# Homework Help: Length of a pulley

1. Nov 9, 2009

### xortan

1. The problem statement, all variables and given/known data

Find the length of the pulley belt system shown if the belts cross at right angles. The radii of the pulleys are 10.5 cm

http://img43.imageshack.us/i/94324138.png/" [Broken]

2. Relevant equations

C=2 pi r

3. The attempt at a solution

(2pi * 10.5) * 2

cant think of any other way to do this and my answer is pretty close to the right one but no cigar..any help would be appreicated

Last edited by a moderator: May 4, 2017
2. Nov 10, 2009

### HallsofIvy

I started to say "there is not enough information" because the distance between the pulleys is not given but then realized that "the belts cross are right angles" is enough.

Draw radii from the point of tangency on one of the circles to the center of the circle. Since radii are perpendicular to tangents, that gives a four sided figure with three right angles. So the fourth angle, at the center of the circle is also a right angle. Now you know that the belt is on the circle for 3 right angles or 3/4 of the circles circumference. Also, the four sided figure, having right angles and 2 adjacent sides of length the radius of the circle, is a square and the other two sides, from the point of tangency to the point of intersection, also have length equal to the radius of the circle.

That is the information you need.