How many physicists does it take to move a boat?
k. This is a physics 1 question that was on the students' homework that we had to help them with.
The Problem: "Two People (with Person A being less massive than Person B) are standing in a boat that is next to a pole. Person A has a mass of 74 kg, the boat is 3.2 meters long, and has a mass of 34 kg. Persons A and B change places in the boat, and after this process, the boat has moved 46.3 cm from the pole. What is the mass of Person B?"
It took 1,2,3,4,5, count them, FIVE physics grad students working on this problem for 3 hours, while the physics 1 students stared aimlessly. And we didn't solve it.
It wasn't until I got to my apartment and wrote stuff out on the markerboard (and trust me, I had chalkboards available all day) that I figured it out.
So, this is the latest cross country game. I'll leave it open until somebody solves it. And whoever does shall get a prize most splendid.
posted by MrB @ 8:34 PM
9 comments
9 Comments:
So here goes:
Assumptions:
1. each person is standing at the very edge of the boat in a point wise fashion (very skinny people)
2. the boats mass is symmetric about its center
3. no external forces
Method:
with the assumption of no external forces, I can conclude that the system as a whole can redistribute itself, but the center of mass about a point of my choice better damn well stay the same. Then I used this to calculate the mass of fatty mcB.
Answer:
93.28 kg (although I suppose my actual answer should be 93 kg with sig figs)
Additional comments:
Just remember that even if I got it wrong, I still answered it before all you guys did...
Josh
WRONG
But good try.
And I'd also like to make a correction to the problem statement: "(with Person A being less massive than Person B)" should read "(with Person B being less massive than Person A)."
kthx
Well of course my answer is wrong if you make that correction (it changes which way the boat moves...which is important.
So, with that in mind, it seems only fair that I get to submit another answer.
Answer:
51 kg
(note, here I have made all the same assumptions)
Josh
Ok, that was alot lamer than I thought it was going to be.
Josh wins, and the next time I'm tempted to waste 3 hours of my life I'm calling Josh.
It gets you back for that time you embarrassed me in my Free Electron Laser presentation... you remember the thing about the gamma factor...
Josh
Hey, that was true! And it wasn't embarrassing, it was just pointing out a discrepancy. :p
So I guess this makes us even then.
We never did build our nitrogen laser either...
83.30 Kg
I didn' read all the postings before putting my answer in, and I also had the wrong approach .. turning the problem around I still get a wrong answer of 63.2 Kg.
Good going Josh !!
Had to remember that this is a second week Physics I problem. It should be an F=ma or conservation of energy problem.
Curse me plopping into bed as soon as I got home, taking a nap, and missing out on all of this physics hilarity.
CURSES!
Oh well, I probably wouldn't have gotten it anyway, since my Physics book is at the rent's and I'm... well... not.
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