Let’s imagine that we have four toy ducks all the same size but each made of different material. The water duck is included to show that, in this example, one pound of water would occupy the volume of each of these ducks.

Rubber Duck

thin skin of rubber, hollow inside, weighted on bottom

1/8 pound

Wood Duck

pine wood

1/2 pound

Water Duck

very thin clear plastic skin filled with water

1 pound

Steel Duck

solid stainless steel

8 pounds

Imagine that any of these ducks is pushed completely underwater. Its volume displaces (pushes aside) one pound of water, an amount equal to that in the duck made of water. This displacement produces an upward buoyant force of one pound on the submerged duck.

Now imagine releasing these ducks one at a time on top of the water. Any duck that weighs less than one pound will sink down into the water until it displaces enough water equal to its weight. Then the upward buoyant force will equal the downward weight of the duck and it will float.

For example, the rubber duck with a weight of 1/8 pound will sink into the water until it displaces 1/8 pound of water, and then it will float. When the rubber duck is floating, the upward buoyant force of 1/8 pound exactly balances the duck’s downward 1/8 pound weight. Similarly, the 1/2 pound wood duck will sink deeper into the water until it displaces 1/2 pound of water, and then it will float.

If the water duck is released on top of the water, it will sink until completely submerged. At that point, it will be displacing exactly its own weight in water, so the upward one-pound buoyant force will balance the downward duck weight of one pound. If the water duck is completely submerged and released at any level underwater, it will remain at that level.

The 8-pound steel duck, on the other hand, will displace only one pound of water when completely submerged, producing a one-pound upward buoyant force. This is not enough to hold up the 8-pound weight of the steel duck, so it sinks to the bottom.

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