Mochi Isle
Raft buoyancy · Bamboo ValleyScience for kidsWhy does a bamboo raft float?
In short: A raft pushes some water out of the way, and the water pushes back up with a force called buoyancy. Bamboo is light, with sealed hollow sections full of air, so it floats well. The more weight you add, the deeper the raft sits.
How it works
Push an empty plastic bottle under water and you can feel the water shoving it back up. That upward push is buoyancy. Anything placed in water pushes some water aside, which is called displacing it. The heavier the water pushed aside, the stronger the upward push.
Bamboo makes great rafts. Most bamboo stems are hollow, and the nodes act like walls that seal air inside a row of little chambers. A bamboo pole is light and slow to fill with water. Tie lots of poles together and they can displace plenty of water.
When people or cargo climb aboard, the raft sinks a little deeper, displacing more water until the upward push matches the weight. Load it too heavily and water starts washing over the top, which is dangerous. If the load is uneven, the raft tips to one side.
Where to see it in New Zealand
Māori traditionally made rafts called mōkihi. They were built from bundles of dried raupō, a kind of bulrush, or from the dried flower stalks of harakeke, New Zealand flax. These materials are full of air, so they float well, and mōkihi were used to cross rivers and lakes.
Try it together
Float a small plastic container in a basin of water. Add coins one at a time and watch it sit deeper. Stick a strip of tape on the side and mark the waterline each time. What happens if you pile all the coins at one end?
Kids also ask
How can a heavy steel ship float?
A ship is hollow and very large, so it pushes aside a huge amount of water. As long as it can push aside water weighing as much as itself, it floats. If it fills with water, it loses that buoyancy and sinks.
How do I stay safe on a raft or small boat?
Always wear a life jacket and have an adult with you. Get on and off slowly, sit still, and do not all crowd to one side, because rafts and small boats tip easily when the weight is uneven.
In the game, “Raft buoyancy” is a nature card on Bamboo Valley.
Your child finds this card by reaching the right place or meeting the right weather on the island, and lights it up by answering the question on the back. Tap the button below and the game shows your child which island it is on and how to find it. Mochi Isle is a browser learning game for ages 5 to 18; the main island and the daily lesson are free.
Find “Raft buoyancy” on the island ↗ All cards from Bamboo Valley