# Homework Help: Finding pulley diameter with speed in rpm

1. Nov 25, 2016

### Tambourine Man

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

In a pulley system, pulley A is moving at 1,500 rpm and has a diameter of 15 in. Three pulleys, B, C, and D, all of different sizes, are attached to a single output axle. Speed and torque output are changed within the system by moving the drive belt between pulleys B, C, and D.

A speed of 2,000 rpm is required when the drive belt is connected to pulley C. What is the diameter of pulley C?

The question is here, how do I find the diameter? I don't know the input or output speeds.

2. Relevant equations

3. The attempt at a solution

2500/1500

(not sure if this is right)

2. Nov 25, 2016

### billy_joule

You sure about that?...

There's no equality so that's not an equation.
You're looking for an equation that relates the diameter of a pair of pulleys with their rotational speed, check your notes and/or textbook. You know both RPM's and one diameter so can solve for the single unknown; the second pulleys diameter.

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