Imagine you are fishing at a lake. You attach your hook to a sinker – a small round ball of metal – that helps it sink to the bottom. While waiting for a fish to bite, you notice a man in the middle of the lake in a small metal canoe. Your sinker and his canoe are both made of metal. Yet, your sinker went straight to the bottom while his canoe held him safely above the surface.
How is this possible?
What does it take to make a boat float?
When you put something into water, the water itself must be pushed out of the way. But, water doesn’t just sit there and take it!
A ship sinks into the water until the weight of the water it moves out of the way equals the weight of the ship. Ships are usually created with a large volume, but very thin sides. This ensures that the water displaced is heavier than the ship.
Because water is denser and heavier than the ship, the ship won’t sink below the surface. In fact, most ships are designed so they displace a huge amount of water and can carry a ton of weight as cargo. If the ship gets loaded with cargo, the extra weight will displace more water and the ship will sit lower in the water.
If you put too much cargo in a ship, the density of the whole package becomes greater than the density of water. In this case, the ship would sink.
A buoyancy experiment you can do at home
For an object to float on water, it has to be less dense than the water itself. If you put a penny into water, it will sink because it’s too dense.
But, a boat can easily distribute the weight of a penny into a larger volume.
Try this: Fill a sink or tub with a couple inches of water. Mold a piece of tinfoil into the shape of a boat and set it afloat on the water. Now add a penny. The air inside the boat offsets the density of the penny, so your boat is still seaworthy.
Now add another penny. Does your foil boat sit a little lower in the water? How many pennies can you add before your boat sinks? Can you change the shape of the bottom of the boat to displace even more water? Now how many pennies can you keep afloat?
Written by Laura McCamy
Edited by Gabriel Buckley, MS Professional Natural Sciences
Illustrated by Pablo Velarde Diaz-Pache