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# Helpp with question... watch

1. ..
2. (Original post by iamspiderman)
I know that rpm ( revolution per min) can be changed to rd/s by multiplying by 6.28/60
Suppose I'm standing at 5m from the centre. I go all the way round in 15secs, and I want a speed in metres per second. By juggling the units ( where x is distance, t is time), I therefore need to know how far I've travelled. How far have I travelled in the 15 seconds?
3. (Original post by iamspiderman)
I know that rpm ( revolution per min) can be changed to rd/s by multiplying by 6.28/60
If you know the distance from the centre, you could calculate the circumference. It'd go that far and then you can use that distance and the time you know. How does that sound?
4. ..
5. (Original post by iamspiderman)

I have found the circumference :

3.14 x 5^2 = 78.53 ( this is the distance)

But for the time how do I put 1 revolution per 15 seconds ?
Doesn't it have to be changed to 1 rev per 60 seconds? The time part of the equation is confusing me..
That's not the distance. Run it past your internal sanity-checker first - there are two reasons 78.53 can't be the distance.

1) really? that's three swimming-pool lengths, just to walk around a 5m circle?
2) the unit of your answer is m^2, since it was calculated as (number) * (distance in m)^2. Distances are never in m^2.
6. (Original post by iamspiderman)

I have found the circumference :

3.14 x 5^2 = 78.53 ( this is the distance)

But for the time how do I put 1 revolution per 15 seconds ?
Doesn't it have to be changed to 1 rev per 60 seconds? The time part of the equation is confusing me..

I bet by now you've found the value of the circumference rather than the area of the circle. So you travel that distance in 15 sec. You will be able to calculate the speed in m/s.

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Updated: January 18, 2015
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