Cartesian Diver!

Curriculum links:

• Physics – Forces – Investigating the law of floatation – How things like ships and submarines float or sink.

Learning objectives:

1. Make predictions about what different objects will sink or float.
2. Learn about pressure, volume, weight, and buoyancy of objects and how these factors may influence floating ability.

Materials:

• A plastic pipette
• Other light objects that will float – e.g. a condiment packet or a small plastic bag
• Different objects to weigh the floating objects down – paper clips, money, metal nut
• A plastic bottle

Safety:

Make sure children are not left alone with water

Instructions:

Activity 1:

The first step in this experiment is to find something that has a little bit of air in it and that floats. A plastic pipette, a condiment packet or even a small plastic bag can work quite well for this.

In order to test this out get a glass of water and drop your object into it to see if it floats or not. If it floats add some weights to it. These can be metal nuts, paper clips or even money into the money bag.

You want it to be just barely bobbing up on the surface so that adding one more weight would cause it to sink.

How it Works:

Most children will be able to identify if objects are floating or sinking. If an object is buoyant, it is less dense than the liquid it is floating in or else it will have something attached to it that will help it float, like a life jacket! An object will sink if it is heavy for its size (paperclip), and float if it is light for its size (log of wood). Objects are heavier or lighter depending on how dense they are (how much air they have inside them.)

Activity 2:

Fill a plastic bottle with water almost to the top and add a plastic pipette to the bottle, allowing it to float at the top of the water. Screw the top on the bottle and by adding a little bit of pressure (a squish) you can amaze everyone with some science! The pipette will sink in the water.

How it Works:

Squeezing the bottle causes the pipette (diver) to sink because the increased pressure forces water up into the pipette, compressing the air at the top of the pipette. This increases the mass, and density, of the pipette causing it to sink. Releasing the squeeze decreases the pressure on the air at the top of the pipette, and the water is forced back out of the pipette, lowering its density and allowing it to float back to the top of the bottle.