In this activity, students will adopt a pet rock and they will design, build, and test a flotation device for it!

Materials Needed

Per student:

One rock (Of similar mass. More mass means harder challenge.)

For the group:

Sharpie markers
Large tub of water
Various construction materials, such as foam, plastic bags, packing peanuts, cork, bubble wrap, aluminum foil, straws, sticks, a pool noodle (if you are willing to sacrifice it for the sake of science), etc.
Various bonding materials, such as tape, glue, string, etc.

Steps:

Using a Sharpie marker or two, give your rock a face and a little personality.
Make sure you name it!
Look at the material options and create a life jacket or boat that will keep your Pet Rock afloat in a tub of water.
Test your life jacket or boat and modify its construction until your Pet Rock safely floats.

Explanation:

Buoyancy is the force that pushes up on an object and keeps things afloat. An object will float if its buoyancy is greater than its weight, but it will sink if its weight is greater than its buoyancy. When you put something in water and it floats, the object pushes some of the water out of its way (displacement). If you could weigh the water that moved, you would be able to tell that it weighs the same as the object did.

As the story goes, this was discovered by a mathematician named Archimedes. He was known for not liking to take baths and oftentimes, he would have to be reminded multiple times before he finally would. One time, when he climbed into his bath, he noticed that the water level rose. This led to the discovery of what we now know as Archimedes Principle.

Real World:

So, how does a big ship not sink? Because a ship is mostly hollow, it weighs less than the amount of water that it displaces. When we talk about the size of a ship, we measure it by its displacement.

Make sure you pay attention to the water level the next time you take a bath!