What causes boats to float?
Displacement, Density, and Buoyancy Lab Investigation
What causes boats to float? Especially a huge and heavy cruise ship!?! Sometimes it is hard for our brain to grasp the concept of something so large (no pun intended). So, let’s break it down. There are actually 3 scientific principles going on: displacement, density, and buoyancy. All three play a role in the floating of a gigantic ship! Be sure to check out our other Science Resources here including our Aluminum Boat Investigation!
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Open up with a few video clips BEFORE lab demonstrations.
This scientific investigation includes some opening video clips followed by a series of lab demonstrations that show each of the principles: displacement, density, and buoyancy. Viewing these short video clips will give your student a little background and vocabulary before you dive into the lab demos. Happy learning!
What makes something float or sink? YouTube Video
The “What makes something float or sink? video is jammed packed with educational information pertaining to density, buoyancy, and Archimedes’ Principle. Worth watching!!!
Why Do Ships Float? YouTube Video
This video does a great job explaining why a massive and heavy ship can float. It also creates the perfect set-up for you to demonstrate some hands-on visuals at home. Look at the next activity!
“Archimedes’ principle, physical law of buoyancy, discovered by the ancient Greek mathematician and inventor Archimedes, stating that any body completely or partially submerged in a fluid (gas or liquid) at rest is acted upon by an upward, or buoyant, force, the magnitude of which is equal to the weight of the fluid displaced by the body. The volume of displaced fluid is equivalent to the volume of an object fully immersed in a fluid or to that fraction of the volume below the surface for an object partially submerged in a liquid.” Brittannica.com
Materials Needed for "What Causes Boats to Float?" Lab Demos
The materials list for the “What Causes Boats to Float?” lab demonstrations is pretty simple. Most of you will have these items on hand… transparent plastic containers, cotton balls, and marbles. If you don’t have marbles, you could use pebbles or any other manipulative that sinks. You will need two identical containers to be on the smaller side (so they can float and sink into the water being held by a larger container).
Displacement Demonstration
It can seem daunting to try to explain to a child what causes boats to float, but once you understand displacement, density, and buoyancy, it is easy. Now, will your child get it completely? Maybe not, but this is the beginning of exposing them to thinking like a scientist.
First, start with a transparent container. Next, fill the container half-way up with water and then using a permanent marker, make a line on the exterior of the bowl to show the level of water.
After drawing your line, ask your child, “What will happen if you put a handful of marbles into the water?” They usually say that it will sink, which is true. Next, ask them, “What will happen to the water?” Some kids will answer, while others will look puzzled not really sure how to answer. Inquire further, asking them if they think the water will rise, fall, or stay on the line. Finally, show them what happens.
This is displacement. Displacement is the removal of something from its usual place or position by something which then occupies that place or position. The amount of water displaced, or water that rose above the line, is equal to the amount of volume taken up by the marbles. This is a great time to inquire: Did we add more water? Why did the water rise?
Buoyancy Demonstration
Buoyancy can be shown by using a small boat (plastic container), marbles, and a larger container filled with water about half-way. Marbles would sink on their own, but in a bowl (boat), their mass is dispersed along a wider base. The bowl has a large surface area to push on the water. In turn, there is a large surface area on the bottom of the bowl for water to push back. If the force pushing up from the water is greater than the force pushing down, the object will float.
After filling your water to the line, place the “boat” into the water. It floats. Now, slowly add marbles to the base of the cup. Notice how the same marbles that sunk in the previous demonstration are now floating. Why? The container has a much larger surface area for the water to push on. This increases its buoyancy, its ability to float.
Also pay attention to how much of the water is displaced. What will happen if you keep adding marbles? Will the water continue to rise? Will the “boat” sink? Why? Every time you add a marble to the boat, you are increasing the amount of mass within the boat. When you increase the mass per volume, you are increasing its density. If the density is greater than that of water, it will sink. Now, let’s move onto the density demonstration!
Density Demonstration
When teaching why objects sink or float, I usually focus on density. Density is the amount of mass in volume. Mass is the amount of matter in something while volume is the amount of space something takes up. To set up your density demonstration, take the two identical transparent plastic containers with lids. Fill one of the containers up with cotton balls and the other one with marbles.
Both containers have the same volume, however, the amount of mass is vastly different. The container full of marbles has a much greater mass than the one with cotton balls. This makes the container with the cotton balls less dense and the one with the marbles more dense. Put the lids on the containers and let your kids feel the difference.
After you and your kids have had a chance to feel the difference in mass between both containers, you are ready to test them out on the water. Will they both float? Why? Why not? What is the difference between both containers? Do they have the same volume? Mass? Density? Ask them lots of questions and try to get them to answer as many things as possible without giving them the answer. Our job is to guide them to the answer, not give it to them.
The boat with marbles will sink. It has a greater density than the water… more mass in its volume.
The boat with the cotton balls floats. It is less dense than that of water… less mass in its volume.
Now, it is time to relate this back to how a cruise ship can float. A cruise ship has a very large base giving the water it sits on ample space to push against. The wide boat also allows for mass to be dispersed within its volume. Even though the ship is heavy, it is also very large. Similar to the container with cotton balls, most of the space within a ship is hollow and full of air or empty space. In conclusion, the boat floats because it is less dense than water. Happy learning!
Looking for a STEM project to go along with what you just taught… Try the Simple STEM Aluminum Foil Boat Lab! Great way to connect the dots for your kids. This activity will help them take what they just learned and put it to use in their own boat creations!
Check it out… Simple STEM Aluminum Foil Boat Lab!
Food for thought...
Looking for a more permanent science curriculum? Even though we pursue various science units that encompass labs and such, we also use a science textbook each year. Some might say that our science units supplement our textbook, while others would say the opposite to be true. The labs, experiments, graphic organizers, videos, and other activities bring the textbook information to life.
We use Abeka Science textbooks. I love that they incorporate God and His sovereignty into their curriculum. When I taught in the public school system, I noticed that many of my students were lacking in vocabulary. This made it difficult for them to understand the why behind many of our investigations. Unfortunately, they were deficient in their background knowledge. Our science time would be lacking too if it weren’t for the rich vocabulary and science language that we find in our science textbook and readers. If you are interested in shopping for a full-time science curriculum or even just a textbook to guide you, click on the following link! Happy homeschooling!
Check out these other FREE resources!
And the light shineth in darkness; and the darkness comprehended it not. John 1:5
