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Hoax? A task not only for high-schoolers... Answered



       Well, exept for being quite a fun thing to watch, it also should work as a great Physics problem for someone like me (or a bit younger)-studying all sorts of Mechanics and Newton's Laws to work on... The task is to estimate the acceleration the motorcycle should have to do it, and whether it's significantly larger than the one stated in some technical data (there is the motorcycle model somewhere in the video's name).
       If you have a go on it and are a) school-aged and b) not sure you are right, PM me. If you simply want to share your results and/or opinion, write here (but try not to put any solution-I mean formulas).

In case the video doesn't work

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rickharris (author)2011-04-27

It wouldn't be possible as offered at any acceleration. The objects would make some movement and fall.

I think they are on a transparent plastic sheet and the cloth is underneath the plastic sheet. The object may or may not be glues to the sheet.

BUT it's not real even if the bike could do light speed.

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user

Do you mean that an old party trick of pulling out a piece of paper from under a full glass is impossible?

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user

No BUT there is a BIG difference between a glass or even a small table and a long table full of crockery etc!

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lemonie (author)2011-03-02


I'll actually have a go at this.
If the pull is fast enough inertia keeps things in much the same place.
But for a long pull the fabric of the cloth makes a difference, and ideally you'd have it pre-tensioned. You also wouldn't want the cloth to tear...
Have you done any research, or is this the point of the topic?

L

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gruffalo child (author)lemonie2011-03-04

Actually, if Kiteman hadn't been so sarcastic, I'd have told him that this task, as long with many other rather-hard-high-school-ones were in a sort of school Physics not-exactly-competition, and there is a site, with lots of other fun things, but unfortunately it's in Russian...

Yes, I had a go on it, but somehow switched to calculating the speed a row of dominoes falls with, which is also a fun task, but somehow not so shiny and inviting as the motorcycle.

Some boys (and mine is an almost entirely boys' school) went a stupid way of estimating the acceleration by considering it constant, estimating the length of the table by comparing it to a normal person standing behind it and measuring the time... We also had a problem with friction between the cloth and SHAKING crockery.

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lemonie (author)gruffalo child2011-03-04


I think this could be tried with steel-sheet, i.e. the rolls used for food-cans. The tether-point on the bike should be low.

L

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Kiteman (author)gruffalo child2011-03-04

What sarcasm? I answered the question you asked!

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Kiteman (author)2011-02-22

Firstly, you can see that all the stuff on the table is fixed together in one piece (watch the way it moves as it is lifted off the table by the cloth.

Secondly, Look at the way the stuff on the table is lifted as the bike drives away - it *has* to be glued together, or that lifting would make the stuff fall off.

MythBusters had a go at this, and found that it required a much greater run-up (ie a far greater length of rope), and even then most of the crockery and ornaments fell over.

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killerjackalope (author)Kiteman2011-03-04

I haven't seen the mythbusters in question but when pulling a sheet from under crockery on a small table the technique is not to pull it away horizontally but pull it down against the edge of the table - at least that's what the literature says.

I may experiment with this and find out...

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gruffalo child (author)Kiteman2011-02-22

Actually, the point is not to tell 'it is total rubbish because...', but to calculate the percentage of rubbishness as needed acceleration /actual acceleration.
it isn't too easy, because even if you know all the frictions, you should be very cautious of the long table and separate pieces with high centres of gravity.
It isn't as easy as pull a plate from under a brick of so-and-so masses and frictions and calculate the dx(t) and dx/dt (t) or x''(t)-whatever...
 

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Kiteman (author)gruffalo child2011-02-22
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gruffalo child (author)Kiteman2011-02-22

Sorry. I was wrong. By 'no formulaE' I meant I've just spent two whole days suffering from people, who prefer to cover a blackboard (YES WE STILL HAVE CHALK GREENBOARDS) with something terrible, which includes thousands of Latin letters, to saying 'I wrote the energy conservation law and got THIS', although I BELIEVE he can manage to do a bit (LARGE BIT) of Algebra.

By the way, x"(t) is not a formula, but a way to write 'how acceleration changes with time' exactly 6 times shorter.

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