# Thread: Circular Disk

1. ## Circular Disk

How many revolutions will a circular disk with a diameter of 4 feet have to complete after it has rolled 20 feet?

Question:

How is this done without trigonometry? Every time I see the word REVOLUTIONS, I quickly think trigonometry.

2. ## Re: Circular Disk

No trig needed here.
What measurement of the circle do you need to calculate first?

3. ## Re: Circular Disk Originally Posted by Debsta No trig needed here.
What measurement of the circle do you need to calculate first?
I must calculate the circumference first, right? If so, the formula C = pi•d comes in handy. I will check your reply in the afternoon today.

4. ## Re: Circular Disk

The distance rolled in a single revolution is the circumference of the disk. This is pi(d), where d is the diameter. So, in this case that is 4•pi.

To get the number of revolutions can I divide 20 by 4pi?

Let R = number of revolutions

Let C = circumference

R = 20/C

R = 20/4•pi

R = 5/pi

Is this right?