Working out RPM using slo motion camera?

[gregmcc]

Serious brain fog this morning and can't work this one out.

I've got a Samsung S21 and filming a rotating wheel using the slow motion camera.

I'm trying to work out the RPM of the wheel. From what I've read the camera shoots at 240fps in slow motion.

Recorded a video and the wheel turns 20 times in 31 seconds which is 38.7 rpm. How do now take the camera slow motion fps into account?

 

[Harmonic]

Is the video playing back at 24 or 30 fps?

 

[gregmcc]

Playing back at 30fps

Guessing here but I think it's 240fps / 30 fps = 8
Then 8 x 38.7rpm = 309rpm. (Which is about right as it should spin at 300fps)

 

[Harmonic]

240/30 = 8. For simplicity sake here let's say the video was 32 seconds long, 32/8 = 4 seconds. 20 rotations in 4 seconds etc...

 

[gregmcc]

Thanks, that makes sense