# What frequency would be required to have twenty 18° segments complete one full revolution on an old vinyl record turntable at various speeds?

#### SpielbergRules

##### New member
Sorry for the long subject, wasn't sure how to condense it. Basically I am working on a project using an old school vinyl record turntable.

For those who may not have had or used a turntable, they rotate at one of 3 speeds: 33-1/2, 45 or 78 RPM.

I thought the 45 RPM would be the easiest to figure out.

Here's what I came up with so far:

45 RPM is 3/4 or a revolution or 270° per second. Another 1/4 revolution (90° obviously) would be a full revolution.

That's the easy part. Unless I'm wrong.

I'd like to get the frequency (in Hz) required for one full revolution 20 segments @ 18° per segment of the turntable.

I hope I explained this right, I know exactly what I want to do, just not sure how to word it. Also hope I didn't add in more information than needed!

I really would like the formulas / equations to figure it out, not just the answer(s), please.

Thank you!

Mike

#### lev888

##### Senior Member
It made sense until the 20 segments part. I can't even parse the sentence. Please clarify.

#### SpielbergRules

##### New member
It made sense until the 20 segments part. I can't even parse the sentence. Please clarify.
Thank you for the reply. I'm sorry I'm not so great at describing things. I can see it in my head perfectly. If you only knew how many revisions that post went thru before I sent it!

Did the attachment show up? Just imagine a pizza sliced into 20 pieces. I'd like to rotate the pizza one full revolution at 45RPM and know the frequency in Hz needed to accomplish that.

I really hope that helped a little! Sorry about the confusion!

Mike

#### HallsofIvy

##### Elite Member
The "twenty segments" is really irrelevant. "Twenty segments at 18° per segment" is 360° or one complete revolution. What is missing is the time. A revolution of "twenty segments at 18° per segment" in what time period?

#### SpielbergRules

##### New member
Thank you again for the reply. As I mentioned, I'm really bad at describing things...

Maybe this will help: At 45 RPM, I would like to know what the [formula for the] frequency is to rotate a 20-slice pizza around 360°.

Or, maybe this: [As an example], I would like to take a picture of the center of each of the 20 slices of pizza's crust, and need to know the f in HZ for one full revolution.

And you know, now that I am typing these, I wonder if the answer is as simple as 20 Hz. 20 'slices' per second would be 20 Hz, right?

Sorry again for any confusion.

Thank you.

Mike

#### Subhotosh Khan

##### Super Moderator
Staff member
Thank you again for the reply. As I mentioned, I'm really bad at describing things...

Maybe this will help: At 45 RPM, I would like to know what the [formula for the] frequency is to rotate a 20-slice pizza around 360°.

Or, maybe this: [As an example], I would like to take a picture of the center of each of the 20 slices of pizza's crust, and need to know the f in HZ for one full revolution.

And you know, now that I am typing these, I wonder if the answer is as simple as 20 Hz. 20 'slices' per second would be 20 Hz, right?

Sorry again for any confusion.

Thank you.

Mike
1 rps = 1 Hz

60 rpm = 1 Hz ,...edited

Last edited:

#### SpielbergRules

##### New member
Thanks everyone for your replies. As I read them all, I realize how bad I REALLY am at describing things!

In my last reply I wrote about "taking a picture of the center of each slice"

I need a strobe to flash at the center of each of the 20 slices every revolution. <- I think that's the important part I didn't mention [properly].

if the turntable rotates at 45 RPM then that is one full revolution every .75 second.

So if I divide .75 by 20 I get 0.0375.

So, every 0.0375 of a second a slice should go by right?

I attached an updated image that I hope will help.

Thanks to all who put up with my terrible explanations!

#### Attachments

• 1,008.7 KB Views: 1

#### lev888

##### Senior Member
Maybe this will help: At 45 RPM, I would like to know what the [formula for the] frequency is to rotate a 20-slice pizza around 360°.

Or, maybe this: [As an example], I would like to take a picture of the center of each of the 20 slices of pizza's crust, and need to know the f in HZ for one full revolution.

And you know, now that I am typing these, I wonder if the answer is as simple as 20 Hz. 20 'slices' per second would be 20 Hz, right?
You have a turntable set at 45rpm. On the turntable you have a pizza divided into 20 slices. You want to set a camera to take 20 shots with such frequency (in Hz) that each slice center is centered in the frame. Sounds right?
One revolution is how much time (in seconds)? Let's say it's T.
You need to take 20 shots during one revolution, which takes T sec.
20 shots - T sec
x shots - 1 sec
Calculate T and solve for x.

#### SpielbergRules

##### New member
You have a turntable set at 45rpm. On the turntable you have a pizza divided into 20 slices. You want to set a camera to take 20 shots with such frequency (in Hz) that each slice center is centered in the frame. Sounds right?
One revolution is how much time (in seconds)? Let's say it's T.
You need to take 20 shots during one revolution, which takes T sec.
20 shots - T sec
x shots - 1 sec
Calculate T and solve for x.

Yes that sounds right. Couldn't have explained it better myself. Seriously.

You're asking a 57 year old man who couldn't do algebra when he [surprisingly] graduated in 1982 to do it in 2021? Just kidding. I'll do my best. I am sure I am doing this the long way, but here goes:

20 shots: .75 sec.

Earlier I divided the .75 second by 20 slices and got 0.0375. Is that on the right track?

Hz, of course is cycles per second. so if it's .75 per rev, to make up that extra .25, it would be 25 slices.

Man I'm confused.

Sorry.

Mike

#### lev888

##### Senior Member
Yes that sounds right. Couldn't have explained it better myself. Seriously.

You're asking a 57 year old man who couldn't do algebra when he [surprisingly] graduated in 1982 to do it in 2021? Just kidding. I'll do my best. I am sure I am doing this the long way, but here goes:

20 shots: .75 sec.

Earlier I divided the .75 second by 20 slices and got 0.0375. Is that on the right track?

Hz, of course is cycles per second. so if it's .75 per rev, to make up that extra .25, it would be 25 slices.

Man I'm confused.

Sorry.

Mike
How did you get .75?

45 rev in 60 sec
1 rev in T sec
Solving the proportion for T:
T = 60/45 sec = 1.3333 sec

20 shots in 1.333 sec
x shots in 1 sec
Solving the proportion for x:
x = 20/1.333 = 15 shots per sec, or 15 Hz

#### SpielbergRules

##### New member
@lev888 Thank you! You have NO idea how hard I struggled with this! Thanks to everyone else who offered help too! I really did try to figure this out myself, just not smart enough I guess!

*THANK YOU!*

Mike

#### lev888

##### Senior Member
@lev888 Thank you! You have NO idea how hard I struggled with this! Thanks to everyone else who offered help too! I really did try to figure this out myself, just not smart enough I guess!

*THANK YOU!*

Mike
Don't forget to post the photos when you are done I'm into photography, need to try something like this.