Why can't this be done?

Question

Place yourself facing the edge of an open door. Position your feet astride the door with your nose and abdomen touching the door's edge. Try to rise on your tiptoes. Why can't this be done?

Please help, this is a physics txtbook question found in a chapter about, momentum, impulse, and center of mass. Thank you!

Answer 1

When trying to get on your tiptoes, your center-of-mass must be OVER the point of support. Typically, you would lean a little forward. However, as you describe the situation, you cannot move any part of your body past the door and so the CM is always behind your toes and you fall backward.

Answer 2

Because your weight is being centered on your heels. If you were to push the front of your feet up you'd fall backwards because the door prevents you from leaning forward. Also, your legs are further forward than the rest of your body, therefore straining more muscles in your legs to push the front of your feet down. NOTE: I'm nowhere near and expert at this but I'm making an educated guess.