PHYSICS - PROJECT no. 2

Physics Build a Motorboat Powered by Surface Tension

Objective

This project has three objectives:

1. develop a qualitative understanding of water surface tension,
2. understand changes in water surface tension under various conditions,
3. make measurements to estimate the net water surface tension.

Introduction

You probably have experience blowing up a balloon. For a balloon made of soft, stretchy rubber, it is easy to blow it up without becoming out of breath. The balloon offers little resistance to becoming stretched out as it is blown up. But for a balloon made of thicker or stiffer rubber, more energy is required to blow it up. The balloon offers more resistance to stretching. We can think of a balloon as a model for surface tension. How “stretchy” the balloon material is determines how much resistance (surface tension) must be overcome by the energy of your blowing in order to inflate the balloon.

Surface tension is defined as the energy required to increase the surface area by a unit amount. Liquids also experience surface tension. The molecules of the liquid experience intermolecular attractions, the details of which depend on the chemistry of the particular liquid. The molecules at the surface of the liquid experience asymmetric forces, since one side of the molecule faces the liquid, and the other side faces air. This asymmetric force is surface tension.

Water may seem ordinary, but is has some extraordinary chemical attributes. One of these attributes is hydrogen bonding. To really understand the surface tension of water, you should do some background research on the chemical structure of water and hydrogen bonding.

The effects of surface tension can be observed easily in our daily life. For example, soap bubbles, water rising inside a capillary tube, floating a needle on a still water surface, floating of water strider on water, using detergent to clean up kitchen utensils, and many others. At the smaller scales of microscopic or nanoscopic worlds, the forces of surface tension play an increasingly important role determining the behavior of molecules.

In this project, you’ll make a small boat from a clear plastic sheet (transparency), and see if you can propel it by taking advantage of the surface tension of water. All right, it’s more of a motor-raft, but maybe you can think of a good way to make a lightweight boat for this project.

Terms, Concepts and Questions to Start Background Research

To do experiments on surface tension, you should do research that enables you to understand the following terms and concepts:

• surface tension
• water molecules
• hydrogen bonding of water molecules
• detergent
• capillary motion
• acceleration
• velocity

Bibliography

• Origin of surface tension:
  http://wine1.sb.fsu.edu/chm1045/notes/Forces/Liquids/Forces03.htm
• Information on bubbles and surface tension:
• http://www.exploratorium.edu/ronh/bubbles/bubbles.html and
• Noddy, Tom. Tom Noddy’s Bubble Magic. Philadelphia, Pennsylvania: Running Press, 1988.
• Information on acceleration, velocity, and force:
  http://physics.webplasma.com/physicstoc.html
• CBSE Blog: http://cbse-sample-papers.blogspot.com

Materials and Equipment

• transparency
• scissors
• several cotton balls, cotton swabs or small pieces of sponge
• marker
• ruler
• glue or tape
• liquid detergent
• soap
• toothpaste
• water
• thermometer (optional)
• a few small pieces of wood to make a straight channel for your raft