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New Re: weeding out
When a department intentionally weeds out students in the intro courses, do they tell themselves they're selecting for interest, or for aptitude? Honest question, I wonder if they even consider the difference when doing it.

Here's the weeding out question from my intro physics final. We had been studying pendulums (pendula?), levers and rotation. Then on the final:
Consider a hockey stick laying on the ice. (To simplify, one-dimensional rod on a frictionless surface.) It is struck by a hockey puck, traveling perpendicular to the stick, transferring 100% of its energy. In terms of length of the stick l, distance from the end of the stick of the impact d, mass of the stick m, mass of the puck m', and velocity of the puck v, describe the resulting motion of the stick as it travels and/or spins on the ice.

Sure, I'd like to have spent some time working with the formulas for two- and three-dimensional problems like that. But this was asking us to derive those formulas on-the-fly during a timed exam.

New Fun. :-/
I don't know if a lot of thought goes into philosophy of the weeding-out questions. The profs know that they have too many people in the program, so they want to cut the numbers down to those able, by hook or by crook, to do the problems. Whether they would actually make good physicists is a question to be answered later (when they need to choose advisers and be graded on their qualifier and dissertation). Engineering schools seem to go through the same process.

On the hockey stick problem - that's a pretty good one. But not one I'd want to see on an exam! I guess I'd attack it by starting thinking about what happens when the puck hits the very end of the stick (pure rotation about its center of mass), and what happens when it hits the center (pure translation of the center of mass). But if the puck transfers all of its energy in the first case, then it will stop while the stick rotates, so presumably the puck will be hit when the stick rotates around. At that point, will all of the stick's energy transfer to the puck? I'd think not in practice, but on a frictionless surface, maybe so. That case is probably a variation of the balls-hanging-from-threads see-saw "Newton's cradle" toy.

It's more complicated if the puck hits between those two extremes - being a combination of translation and rotation.

It's the type of problem one could get lost in on an exam... :-(

I'm glad I don't have to think about those things any more!! :-)

New Why would you expect pure rotation with an edge hit?
I knew the center hit would be pure translation, so I started solving for the end strike. It's definitely not pure rotation.

I think I just figured out the trick! Treat the rod as two point masses at the ends of a massless rod. The mass that is struck is at that instant taking the full force. Solve for that motion, and the other end of the rod is motionless.

The general case formula is still a nightmare, but I can solve for the two extreme cases.

New Ah, ...
If the rod were pinned at the center, it would rotate if hit at the end. If it were not pinned at the center, but were on a frictionless surface, would it just rotate? Hmm... No, I guess it wouldn't. The end point would want to move in a straight line along the direction of the initial impact. It would get more complicated along the length of the rod, I think. It's been too long for me to be confident of constructing a solution...

I'm thinking back on the video out there of the behavior of a slinky that's hanging vertically and then dropped. What happens to the end closest to the ground when the top is released? One can be mislead if one jumps to conclusions based on intuition...


     How to take good photos for under $1000. - (Another Scott) - (30)
         GMTA. - (Ashton)
         Re: How to take good photos for under $1000. - (pwhysall) - (4)
             TL;DR: learn how photography works. :-) - (static)
             Re: How to take good photos for under $1000. - (Ashton) - (2)
                 Re: How to take good photos for under $1000. - (pwhysall) - (1)
                     Last sentence: mandatory for every Pol on the planet! (too) -NT - (Ashton)
         One more thing - (pwhysall) - (19)
             Wait, *that's* what he's selling? Holy crap -NT - (drook)
             On Bryan's book... - (Another Scott) - (17)
                 It's got many, many more glowing ones. - (pwhysall)
                 On "imprecise" explanations - (drook) - (15)
                     Yes. And no. - (Another Scott) - (14)
                         How is that "wrong"? - (drook) - (11)
                             It invites confusion. - (Another Scott) - (10)
                                 Yes, and no, and yes again - (drook) - (7)
                                     You've brought up another important issue - (Another Scott) - (6)
                                         Re: weeding out - (drook) - (3)
                                             Fun. :-/ - (Another Scott) - (2)
                                                 Why would you expect pure rotation with an edge hit? - (drook) - (1)
                                                     Ah, ... - (Another Scott)
                                         Belated comment to (this, buried-in 'Hardware'.) - (Ashton) - (1)
                                             Thanks. I remembered the wrong book. - (Another Scott)
                                 Bingo! Max Jammer's 'Concepts of Force' gives you a qed - (Ashton) - (1)
                                     Gotta move it up my reading list! Thanks for the reminder. -NT - (Another Scott)
                         NO!! ... keep-on 'being sensitive' - (Ashton) - (1)
                             :-) "... of course, you learned this in kindergarten..."!!1 -NT - (Another Scott)
         Re: How to take good photos for under $1000. - (pwhysall) - (3)
             Neat. - (Another Scott) - (1)
                 But they won't, though. - (pwhysall)
             350D and the 18-55mm kit lens - (Bman)

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