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Weightlessness on Earth?

One day, while Einstein was daydreaming at work, he imagined that a housepainter would experience weightlessness if he fell off a roof.

"If a person falls freely, he won't feel his own weight. This simple thought made a deep impression on me." - Albert Einstein


This activity is a physical demonstration of Einstein's idea known as the "equivalence principle." This means that in a freely falling reference frame (that of the falling cup or person), gravitational effects will not be observed. Students will watch a demonstration of a falling cup of water with a hole in it. They will see that as the cup falls, the water stops running out of it.


  • Basin
  • plastic cup with hole punched on its side, near the bottom
  • water


  1. Using a pencil or pen, punch a hole into the side of the plastic cup, about a third of the way from the bottom.
  2. Place your finger over the hole and fill the cup with water.
  3. Holding the cup and water about chest-high over the basin, take your finger off the hole and allow about a third of the water to drain out. Point out to the students that gravity is pulling down the stream of water.
  4. Ask students to closely observe the cup and water for this next step. Drop the cup after a third of the water has drained. As the cup falls, the water stops flowing from the hole. Ask students why they think this happens. Explain that gravity is pulling on both the cup and the water, which means that they both fall together with the same acceleration.

A Little Deeper:

One on level, this is a simple demonstration of the concept of weightlessness in a freely falling reference frame. On another level, it illustrates the concept behind Einstein's Equivalence Principle, which states that, locally, gravity is equivalent to being in a accelerated reference frame; or, gravity and acceleration by an outside force produce the same effect on a local object (in this case, the cup).


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