Bouncing ball I want to rotate -90 x axis between height and floor of bounce

I would like to figure out how to rotate a sphere I have of a face that bounces. I would like to have when it hits the ground it rotated to -90 and when it is up in the air its rotation to be 0 on the x. I think I need Euler angels and a lerp function. It can be assumed the bounce height is the same all the time. Just cannot picture how this might be done.