… but pass it on a retake!
While quickly building an ad hoc washing line pulley assembly from a bag of hooks, eyes, and pulleys, and a 2×4, I put this together at first (blotted out some background for privacy of myself and neighbours – click for larger view):
Huh. Does not want to hang level. Why? A tenth of a second after the thought, I burst out laughing loudly at my error. Ironic since I love teaching about pulleys in basic physics, and for some reason students are scared of pulleys. (Not as scared as they are of torque (why?), but scared nonetheless. I try to help them overcome those fears.) I made an obvious mistake. (Do you see it?)
My mum (staying for a while) comes out and asks why I’m laughing, but I decide not to explain about tension in ropes and so forth at that moment. Later, perhaps. I just say “I need one more pulley on this”, and begin to install it.
Shortly after (click for larger view):
Aha! I love physics!
(The rest of the assembly? Another half built 16 feet away… Beams spaced by two rigid struts.. .five spans of washing line stretched in between from eyes running along the bottom… All to be painted later.)
-cvj
It is nice to know that we all have been haunted by the chapters in Physics 101 that we all thought we resolved fully and casted them aside, thinking they weren’t going to come back to annoy us anymore.
Even the pulleys should have been told that they have the property of being frictionless.
Ahhhh, you should have informed those lines they were supposed to be massless. Annoying real world effects :).
EJ -yes. That was the flaw. Two lines passing over a pulley mounted on the beam to the left. The line was anchored on the beam (to the right), instead of to the ceiling, so only one line.
(Oh, silly me. Tommy, others:- I blotted out a bit more than I intended on the first picture. Upper right there is a ceiling anchored pulley that turns the rope down to the beam.)
Cheers,
-cvj
Did you initially have two lines on the left, but only one dropping down on the right?
Ahh, sweet mechanical advantage. That’s the kind of thing that would trip me up as well. It’s difficult to spot here because the line on the left blends in with the tree; when I first looked at it, I thought that the rope was just attached by eye hooks to both ends of the beam.
Um, is there no pulley on the right in the first picture? Hard to see.
Ah, yes, real life versus classroom. I spent two years as a physics major at university but I was still baffled recently when I couldn’t siphon water out of my garden pond into a bucket sitting next to the pond.
And I learned about conservation of angular momentum for the second time when I was playing volleyball. I jumped with my hands above my head and swung them forward to hit the ball. Then to conserve angular momentum, my feet swung forward and I landed in a sitting position.