# Rate of revolution for a dryer?

• pcandrepair
In summary, the conversation discusses finding the rate of revolution needed for a home laundry dryer to tumble wet clothes uniformly. The solution involves finding the velocity of the dryer tub and then using that velocity to determine the rate of revolution. There is a discrepancy in the equation given by the professor, which is clarified by another user. The correct solution is found and the thread is marked as solved.
pcandrepair
Rate of revolution for a dryer?!?

## Homework Statement

In a home laundry dryer, a cylindrical tub containing wet clothes is rotated steadily about a horizontal axis as shown in the figure below. So that the clothes will dry uniformly, they are made to tumble. The rate of rotation of the smooth-walled tub is chosen so that a small piece of cloth will lose contact with the tub when the cloth is at an angle of 62.5° above the horizontal. If the radius of the tub is 0.385 m, what rate of revolution is needed? (revs/min)

2. The attempt at a solution

First I found the velocity of the dryer tub:

Cos(62.5) = (v^2)/gR
Cos(62.5) = (v^2)/9.8(.385)
v = 1.32m/s

Then I used that velocity to find the rate of revolution:

(v/2(pi)(.385)) * 60sec = 32.74 Revolutions/min

I submitted this answer and it said it was wrong. I can't seem to see what I did wrong. Any help or suggestions would be appreciated. Thanks.

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pcandrepair said:
Cos(62.5) = (v^2)/gR
Where does the Cos(62.5) come from?

mgCos(62.5) = m(v^2)/R
mgCos(angle) is the horizontal component of mg

The masses would cancel and divide g by both sides to get:

Cos(62.5) = (v^2)/gR

Since mg acts downward, it has no horizontal component. Find the component of the weight in the radial direction--that's where the centripetal acceleration points.

Cos(62.5) = (v^2)/gR
That was the equation that our professor lead us up to in class to find velocity. Wouldn't the component of the weight in the radial direction just equal mgCos(62.5)?

I believe your professor made an error. 62.5 degrees is the angle between the radial and the horizontal, not between the radial and the weight.

so to find the velocity i would take the square root of g*R?

v = sqrt(9.8*.385)
v = 1.9424 m/s

then,
(v/(2pi*R)) * 60 = revs/min

(1.9424/(2pi*.385))*60 = 48.178 revs/min

pcandrepair said:
so to find the velocity i would take the square root of g*R?

sin(angle) = v^2 / gR ?

pcandrepair said:
sin(angle) = v^2 / gR ?

Yes.

Ok. The answer was correct. Thanks Doc Al!

Good! (Mark this problem "solved".)

There is no option in my "thread tools" to mark problems as solved. That's where the option should be right...?

pcandrepair said:
There is no option in my "thread tools" to mark problems as solved. That's where the option should be right...?
Yes, that's where it should be. You are not the first to state that can't find that option. (Question: Was this thread moved from another forum into this one? That might explain it.)

## 1. What is the rate of revolution for a dryer?

The rate of revolution for a dryer refers to the number of times the dryer drum rotates per minute. This is also known as the drum speed or drum rotations per minute (RPM).

## 2. How is the rate of revolution measured for a dryer?

The rate of revolution for a dryer is typically measured using a tachometer, which is a device that measures the speed of rotation. Some dryers may also have a digital display that shows the drum speed.

## 3. Does the rate of revolution affect the drying time?

Yes, the rate of revolution can affect the drying time. A higher drum speed can help to circulate the clothes more quickly and evenly, resulting in a shorter drying time. However, too high of a drum speed can also cause clothes to become tangled and take longer to dry.

## 4. What is the recommended rate of revolution for a dryer?

The recommended rate of revolution for a dryer can vary depending on the type and brand of dryer. In general, a speed of 50-60 RPM is considered optimal for most dryers. However, it is always best to refer to the manufacturer's instructions for the specific dryer model.

## 5. Can the rate of revolution be adjusted on a dryer?

Yes, many dryers allow you to adjust the rate of revolution. Some dryers have a specific setting for this, while others may have a knob or button that allows you to change the drum speed. If you are unsure, refer to the owner's manual for instructions on how to adjust the rate of revolution on your particular dryer.

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