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a mirror?

MrKev wrote:4.5.

2.25 is getting there - the maths is along the right lines, but it goes round exactly twice that.

For equal sized circles, the moving one goes round twice.

[edit]
....and seeing as I *know* I'm right:

What beings with 'A' and can be used to look up skirt?

Kev

You're wrong by the way .

moonbeam wrote:Time for another riddle ...... a nice easy one (ish !!)

In the figure below, the radius of circle A is 2 units, the radius of circle B is 3 units.
Starting from the position shown in the figure, circle A rolls around circle B (circle B remains stationary).
At the end of how many revolutions of circle A, will the centre of circle A first reach its starting point?





(Just for the record, this problem is taken from: http://www.projecteureka.org/ ...... so if you like puzzles, go and check it out ).

Uh I got 2.25 counting Pi as 3.14 .....

(2*2)*Pi then (3*3)*Pi ((3*3)*Pi)/((2*2)*Pi) = 2.25 is that right ish.... (if pie went to a further decimal point it might be slightly different.

Just to point people in the right direction - here's a lil' help:

Circumference of a circle = 2 x pi x r (radius of circle).

So, in my example, the smaller circle (the one moving on the outside) has a circumference of 2 x pi x 2 = 4pi
and the circumference of the larger (stationery) circle is 2 x pi x 3 = 6pi.

Therefore, when the smaller circle travels around the larger circle, it has to travel a distance of 6pi, which is equal to 1 and a half times its own circumference, so it would appear that it makes 1 and a half revolutions ..... this is the most popular incorrect answer - there's more to it than that:

Take 2 identical size coins and place them on a flat surface.
Keep one stationery and roll one around the other - how many revolutions will the coin make when it reaches its starting point ??
The obvious (and incorrect answer) is one - try it for yourself and see and the try the original problem again.


EDIT: I think some people are confusing the area of a circle and the circumference of a circle.

Area = pi x r x r

Circumference = 2 x pi x r

moonbeam wrote:

MrKev wrote:4.5.

2.25 is getting there - the maths is along the right lines, but it goes round exactly twice that.

For equal sized circles, the moving one goes round twice.

[edit]
....and seeing as I *know* I'm right:

What beings with 'A' and can be used to look up skirt?

Kev

You're wrong by the way .

course he is...and slightly arogant in that edit two...appologies if it was a joke but it does nto coem across that way......too correct you..... with two circles of exact same circumfrence and one rolling aorund the other ther will be a total of one revolution.....it is a simple cog theory...two cogs of exact same sirxe conected turn at exact same speed contacting at the exact same spots each time.

moonbeam wrote:Just to point people in the right direction - here's a lil' help:

Circumference of a circle = 2 x pi x r (radius of circle).

So, in my example, the smaller circle (the one moving on the outside) has a circumference of 2 x pi x 2 = 4pi
and the circumference of the larger (stationery) circle is 2 x pi x 3 = 6pi.
but the circumfrence of a circle is R2 * pi (sorry no small squared button) not 2 * PI * 2. plus the answer too 2 * pi * 2 would be 2pi2 (second two is little squared one.

but the circumfrence of a circle is R2 * pi (sorry no small squared button) not 2 * PI * 2. plus the answer too 2 * pi * 2 would be 2pi2 (second two is little squared one.

MagicalSmithy wrote:[with two circles of exact same circumfrence and one rolling aorund the other ther will be a total of one revolution.....it is a simple cog theory...two cogs of exact same sirxe conected turn at exact same speed contacting at the exact same spots each time.

Try it out - you'll be surprised - the answer is not 1 (I'll admit - I thought it was 1 until I tried it)
When the moving coin is half way round the stationery coin, you'd expect the moving coin to have made half a revolution yea ?? Look at how many it's done when it's half way round.

MagicalSmithy wrote:but the circumfrence of a circle is R2 * pi (sorry no small squared button) not 2 * PI * 2. plus the answer too 2 * pi * 2 would be 2pi2 (second two is little squared one.

You're confusing area with circumference.

Area = pi x r x r

Circumference = 2 x pi x r

moonbeam wrote:

MagicalSmithy wrote:but the circumfrence of a circle is R2 * pi (sorry no small squared button) not 2 * PI * 2. plus the answer too 2 * pi * 2 would be 2pi2 (second two is little squared one.

You're confusing area with circumference.

Area = pi x r x r

Circumference = 2 x pi x r

My appologies on that one I just rememebr using pi tp wprk out circumfrence lol.....and I know that you need to divide the circumfrence of the larger by the smaller to get the number of required revolutions...

ad for the coin thing brb,.

Well I roled one ten pence round another and it took two full turns to get round....thats strange but makes sense..

if hte second ten pence was spinning also then it would be half the revolutions required so 1......that is if it travels in the opposot direction such as with cogs....ahhhh..... if a single cog ran round the outside of another it would take two full spins but because two cogs share the load they perform one full spin each....

Surprising isn't it ..... although it makes sense when you know the answer .

Sooooooo, back to the original puzzle - any takers now ???

Ok wll if the logical answer it 1 and a half times the seemingly daft answer would be uh 3 times

MrKev wrote:
What begins with 'A' and can be used to look up skirt?

Kev

A dictionary.

Mr Grue...we are yet to solve the last riddle as i though we had it wrong. (from whath ad been said...but I agree with a dictionary lol...although A mirror or A little slave child are also adequate answers.

Ah, but the "a" in question is the first entry in the dictionary, rather than the indefinite article, so it's a much neater answer.