Concepts

Rotational motion, free fall

Overview

A ball on a hinged plank falls into a cup mounted in the plank. For the ball to fall into the cup, the cup and nearby plank segment must accelerate down faster than g.

Details

Equipment

• [1] Hinged plank with cup
  
• [2] C-clamp
• [1] Specially sized wooden stick
  

Classroom Assembly

1. Clamp both ends of the lower plank to a table.

Important Notes

• Make sure nothing obstructs the removal of the stick or the follow-through motion of your hand in removing the stick.
• Doing this demo correctly requires a bit of finesse. If you move too quickly, you may jerk the top plank around and cause the ball to fall unpredictably.
  

Script

1. Note the location of the cup when the plank is in the down position.
2. Support the plank using the stick and put the ball in the small depression on the end of the plank.
3. Note that the ball is now directly above where the cup will be in the down position.
4. Smoothly remove the stick from under the plank.
   

Additional Resources