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WHAT IS BUOYANCY? WHY DO THINGS FLOAT?

How can it be that things float in liquids? Is there an invisible string hooked onto the sky that holds things at the top of the water? Is there an invisible pusher-upper that is on the bottom of the body of water that keeps things at the top? Is it magic?

The truth is that objects float in water because the water pushes up against them. I guess it isn't an invisible holder or pusher-upper. There are several things to do that let us feel the push of a liquid. That push is called buoyancy (pronounced "BOY-ann-see").

a. Hold a block of wood under water, then describe how it feels to your hand. It almost feels like the wood is struggling against you to get to the top of the water.
That's buoyancy.

b. Float an empty can in a pan of water. Make sure the can does not have sharp edges. Put one hand into the can and push it downward without sinking the can. What does that feel like? What does it mean?

c. Let's take turns pushing these large corks to the bottom of the sinkful of water. How does that feel? How much strength does it take to keep the cork on the bottom?
Let's put the cork into a bottle and try it again. Does it feel any different than just holding down the cork?

d. Now we'll try it with a balloon. Is this different from holding down the wood, the can, or the cork?

e. This time we'll take turns with this empty, one-pound coffee can, with its plastic lid on, of course, and with a small plastic pill bottle with a tight-fitting lid.

Push the covered can, top down, to the bottom of the sinkful of water. Describe how it feels, and what the can does. Now let it go, quickly, and watch it! Next, do the same with the little bottle. Does it behave the same as the coffee can?

Wonder if it makes any difference whether the water is deep or shallow? After we've finished with this part, we can let out half the water in the sink, and try the same
experiments in the shallow water.

 

 
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