At how many rpms will a blade tip break the sound barrier

In summary, to find the rpm at which a blade tip will break the sound barrier on a 400-foot windmill with 130-foot blades, we use the equation v = rω and solve for ω. This gives us a value of approximately 9 radians per second. To convert this to rpm, we use the conversion factor of 1 revolution = 2π radians and calculate 9 radians per second = 9 * 60 seconds = 540 revolutions per minute. Therefore, the blade tip will break the sound barrier at approximately 540 rpm.
  • #1
natux
3
0

Homework Statement


The windmill is 400 feet tall and each of it's three blades are 130 feet long. At how many rpms will a blade tip break the sound barrier? Speed of sound: 1125ft/sec


Homework Equations


Linear speed: v = rω
Angular speed: ω = Θ/Time


The Attempt at a Solution


1125ft/sec = 130ft(ω)
ω = 1125ft/sec * 1rad/130ft ≈ 9
 
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  • #2
What are the units of the number 9? Are they RPMs like what was requested in the OP?

Hint: RPM usually means 'revolutions per minute'.
 
  • #3
Sorry, the 9 is in radians per second, but the problem is I get stuck there. I know I need the linear speed...but I don't how to go about doing that in this situation.
 
  • #4
You have already solved this problem! You are looking for an answer in rpm's. You have a solution in radians per second. There is only a conversion to do from here.
 
  • #5
natux said:
I know I need the linear speed...but I don't how to go about doing that in this situation.
You already have the linear speed, 1125 ft/sec, as used in your originally posted attempt at a solution.
 
  • #6
Thanks, I got it figured out, I don't know why this confused me so much...
 

1. What is the speed of sound and how is it related to RPM?

The speed of sound is approximately 343 meters per second (or 767 miles per hour) in standard atmospheric conditions. It is directly related to RPM (revolutions per minute) in the sense that the faster an object rotates, the faster the sound waves it generates will travel.

2. Can a blade tip actually break the sound barrier?

Yes, it is possible for a blade tip to break the sound barrier. However, this is only achievable at extremely high RPMs and in specific conditions, such as in a vacuum or at high altitudes where the speed of sound is lower.

3. How fast would a blade tip need to rotate to break the sound barrier?

The exact RPM needed to break the sound barrier would depend on various factors such as the blade length and shape, air density, and temperature. Generally, it would require tens of thousands of RPM to achieve this speed.

4. What are the dangers of reaching supersonic speeds with a blade tip?

Reaching supersonic speeds with a blade tip can cause extreme vibrations and stresses on the blade, potentially leading to failure and catastrophic consequences. It can also generate intense noise and cause damage to surrounding structures.

5. How is the speed of a blade tip measured?

The speed of a blade tip is typically measured using a tachometer, which is a device that measures the rotational speed of an object. It works by detecting the number of revolutions per minute and converting it into a numerical value.

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