If so, I have a dynamics problem for you that I cant figure out...

Ok so there is a picture of a box fan, with radius = 1.75ft.
The question reads, Just after the fan is turned on, the motor gives the blade an angular acceleration alpha=(20*e^-0.6*t) rad/s^2, where t is in seconds. Determine the speed of the tip of the blade when t=3 seconds. How many revolutions has the blad turned in 3 seconds? When t=0, the blad is at rest.

Ok so to get the angular velocity, you have to integrate 20*e^-.6*t with respect to t from 0 to 3 seconds. That result yields 27.82337. So that number times the radius gives the speed at the tip, so (27.82337*1.75)= 48.7 ft/s. This part is right.

The part I cant figure out is the last bit about how many revolutions.

I know you have to integrate alpha twice to get to theta (position), and doing so and evaluating from 0 to 3 seconds, yeilds 46.372284 rad/s.
So to get revs you divide theta by 2*pi, so 46.37/(2*pi) = 7.38 revs.

This answer is wrong, and the correct answer is 8.54 revs.


help?

 

No one?

 

Disclaimer:OK I am a construction worker with a buzz that is not too bright.


Try pi x r squared=d ft.

48.7x3=a


d/a=revs in 3 secs.

 

That wouldnt work because the area would be static after 1 revolution, you would need a double integral but the parameter of the second one is what I am trying to find.
Thanks though

 

Why not just count them????

 

physics is still the hardest course I've ever taken. In fact, that's the only course I've taken that was a true test of intelligence. A lot of abstract thinking is required, not just rote memorization. I've got a lot of respect for engineers.

 

I have ADDDDDDD i think....
for me to wire something up i have to sit down and force myself to draw it out on paper and then go over it in my head to make sure its goin to work... then i can relax and run one wire at a time.

 

my roommate said something about your integration being wrong??

 

The integration looks correct to me, I am curious if you found the correct way. When I look at your work I don't see anything wrong, but I am terrible about seeing something and then getting it stuck in my head that it is the right way. Good luck.

 

I did the integration by hand and on a calculator, im sure its right. I still dont know how to get the answer.