What is the parallel axis theorem and how is it used? Please explain the equation.

Explanation:

If an object has a moment of inertia I₀ about an axis, then the moment of inertia about a different, parallel axis is I = I₀ + md², where d is the distance between the axes.

For example, consider a horizontal thin rod rotating about a vertical axis passing through its center.  It has mass m and length L.  Its moment of inertia is known to be I = 1/12 mL².

Now consider the same rod, but this time we move the axis of rotation L/2 to the end of the rod.  We can use parallel axis theorem to find the new moment of inertia:

I = I₀ + md²

I = 1/12 mL² + m (L/2)²

I = 1/12 mL² + 1/4 mL²

I = 1/3 mL²

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.