Brain Teasers
Rotating Coin
How many times does a coin rotate in rolling completely about another coin, of the same size, without slipping?
Answer
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can you explane? i think you may be wrong.
Could you EXLPANE?!?!?!?!?!?
And I think it's only once because the fact it's not only rotating but also revolving.
And I think it's only once because the fact it's not only rotating but also revolving.
All I can say Mogmatt is that you should try it with two coins of the same size, it really does work, i just can't explain it. sorry.
well if the distance of the edge were Straight it would only turn around once. It's only because it goes in a circle.
could you spell?
could you SPELL???
Now how does that sound as an admission of fault?: "
well if the distance of the edge were Straight it would only turn around once. It's only because it goes in a circle."
The distance is not straight, and of coarse it goes in a circle, thats the point.
Although an explaination should have been goven.
How about imagining it like you have a circular ashtray. If you have two coins, one in the centre, each have a diameter of ONE THIRD that of the ashtray. So if you put one in the centre, one can just circle it, while being in contact with the edge of the ashtray.
This is the same as before, only a side around the whole thing. Think of the outside coin or rotating around the edge on the coin backwards, rather than rotating around the coin forwards.
The inside diameter is 2 pi r, the outside is 4 pi r.
Can you see how it works?
I hope this helps, sorry if it doesn't
well if the distance of the edge were Straight it would only turn around once. It's only because it goes in a circle."
The distance is not straight, and of coarse it goes in a circle, thats the point.
Although an explaination should have been goven.
How about imagining it like you have a circular ashtray. If you have two coins, one in the centre, each have a diameter of ONE THIRD that of the ashtray. So if you put one in the centre, one can just circle it, while being in contact with the edge of the ashtray.
This is the same as before, only a side around the whole thing. Think of the outside coin or rotating around the edge on the coin backwards, rather than rotating around the coin forwards.
The inside diameter is 2 pi r, the outside is 4 pi r.
Can you see how it works?
I hope this helps, sorry if it doesn't
good explanation, cathalmccabe. It's basically the concept of GEARS, and to find out more about how gears and gear ratios work, see this article: http://www.howstuffworks.com/gears.htm
Excellent teaser. I'm going to add this one to my favourites until I can figure out a suitable explanation. Oh, I know it's true alright but logic tells me that each point on the circumference of one coin should map onto a corresponding point of the other so my logic has a fault in it.
Jimbo, your logic does make sense but you have to see that with respect to the point of contact, both coins are making one rotation in opposite directions. But actually, one coin is stationary. So, with respect to it, the other coin is making two rotations.
Bret's comment cracked me up...but it is good to know that I am not the only one guilty of typos in my comments. Of, course I don't think we need to be so intense in our comments, so take a deep breath Mogmatt.
it was realy random i loved it
Try it with two quarters. If the top quarter begins right-side up is rolled around the bottom one, when it reaches the bottom, it will be right-side up again. When it reaches the top again, it will have gone around again.
I'm sorry but i believe you are wrong
Kids, let this be a lesson that things you put on the internet can stay there forever. I am reading the comments I posted more that nine years ago, and I am embarrassed about the way I didn't bother to think this brain teaser through (or even just try it), not to mention my terrible spelling.
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