On Finding Torque & RPM

[SpaceLight]

Presently, two things I need clarity on, please.

Thing 1:
I found this on youtube

I organized it in the following and hope it's good for arriving at fair approximations of torque and rpm...

T = I x alpha = F x r x sin(theta):

By example...
A rotor Mass of 100 kg,
'r' of 2 meters,
A force of 75 Newtons,
point of force at a 90 degree angle.

First filling in T = F x r x sin(theta), as such:
T = 75 N x 2 m x sin(90) = 150 N*m
T = 150 N*m

Next, add the other part of the equation, T = I x alpha; I = 1/2Mr^2, hence:
T = 150 N*m = 1/2M x r^2 x alpha
Resolved progressively...
T = 150 N*m = 50 kg x 2^2 x alpha
T = 150 N*m = 50 kg x 4 x alpha
T = 150 N*m = 200 x alpha

Next, for alpha:
alpha = 150 divided by 200 = .75 rad/s^2

Next, convert .75 rad/s^2 to RPM using this equation:
([rad/s]/6.28318530718) x 60 = RPM
(Note: 6.28318530718 was Pi x 2, I just filled it in....)
Solved progressively...
([.75 rad/s]/6.28318530718) x 60 = RPM
.11936620731891365 x 60 = RPM
.11936620731891365 x 60 = 7.161972439134819 RPM
.75 rad/s^2 = 7.161972439134819 RPM

Correct?


Thing 2: Please refer to attachment "thing 2".

The only known variables:
1) One angle (at the location depicted in the diagram).
2) Distance between A & B.

Objective: Find line 3 (the dotted line) given the two known variables; line 3 needs to be perpendicular to line 2. Note: The one known angle is not 90 degrees.

If you could write the solution out simply enough with an example, that would be helpful.

Sincerely grateful for your guidance.

Thing2.bmp

[imatfaal]

!

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So SpaceLight would you explain where you have got to in solving this problem and what has stopped your progress - hopefully members can then help you to overcome your difficulties yourself.