in the air.
- We drew new BLUE arrows to represent this change and I thought perhaps this might be related to entropy??
- Overall Effort from the Human: We erased some of the C-->T transformations in the hand during the lowering process based upon our observations that the body gets warmer when "walking up stairs" as opposed to "walking down stairs" at the same rate. We are still unsure about this and how it relates (or if it relates) to the fact that the force exerted by the arm/body on the hand is the the same for both situations!
- How can it be that you are exerting the same force (doing the same work) but using more energy?
Lowering the Ball at constant speed:
During our meeting today, I started to wonder (again) about the friction-related energy transfers/transformations in your diagrams. (I know you may ultimately just throw those away, but I thought I'd comment anyway.) My question became: should these frictionful energy exchanges be between the _ball_ and the _air_ and not involve the hand? (I'm sort of imagining the thermal energy arising out of rubbing between the air and the ball, so I'm wondering if the kinetic energy of the air becomes the thermal energy in the ball and vice versa?)
ReplyDeleteJust so folks know - my graduate level PER course has 2 PhD students and 3 of our MST (Master of Science in Teaching) students. We are doing this problem in class today. Last week, we talked about the insanely difficult problem of a hand that gets a box moving from rest - today, we are comparing the constant speed motion up and down.
ReplyDeleteI wish I could record their conversation, but that feels... unethical right now!
At some point, I should write up (for someone else to post here) a description of the course we have been designing and how it's going. Right now, we're doing ET. Soon, though, we'll switch to talking about Eric Brewe's Modeling Instruction in Energy. We already worked through the UW energy tutorials, and are going to spend time on Ranking Tasks as well as RealTime Physics. Readings include the K12 framework in which energy is a cross-cutting concept... If there's an audience and interest, I'll write it up.