A huge ship under sail

What makes a heavy ship float?

How does a boat or ship carrying hundreds of pounds worth of stuff float while that same stuff would sink to the bottom of the ocean if dumped overboard? How come when you’re in a pool and you stretch your body out flat you float. But, if you wrap your arms around your legs and curl up into a ball you sink? Well, it all has to do with how much water is pushing against you and a little scientific principle called buoyancy or floatation. When you stretch out flat more water pushes against you since your body is laid out flatter. When you curl up into a ball, less water is pushing against you. Want to test this for yourself? Try this experiment:

  1. Take a piece of clay and split it into 2 identically sized pieces. Take one of the pieces and roll it into a ball. Take the other piece and fashion it into a flat boat shaped object (if needed, get mom or dad to help – that’s what they’re there for).
  2. Now place both pieces into a sink full of water. Which one floats and which one sinks? Both? Neither?

So you see, if the total area of the object that makes contact with the water is large enough, the object floats. The object must make room for its own volume by pushing aside, or displacing, an equivalent (or equal) volume of liquid. The object is exerting a downward force on the water and the water is therefore exerting a upward force on the object. Of course the floating object’s weight comes into play also. The solid body floats when it has displaced just enough water to equal its own original weight.

This principle is called buoyancy. Buoyancy is the loss in weight an object seems to undergo when placed in a liquid, as compared to its weight in air. Archimedes’ principle states that an object fully or partly immersed in a liquid is buoyed upward by a force equal to the weight of the liquid displaced by that object. From this principle, he concluded that a floating object displaces an amount of liquid equal to its own weight.

Experiment Supplies

Supplies: Clay


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