## Materials

### Per Class:

• Large container of water
• Dishwashing liquid

### Per Student or Team:

• A small (about 1-inch long) piece of cardboard cut into a boat shape, with a “v” shape cut into the stern
• Small piece of soap

## Instructions

Students experiment with different soap products to see which one moves a cardboard boat across the water most effectively.

1. Put a cardboard boat on the water.
2. Place a few drops of dishwashing liquid on the top of the water in the “v” of the boat. See what happens.
3. Try again, but this time insert a wedge of soap into the “v” and don’t use any dishwashing soap. See what happens.

## Guiding questions

• Engineers have to know chemistry—the properties of elements and compounds—in order to solve certain kinds of problems. In this activity, how can you make a model boat move in water if all you’ve got is soap? The dishwashing soap mixes with the water and weakens the attraction between the water and the back of the boat, which means that the front of the boat is more strongly attracted to the water (called adhesion). The difference in attraction forces between the back and front of the boat creates a pulling force that moves the boat forward.
• Some insects, like water striders, are able to walk on water due to a force called surface tension. The surface of the water forms an invisible membrane to act like a stretched elastic cover that holds the water together. Surface tension is what enables water to support the weight of objects like water bugs, leaves, and even a paper boat. Engineers who study surface tension have been able to design small water-walking robots will be used to clean up oil spills.

## Engineering & science connections

• Friction is the resistance of one surface or object upon encountering or moving on another surface or object.
• Engineers use the principle of friction to make things safe. They thought of putting rubbery dots on the bottom of footy pajamas and rubber soles on sneakers.
• Aerospace engineers design airplanes to land and stop safely by making use of friction. Planes slow down using friction within the brakes, as well as from the rubber tires on the landing strip.
• In some cases, engineers need to reduce the amount of friction. For example, a water slide is designed to reduce friction for speeding up the slide action.

Adapted from “365 Sience Experiments”, with permission from Hinkler Books Pty Ltd. 2007. www.hinkler.com.au