1. The ball and hand start with no G's (gravitational energy)
2. The air starts with no energy. :)
3. The hand starts with as many C's as we will need
4. The ball and hand will both wind up with the same G's (assuming same mass).
5. The ball and hand have the same K's throughout the scenario.
Here is what we came up with initially. The arrow color indicates "the reason" behind the transfer or transformation. The key to the colors is at the bottom of the picture. Metabolism (orange) indicates that there is some form of bodily process going on to transform the energy. Force (purple) here indicates a contact force (push from hand or ball) or friction (between air and ball). Conduction (red) indicates thermal energy transferring by touching (hand to ball or hand to air). Finally, gravity (green) is sort of an uncomfy one for us. It is our only non-contact force. More is said about this below. The circled numbers correspond with the numbers in the list. The list is also retyped here to make sure it is readable:
1. Hand warms ball - this assumes that the hand is at a higher temperature than the ball. AND probably we should have drawn a C --> T for the first T in this part like we did for number 8.
*2. Hand "moves" "up"
3. Hand move
4. Hand moves ball
*5. Hand moves ball "up"
*6. Hand moves ball and friction warms ball
*7. Hand moves ball and friction warms air
8. Hand warms air
9. Hand warms with effort
One note here is that for a given instant, the K's would be the same for both the ball and the hand, but here we have shown all of the transfers and transformations so if you look at all of the end results you will see that the hand and ball only have one K each, one G each, and different amounts of T.
So here is where we need to think more:
A. For numbers 2 and 5, our question became where does the earth come into this diagram? How do we talk about a force transforming energy without the object that is causing that force in the diagram?
B. For numbers 6 and 7, we talk about friction warming both the air and the ball as they move against each other, but how does the friction between the air and the ball make kinetic energy in the ball transfer to thermal energy in the ball? It seems like the air should be involved in this transfer inside the ball. Additionally, I struggle with the idea that you can't have one without the other because they happen simultaneously.
These questions seem to be related to each other. They both address an internal transformation of energy that is caused by an interaction with or force by an outside object. E2 talked a lot about forces and their role in ET, but both Rachel and I were focusing on E1 this summer. Is there an easy explanation to these questions?
Near-simultaneous posts! Jinx!
ReplyDeleteI'm on my lunch break (6 hours of lab teaching today - not too different from the Energy Project days - even have my trusty videographer here with me at lunch!)... Here's my attempt at an answer. We tried to think through these questions some this summer in E2.
ReplyDeleteA. We could think of the Earth as an external force on the diagram and all of the GPE is being stored in the ball. At least, I think that's fair for an earth-ball scenario (where the earth is way bigger than the ball and so will never change its KE as a result of that interaction). So the earth doesn't show up in the diagram b/c it's not receiving energy. But I think if we want to be complete, the energy diagram needs both earth & ball, and the "container" for PE must be the pairing (as an abstract container).
2. I think the way to reconcile this is to think microscopically - little balls speeding up and slowing down resulting in changes to *average* KE of both... I'd say more but it's almost time to teach.
(Today in inquiry class-- why big pinholes make blurry images, and how to construct reasonable representations to show this.)
I like the way this diagram seems to infuse the energy story with information about the mechanisms associated with each transfer or transformation. I am a unclear on one point. Why are there 6 tranformations to K's in the hand and only 5 transfers/transformations from K's in the hand. Wouldn't this imply that a K is accumulating in the hand?
ReplyDeleteAbby, I think Lane is right: we've represented a set of processes that adds kinetic energy to the hand and the ball. I think we fix this by erasing (3) and (4).
ReplyDeleteThis comment has been removed by the author.
ReplyDeleteI see what you are saying. Originally, we put those in there because if you look at the energy in the end (after all transformations and transfers have occurred) then no K's are left. Should we get rid of those processes and instead have a random 'K' in each box to signify that it is moving?
ReplyDeleteAbby, yes, we could do that. We could also stick a G in each box to signify that the hand and ball are already at a certain height, if we want to; or a C in the hand to show that the hand has some baseline level of chemical energy that is always there... I think it just depends on what we want to make explicit. It does feel especially odd, to me, to not signify that it's moving. I guess I could choose to see the movement as the production of G's, instead of the presence of K's, since more G's means the object is going up.
ReplyDelete