Hero's Engine: Example of Newton's Third Law

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Updated on Oct 24, 2013

Sir Isaac Newton, a scientist and philosopher of the late 1600’s, spent many years trying to come up with codified rules that describe how all stuff in the universe behaves. In the experiment below, we will explore Newton’s third law, which states that “for every action there is an equal and opposite reaction”—but what does this mean, exactly?

Imagine dropping a tennis ball to the ground. What happens in response to the action of the ball striking the ground? It bounces back up towards you. This is due to the reactionary force of the floor acting against the ball, which pushes it upwards into the air.

To explore this idea more fully, you can easily construct your very own device called an aeolipile (sometimes referred to as Hero’s Engine or a Hero engine). Created by an engineer named Hero of Alexandria about 2000 years ago, this invention was able to show one way in which an action can lead to an equal and opposite reaction: an example of Newton's third law.



  • Plastic cup
  • 2 plastic bendable straws
  • String
  • Craft knife
  • Water and sink
  • Modeling clay


  1. Take plastic cup and have an adult help you poke two small holes near the top rim on opposite sides from one another.
  2. Thread string through the holes and tie a knot so that the cup can be suspended from the string.
  3. Have an adult make two slightly larger holes near the bottom of the cup as seen in the picture below (make sure these holes are just large enough for the straws to fit through)
  4. Cut each straw about 1.5 inches below its bendable portion.
  5. Slide the straws into the holes. Make sure that they both point in a clockwise direction.
  6. Use your modeling clay to seal the space between the cup and the straw so that no water leaks out when you fill the cup.
  7. Hold the finished Hero engine away from your body. Pour water into the cup and observe.


Gravity draws the water downward and out through each straw. This causes the engine to spin in a clockwise direction.

Hero's Engine Results


The water being forced by gravity to leave the cup in a clockwise direction pushes back on the cup in a counterclockwise direction, causing the cup to turn. This is the same principle that enables rockets to work—gas that’s forced out of the nozzle pushes back on the rocket, propelling it forward!

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