# Homework Help: Ceiling Fan

1. Mar 12, 2010

### eiriyuki

1. The blades of a ceiling fan have a radius of 0.318 m and are rotating about a fixed axis with an angular velocity of +1.48 rad/s. When the switch on the fan is turned to a higher speed, the blades acquire an angular acceleration of +2.22 rad/s2. After 0.394 s have elapsed since the switch was reset, what is (a) the total acceleration (in m/s2) of a point on the tip of a blade and (b) the angle between the total acceleration and the centripetal acceleration (See Figure)?

http://edugen.wiley.com/edugen/courses/crs2216/art/qb/qu/c08/w1590int.gif

2. Relevant equations
I tried using equation $$\omega$$$$_{2}$$ = $$\omega$$$$^{2}_{o}$$ + 2$$\alpha$$$$\theta$$

3. The attempt at a solution
This is the first time I have used the forums and I'm having a hard time putting in equations and things, so I apologize if the format is silly. I solved for theta in the above equation and my answers were 3.6509 (part a) rev and 14.153 (part b). I do not know if this is right, and if it is, I am not sure how to convert it into degrees so I can figure the position on the wheel.

2. Mar 12, 2010

### rock.freak667

Why did you solve for θ? You don't have ω2. You need to find ω2.

Use the other equations of rotational motion.

Also, how does the tangential acceleration aT relate to the angular acceleration α?

How does the centripetal acceleration relate to ω2?

3. Mar 12, 2010

### eiriyuki

Wow, that is silly. I put my work for another problem I was having trouble with.

For this problem I used, total a=$$\sqrt{(rw^{2})^{2} + (r\alpha)^{2}}$$

I worked out the problem and got 0.99m/s$$^{2}$$, which was incorrect.

4. Mar 12, 2010

### rock.freak667

What value did you use ω?

5. Mar 12, 2010

### eiriyuki

6. Mar 12, 2010

### rock.freak667

That's where you went wrong. You need to use the final angular velocity. It started out at 1.48 rad/s and then allowed to accelerate at 2.22 rad/s for 0.394s. How would you find the final angular velocity given the information?

7. Mar 12, 2010

### eiriyuki

Would I use average angular acceleration?

w - w0 / t - t0

8. Mar 12, 2010

### rock.freak667

Now try it back in your equation.

9. Mar 13, 2010

### eiriyuki

I got the correct answer for both parts now! Thank you so much for you help! :)

Last edited: Mar 13, 2010
10. Jan 25, 2011

### technomaze

Use the other equations of rotational motion.

Also, how does the tangential acceleration aT relate to the angular acceleration α?

How does the
ceiling fan relate to ω2?