Weird Science: Dropping Mugs

Weird Science: Dropping Mugs

HONOLULU (HawaiiNewsNow) - Dr. V joins us for another weird science experiment. This one involves dropping mugs. Check it out!

Dr. V Show: Dropping Mugs

May 11, 2010

Things you need:

  • Coffee mug, or something of similar weight
  • String
  • Nut (e.g. hex nut) weighing about 5-10g
  • Pencil

The experiment

Please do this and all experiments with the help of an adult. You may want to practice with something less breakable than a mug, or practice over a cushion. Cut a length of string that does not quite touch the ground from shoulder height. Tie the mug to one end of the string, and the nut on the other. Holding the nut in one hand, and the pencil in the other, Put the string over the pencil and hold the pencil at about shoulder high. Tilt the far end of the pencil higher than the end you are holding so the string won't slip off. Pull the nut away until the mug is close to the pencil and the string is horizontal. Then, let go of the hex nut! What happens?

How does it work?

What happens is that the string wraps itself around the pencil before the mug can hit the ground. As soon as you let go of the nut, it begins to fall downward under the force of gravity as well as getting pulled toward the pencil as the mug falls. This causes the nut to rotate around the pencil like a pendulum, but the length of the pendulum is getting shorter at the same time. When something, in this case, the nut, is rotating around a point and you move its mass towards the center of rotation, it will rotate faster. As the radius of the circle shrinks, the distance around the circle the nut has to travel decreases, so it takes less time and the nut rotates faster. In our case, the nut rotates fast enough to catch the mug before it hits the ground.

This is the same effect you see when ice skaters start spinning with either their legs or arms spread out, and then they pull them in close to their bodies. They start spinning incredibly quickly!