Moment of Inertia

I have been calculating the moment of inertia for a con-rod, does a value of 0.075Kgm^2 sound reasonable. I used the formula for the periodic time for a compound pendulum, and rearranged for k, and put this into the formula for the moment of inertia
- Gareth (age 19)
Wales

I guess a con-rod is a connecting rod. You don’t say what it’s from- a little motorcycle or a big tractor, etc. We could tell if the answer is close if we knew the dimensions etc. Still, the number you give sounds very reasonable for the moment of inertia of a medium-size part from a car engine. However, if you took some part and, say, doubled each of its dimensions, the moment of inertia would be 32 times bigger. So there’s no way we can be at all accurate without knowing detailed specs. You also have to be careful that the moment of inertia depends strongly on which point in the object it’s measured around. It also depends on the direction of the axis chosen, but I’m guessing you’ve picked the direction to get the largest moment.

Mike W.

(published on 10/22/2007)