# Will it sink or float?

Mass, Weight and Density:  How Heavy Boats Stay Afloat

Today at CHA we talked about mass, weight, and density, and what makes things float.  We first broke down the difference between mass and weight – how mass is the amount of MATTER in an object (without the influence of gravity) and weight is GRAVITY-dependent mass. The students were already experts on this and even had a song to go with it! We also reviewed volume, which is the amount of SPACE an object takes up.  These concepts are important when trying to understand density. We used our knowledge that objects LESS dense than the liquid they are in will FLOAT, while objects that are MORE dense will SINK.

First we looked at a single bean, a plastic bottle about 1/4 filled with beans, and the same bottle completely filled with beans and guessed if each would sink or float. The single bean and filled bottle both sank while the mostly empty bottle floated! But if one bean sinks, how could a bottle containing many beans float? Because the bottle also contains air, making it less compact (and less dense) than a single bean.

Next, the students observed a “poly-density bottle”. The bottle contains salt water, rubbing alcohol, and blue and white plastic beads. At first glance, the white and blue beads appear to be positioned between the two liquids. But shake it up and the white beads quickly rise to the top while the blue beads sink to the bottom of the bottle! Let it sit undisturbed and the beads will slowly meet up again at the center, where the salt water and rubbing alcohol layers meet. The students used their observations to discuss the general densities of the two types of beads and liquids and explain what happens to the density of the liquid when you shake the bottle. They were amazed that they saw the same results if they held the bottle sideways and even upside down!

For our final experiment, the students constructed small rectangular “boats” out of aluminum foil and had to predict how much gravel they could carry before sinking. We did this by determining the volume of our boats and calculating the density. Just like engineers designing the next great cargo ship, we needed to make very careful calculations of the volume of the boat and how much water the boats would displace so we could correctly calculate the amount of gravel the boats could carry without sinking.