MHB Finding a radius when an angular speed in RPM is given

AI Thread Summary
To find linear speed when angular speed in RPM is given, the formula used is Velocity = RPM × 2π × Radius. In the discussed scenario, the radius is not provided, making it impossible to calculate linear speed directly. The previous example with a known radius of 60mm yielded a linear speed of approximately 75,000 mm/min. The relationship between angular speed and linear speed is based on the distance traveled in one revolution, which is the circumference of the circle (2πr). Without the radius, the calculation cannot be completed, but assuming a consistent radius across different disc types can provide a workaround.
bsmithysmith
Messages
23
Reaction score
0
When being burned in a writable CD-R Drive, the angular speed is often faster when playing audio... When writing along the outer edge, the angular speed of one drive is about 4800 RPM (Revolutions per minute). Find the linear speed.

The question above it, or before this one was with CD's radius is 60mm and the RPM is 200 but this time I'm not given the Radius. On that previous problem, it took me awhile to figure out after seeing the answer at the back of the book that the linear speed is approximated to 75,000.000 mm/min. (just giving part of the final answer). What I did to get that answer was multiply $$2pi$$ by the RPM, 200, because an entire revolution IS 2pi. Then I multiplied the answer by the radius. All in all it was pretty much:

$$Velocity = RPM(2pi)(Radius)$$

I still don't understand how that equation links to the answer; it's not clicking in my head why and how velocity is linked to circumference multiplied by the RPM.

Back to the main question, how do I find the linear speed when I have not been given the radius?

$$Velocity = 9600pi(radius)$$
 
Mathematics news on Phys.org
bsmithysmith said:
When being burned in a writable CD-R Drive, the angular speed is often faster when playing audio... When writing along the outer edge, the angular speed of one drive is about 4800 RPM (Revolutions per minute). Find the linear speed.

The question above it, or before this one was with CD's radius is 60mm and the RPM is 200 but this time I'm not given the Radius. On that previous problem, it took me awhile to figure out after seeing the answer at the back of the book that the linear speed is approximated to 75,000.000 mm/min. (just giving part of the final answer). What I did to get that answer was multiply $$2pi$$ by the RPM, 200, because an entire revolution IS 2pi. Then I multiplied the answer by the radius. All in all it was pretty much:

$$Velocity = RPM(2pi)(Radius)$$

I still don't understand how that equation links to the answer; it's not clicking in my head why and how velocity is linked to circumference multiplied by the RPM.

Back to the main question, how do I find the linear speed when I have not been given the radius?

$$Velocity = 9600pi(radius)$$
Are the two problems linked? As you say, this cannot be done without the radius.

-Dan

PS Velocity has an associated direction. Speed does not. You are looking for the speed, not velocity.
 
topsquark said:
Are the two problems linked? As you say, this cannot be done without the radius.

-Dan

PS Velocity has an associated direction. Speed does not. You are looking for the speed, not velocity.

My mistake, linear speed is not velocity since its distance/time whereas the one in my book is arclength/time.

The two problems are not linked despite the one I'm asking does not have a radius given. I'll ask my teacher tomorrow about it.

On the other hand, I'm still curious about how [MATH/]2pi(r)(rpm)$$ turns angular speed to linear speed.
 
bsmithysmith said:
how do I find the linear speed when I have not been given the radius?
If the disk radius has not change between CDs, DVDs and Blu-ray disks, we can assume that it has not changed from one problem to the next. :)

bsmithysmith said:
I still don't understand how that equation links to the answer; it's not clicking in my head why and how velocity is linked to circumference multiplied by the RPM.
Let $\nu$ be the frequency measured in RPM and $r$ be the radius. Fix some time interval $t$ measured in minutes. During this time, the disk has made $\nu t$ revolutions. The point has traveled distance $d=2\pi r\cdot\nu t$. Therefore, the point's speed is $d/t=2\pi r\nu$.
 
Well as you pointed out, there is pretty much no way to finish it without a radius, so I just used the radius of the CD above (60mm) and I'm satisfied!
 
Suppose ,instead of the usual x,y coordinate system with an I basis vector along the x -axis and a corresponding j basis vector along the y-axis we instead have a different pair of basis vectors ,call them e and f along their respective axes. I have seen that this is an important subject in maths My question is what physical applications does such a model apply to? I am asking here because I have devoted quite a lot of time in the past to understanding convectors and the dual...
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...

Similar threads

Replies
2
Views
3K
Replies
4
Views
2K
Replies
6
Views
4K
Replies
3
Views
7K
Replies
7
Views
3K
Back
Top