Moment of inertia converter
Convert rotational inertia between metric and imperial units: how hard something is to start or stop spinning.
That's 7.1× as hard to spin as a bicycle wheel, while a car wheel is 1.2× harder. Spun at 100 rad/s it would store about 5,000 J.
What it means
How hard is that to spin?
Moment of inertia depends on where the mass sits, not just how much there is. The same wheel about a different axis gives a different number.
Every unit
Your moment of inertia in every unit
Learn
Three things worth knowing
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Radius counts twice
Doubling the radius while keeping the mass gives four times the inertia. Skaters pull their arms in to spin faster for exactly this reason.
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Euler put a number on it
Leonhard Euler formalised rotational motion in 1765, defining the quantity that turns torque into angular acceleration.
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Light rims beat light frames
Cyclists pay for light wheels before light frames: mass at the rim costs four times as much in acceleration as mass at the hub.
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Popular moment of inertia conversions
Background
About moment of inertia
Moment of inertia is rotation’s version of mass: kg·m². Mass far from the axis counts much more, because the radius is squared. A bicycle wheel is hard to spin up not because it is heavy but because its weight sits at the rim.