Projects‎ > ‎(1) Discrepant Events‎ > ‎

The Unreachable Cup- Jenny Green


Jenny Green

Principle(s) Illustrated

  1. Center of Gravity
  2. Pivot points
  3. Shifting the Center of Gravity 
  4. Effect of motion and gravity


  • PP.6.h: Students know how to compare joints in the body (wrist, shoulder, thigh) with structures used in machines and simple devices (hinge, ball-and-socket, and sliding joints).
  • PP.6.i: Students know how levers confer mechanical advantage and how the application of this principle applies to the musculoskeletal system.

Questioning Script

Prior knowledge & experience:

Students think they can bend over and reach the cup

Root question:

Why do we fall forward when reaching for the cup?

Target response:

When picking up an object from the floor without bending our knees, our legs and lower part of our body has to move backwards to stay balanced and the wall prevents this.  The center of gravity of our body has to remain above our feed which is the pivot point, supporting our body.  Standing straight with our body against the wall makes it impossible to move the lower part of our body backwards.  The forward bending of the upper body shifts the body's center of gravity towards the frong of the pivot point (our feet) and our whole body falls or topples forward.  You can also try this back to back with students and let them pick up cups at the same time.  

Photographs and Movies

Common Misconceptions:

Students are under the impression that center of gravity stays in one place and don't realize we go back to put center of gravity over feet.

You can see that as you bend, your center of gravity shifts backwards.  The wall above would prevent this and your center of gravity would shift out in front of you, causing you to fall over.  


This is a really cool site about circus acrobats and their center of gravity, pivot points and torque