Rotating half cylinder energy and power calculation

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SUMMARY

The discussion centers on calculating the energy and power output of a rotating half-cylinder mass on a shaft. The user provided parameters including mass (18.6 kg), length (0.305 m), radius (0.07 m), and rotational speed (60 rpm, equivalent to 0.159 rad/s). The initial calculation of rotational kinetic energy (Er) was found to be incorrect, with the correct value being 0.90 J based on the mass moment of inertia (I) calculated as 0.046 kg-m². The conversation emphasizes the importance of accurate unit conversion and the relationship between energy and power output in mechanical systems.

PREREQUISITES
  • Understanding of rotational dynamics and kinetic energy
  • Familiarity with mass moment of inertia calculations
  • Knowledge of unit conversions, particularly between rpm and rad/s
  • Basic principles of energy conversion in mechanical systems
NEXT STEPS
  • Study the derivation of mass moment of inertia for various shapes
  • Learn about the relationship between rotational kinetic energy and power output
  • Explore the principles of energy conversion efficiency in alternators
  • Investigate the effects of load on rotational systems and power calculations
USEFUL FOR

Ocean engineers, mechanical engineers, and students studying rotational dynamics or energy conversion in mechanical systems will benefit from this discussion.

crf59
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Hi folks,

I'm an ocean engineer who's getting old and slow. If I have a half-cylinder mass on a shaft where I'd like to be able to quantify energy (which I think I have right) and then have some way of relating that to potential power output.

What I have so far:
m=18.6 kg
Length=0.305m
radius=0.07m
Mass moment of inertia (I) = 0.5mr^2
Rotational KE (Er) = 0.5Iw^2
w=60rpm=0.159 rad/s

I calculate the Er as 0.00747J

Does this jive? Is there any meaningful way of relating that to power? I do not have a load value (yet), so am just trying to understand what this might be in terms of power if I use some conversion efficiency (thinking alternator load). I am assuming the same motive force that accelerated it to 60 rpm is constant (in some sort of resonance). If I have my calcs/assumptions wrong so far, please correct.
 
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Welcome to Physics Forums.

You're general approach is correct. However, 60 rpm is actual 2*pi rad/s. I get 0.90 J for the energy. (I=0.046 kg-m2)
 

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