Density: What sinks and what floats?

Today at Bayside 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 did a excellent job differentiating the two. 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 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  filled bottle 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.

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.




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