# Flywheel rotation question.

1. Feb 7, 2017

### Arman777

1. The problem statement, all variables and given/known data
A flywheel with a D=1.2m is rotating at an angular speed of 200 rev/min
(a) Whats the angular speed in rad/s ?
(b)Whats v=? in the point on the rim ?
(c)What const. ∝ ( in rev/min^2) will increase its angular speed to 1000 rev/min in 60 sec ?
(d)How many revolutions does the wheel make during that 60 sec ?

2. Relevant equations
Rotational Kinematics Equations

3. The attempt at a solution
I found a,b,c correctly (for long tries ) but I stucked at d.

I guess I ll use here $Δθ=wt-1/2∝t^2$

(b)v=12.5
(c)=800 rev/min^2

Now for d,

lets to write all equaiton in rev/min form so
I dont know what will be the left part cause Δθ in units its normally just rev I guess so
θ-200rev/min=7879rev/min.1min+1/2 (800rev/min^2).(1 min)^2
Here I get a huge number...

Thank you

2. Feb 7, 2017

### Staff: Mentor

You established before that 200 rev/min corresponds to 20.9 rad/s (which is correct). How can it correspond to a larger number of revolutions per minute now?
You cannot subtract an angle and an angular velocity. That's like adding 3 meters to 1 minute: It does not make sense.

You can use the formula you posted, but it is probably easier to find the average angular velocity (in rev/min) and to use that.

3. Feb 7, 2017

### Arman777

ohh I see...so I can convert 200rev/min to rev which its just 200 or ..I dont know ?

4. Feb 7, 2017

### Staff: Mentor

You cannot "convert an angular velocity to revolutions" in the same way you cannot convert apples to minutes. There is no need for such a conversion. Your formula multiplies the angular velocity by a time. The product of those two is a number of revolutions or an angle, depending on which units you use.

5. Feb 7, 2017

### Arman777

ok then how can I found inital rev ? If other parts are correct ?

6. Feb 7, 2017

oh I see...

7. Feb 7, 2017

### Arman777

Here my new try

8. Feb 7, 2017

### Staff: Mentor

Your angular acceleration seems to be too low.

9. Feb 7, 2017

### Arman777

yeah,sorry it would be 1.40 rad/sec ?

10. Feb 7, 2017

### Arman777

I found yeyyyyy

11. Feb 7, 2017

Thanks