Density

What happens when you put a golf ball in water? It sinks. What happens when you put a tennis ball in water? It floats! Why do some objects sink and some objects float? It has to do with density. Density is how tightly packed the matter is inside an object. Dense objects have more mass per volume than less dense objects. That means there is more matter packed into a smaller space.

Image summary: A digital illustration featuring two overlapping cubes. The larger cube in the background is lime green and displays a texture resembling a tennis ball. The smaller cube in the foreground is white and grey, featuring a dimpled texture resembling a golf ball.

Think of the golf ball and the tennis ball. A tennis ball is larger than a golf ball. But if you were to hold them both at the same time, it actually feels lighter.

A golf ball is more dense than a tennis ball. It has more "stuff" packed inside it. A tennis ball is hollow inside. It is full of air which makes it less dense.

Density is measured by dividing the object's mass by its volume. Mass is the amount of matter that makes up an object. It is usually measured in grams.

Volume is how much space the object takes up. It is usually measured in cubic centimeters. So density is measured in grams per cubic centimeter or g/cm.

The density of water is 1 gram per cubic centimeter. What is a cubic centimeter? Think of a cube made of squares with sides measuring 1 centimeter. If you filled that cube with water, it would hold 1 gram of water.

Image summary: A diagram of a cube labeled 1 g. water, with a bracket indicating the height of the cube is 1 cm.

Density equals 1 gram per cubic centimeter

If an object is more dense than water, it will sink. If it is less dense than water, it will float. Imagine cutting a cube out of the golf ball with sides measuring 1 centimeter. This cube of golf ball would have a mass greater than 1 gram. The golf ball is more dense than water and will sink. If you could cut a cubic centimeter sized piece out of the tennis ball, it would have a mass less than 1 gram and will float.