How Do You Calculate the Tangential Speed of a Fan Blade?

[Chica1975]

Homework Statement

a ventilation fan has blades has 0.25m in radius rotating at 20rpm. what is the tangential speed of each blade tip?

Homework Equations

Vtangential - Omega *r
Conversion to radians is 2Pie radians

The Attempt at a Solution

I have tried everything I know and I am really hating physics right now. I can't seem to get this out.

I converted the rpm to radians like this
20(2pierad)*60
then I used the Vtangential equation

Its out - can someone please shed some light. I can't wait for the exam on Tuesday to be over. I have a teacher who can't even speak proper english let alone teach physics.

Thank you!

 

[rock.freak667]

Your method is correct.

 

[Chica1975]

Thanks - but when I do the whole thing I come out with a totally different answer to the one they have suggested is correct apparently the answer is .52m/s (I get a ridiculous number).
where am I going wrong?

 

[rock.freak667]

Their answer is correct. You need to post your work for me to see. However, I think it is likely that you are calculating the value of ω (omega) wrong on your calculator.

EDIT: I now see what you did, you are multiplying by 60, you need to do the opposite.

EDIT2: What ehild is saying.

 

[Chica1975]

I am entering it the exact same way as I wrote it but I get some ridiculous answer - here it is 7539.822

 

[Chica1975]

then I * that by the radius

 

[ehild]

I converted the rpm to radians like this
20(2pierad)*60

20 rpm is 20 revolution in 1 minute. How many revolutions is it in 1 second, less or more than 20?

ehild

 

[Chica1975]

more I thought I had taken care of that by * by 60? Where am I going wrong?

 

[verty]

This question is really one of intuition. If the intuition is wrong, plugging into formulas will be risky and prone to error.

The first question to ask is, what path do the blade tips follow? And that is, they travel around the circumference of a circle defined by the blade length. One knows that the length of the circumference is 2pi*radius and one is given the radius.

The second question is, how many revolutions are there in a second? We are told 20 in one minute. That's 40 pi * radius in 1 minute. How far in one second?

 

[rock.freak667]

more I thought I had taken care of that by * by 60? Where am I going wrong?

rpm = revolutions per minute.

What does the word 'per' imply?

 

[Chica1975]

divide

 

[Chica1975]

i will try again

 

[rock.freak667]

divide

So instead of multiplying by 60, divide by 60.