# How do you convert flash/min to rev/s using stroboscope

1. Jun 29, 2013

### shin777

mass of color wheel is 133.4g and raidus is 12.7cm. The moment of inertia for disk is I = (1/2)mr^s. We used strobescope and color wheel and measured the time of how long it took till color wheel's yellow part looked stop. It took 2.1s for speed up and 1.5 second for speed down. Maximum value we could go with stroboscope was 525 flash/min.
second highest 475 flash/min
third highest 425 flash/min

question 1) convert maximum angular speed of wheel to rev/s.

question 2) convert maximum angular speed to wheel to rad/s

question 3)Find angular acceleration in rev/s^2 auuming it is constant.

question 4)Repeat question 3 when the wheel was turned off.

question 5)find the angle turned through during the acceleration in question 3, in rev.

question 6)Find I, the moment of inertia for the color wheel in kg m^2.

question 7) find torque exerted by motor during the acceleration of the color of wheel in units of
N m.

question 8)Multiply highest strobe setting which "stopped motion" by one. Multiply the second highest setting by two, and the third by three. Compare these products and then explain what is happening physically for the three highest settings which "stopped" the motion of the wheel. What is the wheel doing compared to the strobe flash rate for each of these three speeds?

question9) State the true direction of motion of the disk.

question 10)How will the disk appear to move relative to its true motion if the strobe is flashhing slightly faster than the true rate of revolution, in the same direction or in the opposite direction?

question 11)Repeat question 10 for the strobe flashing slightly slower than the true rate of revolution.

Last edited: Jun 30, 2013
2. Jun 29, 2013

### SteamKing

Staff Emeritus
Was the wheel traveling at constant speed 1.5s after it was started?

Did you strobe it when it was at constant speed?

Do you care how long it takes the wheel to speed up and slow down?

If the wheel started from rest and then returned to rest, you know that Wi and Wf are going to be equal to zero before the wheel started turning and after the wheel stopped, respectively. How would that relate to the speed of the wheel at times in between?

3. Jun 30, 2013

### Permanence

Why don't you try providing an answer or an attempt first? Instead of asking 'if' you should do it, why don't you give it a go instead? Post your thought-process/work along with an answer and I, or possibly another user, would be happy to help you.

4. Jun 30, 2013

### shin777

This was what I was thinking. Correct me where I am wrong. :(

question 1) convert maximum angular speed of wheel to rev/s.
so the maximum speed we could go for strobescope and color wheel was 525 flash/min.

so 525/60 = 8.75 rev/s

question 2) convert maximum angular speed to wheel to rad/s

question 3)Find angular acceleration in rev/s^2 auuming it is constant.

σ = 8.75/2.1 = 4.17 rev/s^2

question 4)Repeat question 3 when the wheel was turned off.

σ = 8.75/1.5 = 5.83 rev/s^2

question 5)find the angle turned through during the acceleration in question 3, in rev.

Wa = (53.4 + 0)/2
26.7 = θ/2.1
θ = 56.7
56.7/2π = 9 rev/s

question 6)Find I, the moment of inertia for the color wheel in kg m^2.
I = 1/2 mr^2
I = (1/2)(133.4)(12.7^2)
I = 10758/1000 = 10.758 kg m^2

question 7) find torque exerted by motor during the acceleration of the color of wheel in units of
N m.

T = Iσ
T = 10.758*5.83
T = 62.7 N m.

question 8)Multiply highest strobe setting which "stopped motion" by one. Multiply the second highest setting by two, and the third by three. Compare these products and then explain what is happening physically for the three highest settings which "stopped" the motion of the wheel. What is the wheel doing compared to the strobe flash rate for each of these three speeds?

525x1 = 525
475x2 950
425x3 = 1275

have no idea what to do next. :(

question9) State the true direction of motion of the disk.

*seems to go clock-wise to me but I don't think this is answer. :(

question 10)How will the disk appear to move relative to its true motion if the strobe is flashing slightly faster than the true rate of revolution, in the same direction or in the opposite direction?

*I have no idea on this one either. :(

question 11)Repeat question 10 for the strobe flashing slightly slower than the true rate of revolution.

*no idea on this one too. it seems like this is guess question? :(

Last edited: Jun 30, 2013