Center of mass
It's time to set sail! Even if you don’t live near a lake or ocean, you will get to do some sailing in this science activity as you build your own toy sailboat. But first you have to make sure your boat doesn’t capsize! Are you up for the challenge?
Do you remember playing with toy boats in the bathtub—or have you ever been on a real boat? Boats can float because of buoyancy. At the same time they are pulled down by the force of their own weight (caused by gravity) they are pushed up by the buoyant force, which is equal to the weight of the volume of water they displace. (You can find a more detailed explanation of buoyant force in the “More to explore” section.) Some boats are made of materials that are less dense than water, meaning they have less mass per unit volume. These materials will always float. Other boats, however, are made of metals such as steel, which are much denser than water. So how do they float? They can because they’re hollow, so there is a lot of empty air space inside the boat’s hull. The average density of the boat (including both the metal and the air) is lower than the density of water.
Boats don’t just need to float—they also need to stay upright and avoid capsizing, or flipping over. To do this they need a low center of mass, meaning their weight is concentrated toward the bottom of the boat, not the top. That might seem like it would be a problem for sailboats, which have very tall sails that stick way up into the air. How do they stay balanced with so much mass concentrated way up high? They do so with another part called the keel, which is on the bottom of the boat. (If you’ve only ever seen a sailboat from above the water, you might not even know the keel existed!) The keel is a big part under the boat, shaped like a fin, which serves two purposes. It holds the ballast, or heavy weight, that helps lower the boat's center of mass. It also helps prevent the boat from being blown sideways by the wind. In this project you’ll see how a keel can help keep a sailboat from flipping over and help it go straight.
- Three wine corks
- Two rubber bands
- Several screws or nails
- Waterproof material, such as craft foam, wax paper or paper milk carton to make a sail
- Aluminum foil
- Sink, bathtub or a large container you can fill with water (The container should be deeper than the length of your nails/screws.)
- Fill your container with water. Make sure the water is deep enough so you can vertically submerge your longest nail/screw.
- Line up three corks (side by side, not end to end).
- Use two rubber bands to hold the corks together, forming a “raft.”
- Poke a toothpick into the center cork, so it sticks straight up. This is your boat’s mast (the part that holds the sail).
- Cut a square of your thin waterproof material to make a sail. It should be about six centimeters square.
- Poke the toothpick through opposite ends of the sail (near the edges) to hold it in place.
- You’ve made your first sailboat! Put it in the water. What happens?
- Blow on the sail from behind. What happens?
- Now make a skinnier boat by removing the rubber bands and the two outer corks. Keep the sail in place. Depending on how you attached the sail initially, you might need to rotate it 90 degrees.
- Put your new sailboat back in the water. What happens?
- Uh-oh! Your sailboat probably tipped over! That’s not good. To fix it, try adding a keel. Carefully stick a nail or screw into the bottom of the boat, directly opposite the sail.
- Try putting the boat back in the water. Does it stay upright this time?
- If your boat doesn’t stay upright, keep adding nails or screws (in a straight line with the first one) until it can float without tipping over.
- Now try blowing on the sail again. What happens? Does your boat move in a straight line?
- Right now, your keel is made of one or more nails/screws, but they are not connected to one another. Cut a rectangular piece of aluminum foil and tightly wrap it around the nails/screws to make a fin shape.
- Put your boat back in the water and try blowing on the sail again. What happens this time? Does it go straight?
- Extra: Try making a bigger sail and using part of a wooden skewer for the mast instead of a toothpick. How heavy does your ballast need to be to balance the boat with a bigger sail? Hint: Try attaching a horizontal nail/screw to the bottom of your keel to act as ballast. That way you don't have to keep poking more nails/screws into the cork.
Observations and results
Your first sailboat was probably pretty stable because it was very wide (made from three corks). When you removed two corks to make it skinnier, however, your sailboat probably became unstable and tipped over. It’s similar to standing with your feet tightly together instead of spread out slightly—it’s harder to balance. When you added nails/screws to the bottom of your sailboat you lowered its center of mass and made it more stable. Individual vertical nails, however, don’t do a very good job pushing against the water—the water can flow right around them. That means the keel doesn’t do a good job making the boat go straight. If you blew on the sail, your boat might have curved off to one side or spun in circles. When you wrapped the nails in aluminum foil you made the keel more like a fin. It can cut through the water very easily in one direction, but it provides a lot of resistance against the water in the other direction. That makes it easier for your boat to move forward, and harder for it to move sideways. This is why real sailboats can be long, skinny and have tall sails—the keel prevents them from tipping over and helps them go straight!
More to explore
What Is Buoyant Force?, from Khan Academy
Buoyant Science: How Metal Boats Float, from Scientific American
DIY Toy Sailboat, from Science Buddies
Shipping Science: Building a Boat That Can Carry Cargo, from Scientific American
Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies