# How can I engineer a raft so that it holds the most weight?

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#### How does this align with my curriculum?

Explore the forces of gravity and buoyancy while designing a raft to hold the most weight possible.

## What You Need

• Scrap paper for designing
• Pencils
• Straws - 5 per group
• Craft sticks - 1 per group
• Paper towel - 1 per group
• Pipe cleaners - 3 per group
• Aluminum pan filled with water for testing
• Scissors
• String
• Wax paper - 1 piece per group
• Plasticine - 1 small ball per group
• Cork - 1 per group

## What To Do

### Individual Activity

1. Design your raft, using the scrap paper and a pencil. Consider how you could use the various materials.
2. Build your raft. The goal is to build a sturdy raft that will hold as much weight as possible.
3. Fill your pan with water.
4. Once finished building your raft, float the raft in the pan of water. Gradually add coins or small washers to your raft to see how much weight it can hold while staying afloat.

### Group Challenge Activity (for two or more participants)

1. Give each group scrap paper and pencils to first design their raft (they may look at the materials that will be provided).
2. Within a designated time period (i.e. 30 minutes), have each group build their raft. Remind them that their goal is to build a sturdy raft that will hold as much weight as possible.
3. Fill your pan with water.
4. Once groups are finished building, have them bring their raft and float it in the pan filled with water. Gradually add coins or small washers to see how much weight their raft can hold while staying afloat.

There are two forces acting on your raft: gravity and buoyancy. Gravity is working to pull the raft down and buoyancy is working to push the raft up. Gravity is dependent on the mass (or weight) of the raft. The heavier the raft, the more gravity it has. Buoyancy is dependent on the density (mass ÷ volume) of the raft. The less dense your raft is (with mass/weight more spread out), the more the water will push on the raft. The forces of gravity and buoyancy must be equal or balanced in order for the raft to float.

Engineers use the same principles illustrated above when building ships. Because gravity is pulling these ships down, large buoyant forces are needed to keep them afloat. This is achieved by constructing ships that have a lower density than water. In order to compensate for the weight of the steel hull, ocean liners are very large (increasing their volume) and have large air-filled cavities (decreasing their mass). When cargo is loaded onto these ships, they may float lower in the water because of an increase in mass and an increase in density. If a ship is too full, it might sink!

In contrast, engineers build submarines so that they actually do sink. To control the depth of a submarine, the crew controls the density of the ship. If the submarine crew wants to surface, they need to decrease the density of the ship. If the crew wants to sink, they need to increase the density of the ship. These changes in density occur by pumping water in and out of huge tanks onboard called ballast tanks. When the ballast tanks are filled with water, the ship will sink because of an increased density. To bring the submarine back up to the surface, the ballasts are emptied of water and filled with air, making the ship float!

For more information on this topic check out these Let's Talk Science resources:

• Why do Ships Float? (STEM in Context) - Have you ever been on a ship and wondered how you’re staying afloat? The answer is buoyancy!
• How do scuba divers sink? (Hands-on Activities) - Learn how scuba divers manipulate density to move up and down in this hands on activity.