# Sink or Float?

Mass, Weight and Density:  How Heavy Boats Stay Afloat

Today at Saltonstall the 5th and 6th graders talked about mass, weight, and density, and how you can determine if an object will float.  We first broke down the difference between mass and weight – mass is the amount of MATTER in an object (without the influence of gravity) and weight is GRAVITY-dependent mass. Traveling to the moon will change your weight, but your mass will remain the same! 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. The students had a great time figuring out the mystery behind the poly-density bottles!

For our experiment, the students were given the task of constructing a rectangular “boat” out of aluminum foil and calculating how much gravel cargo it could hold before sinking.  Using rulers they were able to determine the volume of their boat, which allowed them to determine the maximum amount of gravel it could carry.  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.  Many of the kids were surprised to see their boats float with the amount of gravel they calculated!  Some sank, but after carefully taking their measurements again they were able to get it to float.  We have some future engineers on our hands!