# How does a steel ship float in water?

Mass, Weight and Density:  How Heavy Boats Stay Afloat

On Friday 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. We also reviewed volume, which is the amount of SPACE an object takes up.  These concepts are important when trying to understand density. We looked at different demonstrations, which showed that LESS dense objects FLOAT, while MORE dense objects SINK.

For our experiment, the students were given aluminum foil, small “ponds,” and a cup of aquarium gravel. Each team constructed a rectangular boat out of the foil and determined its volume. By calculating the density of the boat (mass divided by volume), we were able to predict how much gravel could fit in our aluminum foil boats before they would sink. 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.

We also examined small plastic bottles, one of which had been filled with beans until it was six times as heavy as the other, to illustrate the difference in weight of an object on Earth and on the Moon. The students also observed a “poly-density bottle.” Filled with salt water and less dense isopropanol, and with pony beads of two densities, the bottle provided a puzzle for the students to figure out.