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Soap Powered Boat Experiment

The movement of the soap powered boat has combination of three principles.


Surface Tension

1) All liquids have a little thing scientist call surface tension! Water molecules are strongly attracted to each other and stick close together. This creates a strong but flexible “skin” on the water’s surface, which is able to support small insects and materials like our paper boat on their surface.


2) Isaac Newton’s third law of motion says for every action there is an equal and opposite reaction. The movement of the soap provides a ‘pushing force’ backward into the water surface as the soap molecules spread out, sending the boat forward… i.e. an equal and opposite motion.


3) The Marangoni effect is also acting upon the boat. This affect describes how a liquid with a high surface tension pulls more strongly on the surrounding liquid than a liquid with a low surface tension. If you change the surface tension of some parts of the liquid you introduce a difference in surface tension. A presence of a surface tension gradient will cause the liquid to flow from areas of low surface tension to high surface tension.


Your soap molecules reduced the water surface tension behind the boat, causing the water and the boat to move forwards.


To repeat the experiment you will have to replace the water in the pan.


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