# Newton's Third Law

## For every action, there is an equal and opposite reaction.

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Flight by Force

### How does Newton's third law allow a balloon to propel itself much like a rocket does?

Credit: NASA Goddard Photo and Video
Source: http://www.flickr.com/photos/gsfc/3640177668/sizes/o/in/photolist-6xERZW-6xESaQ-6yL5Dv-6yLmqM-6BLYei-6GeGxH-6GfiwT-6Ng4nu-6Ng7x3-7cN3fc-7gYWjk-7gYWnn-7h3Ux5-7hoVLs-7m14pa-a2W7sE-e3R82c-bbibXF-a2TfgR-9ZT3gD-9Yf9JP-9Yf9Kk-97htpQ-9S6d4F-dRhQkY-eEzK7P-7VqqZ5-dR6oM8-9tY24B-9FpHz7-ee5y7i-bo49DJ-bAY1qT-9XsGL7-9XsGPA-cVC1a7-8xNz8n-cA5Lw3-8xNyWz-8xRAcs-ecXWFz-btu7MA-btu7A7-bGoWeX-bGoWnD-btu7F1-bGoWje-9W9kJR-eebfaw-eebf2L-arUdQa/

[Figure1]

Have you ever seen a rocket launch, either live or on television? You would have noticed that it takes off by releasing a stream of gases. However, it doesn’t leave the ground because these gases push on the ground. If that were true, what would propel the rocket when it is up in the air, with no ground directly beneath it? The rocket actually takes off due to Newton’s third law of motion. It exerts a force on the gases and releases them, while the gases exert an equal and opposite reaction on the rocket itself. This allows the rocket to leave the ground and begin its flight. The more fuel that is combusted, the more additional acceleration that the rocket achieves.

Credit: matthewthecoolguy
Source: http://www.flickr.com/photos/matthewthecoolguy/6292985963/sizes/o/in/photolist-aA6ctZ-7ZRx7o-7ZNpky-81sZ1y-82U47g-b64PNp-euXDJU-egx62y-8wNfsS-89LJQB-d5AzPW-cXFrZW-bJi2jr-81qGBQ-82U4tV-82U9FV-7ZSnLj-9YjSVj-aL9JYv-euUxxp-am3Uo2-aTMJDv-am6MwU/

[Figure2]

This principle can be tested with a simple home experiment using a makeshift “rocket” that is nothing more than a balloon. Fill up a balloon, but do not tie it. Then, when you are ready, release it and observe what occurs. The balloon should propel itself through the air just like a rocket!

### Creative Applications

1. The more fuel that a rocket burns, the more it accelerates. What allows for an untied balloon to accelerate more?
2. What would happen if there were a tiny hole at the front of an untied balloon as it flew? (Assume the balloon did not have the hole when it was being filled up.)
3. Let's say a long strip of cardboard is balanced flat on several marbles. If a wind-up toy car were placed on the cardboard and allowed to move forward, what would happen to the cardboard?
4. If every action has an equal and opposite reaction, how is motion even capable of existing, such as when you push a shopping cart and it pushes back on you?

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