You know that Density is a measurement of Mass ÷ Volume. Therefore Mass and Volume amounts affect the density. If individual particles have more mass, density is affected: Styrofoam and gold. If particles take up more space, density is affected: temperature and pressure. You also know that pure substances have a specific density, no matter how much of the substance you have. But what happens when you have an object with a variety of substances in it? For example, a boat. This sailboat has rope, wood, cloth, metal, plastic, etc. Each of these individual parts will have its own density.

Just as you have individual assignments in your courses that have a total mark, objects in our daily life can have individual parts that make up the total object. When you talk about your “mark” in a course, you are talking about the AVERAGE of all the parts calculated together. When we talk about the AVERAGE DENSITY of an object, we are also talking about the average density of all the parts calculated together.

The sailboat is designed to float, even when it’s average density is changed……… What does this mean? Well, when I add more rope, a few lifejackets, and some people to the boat, I change its’ mass. Now because I have changed something in our Density formula, I have changed the average density of the boat. When I changed the average density, I also changed how far down in the water my boat will float or sink. There is a direct relationship between density and floatation.

Can I “add” something to you to change your mass? YES ! I can give you a chair to hold, I can give you a sack of bricks to hold, etc. On a scale, your mass has changed. Can I “add” something to you to change your volume? YES AGAIN! When you put on clothing, especially a big winter jacket, you are adding volume to yourself (check out how much volume your winter jacket takes up in your locker). So then, what does a lifejacket do? Talk about it with a partner and explain it to me…….(fantastic test question by the way!)* Use some science vocabulary words: Average Density, Volume, and Mass. Try this: Why a lifejacket and NOT a winter coat?

A lifejacket adds very little mass to a person because of the light-weight material it is made of and because it generally does not absorb water. However, it adds a fair bit of volume to you compared to not having it on at all. The SIZE of a lifejacket is dependent upon your weight because the more weight you have, the more volume that will be necessary to change your density…………. Your AVERAGE DENSITY. (You are a combination of many pure substances)

Most of the time when you are wanting to work with Average Density, it is because you want something to float or to sink. You want to affect the BUOYANCY of an object. So what types of “things” control the rate of floating and sinking? Submarine Scuba Diver Jelly Fish Fish Shark (different!) Next we’re going to get to the handouts to relate Average Density to Water Displacement, and Buoyancy.