why do some objects float when placed in water

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Yay, summer! Yay, water! Yay, playing in water during the summer! While youБre having fun splashing in a pool, lake or at the shore, think about, БWhy do you float, but a rock that could fit in your hand sinks? Б Or: БWhy does a boat float while its anchor sinks? Б
Obviously, you weigh more than the rock and a boat weighs more than its anchor. So what makes the difference? ItБs not about weight, is it? Did you ever notice how water in a bathtub rises as you lower yourself in? You displace water (move it out of the way) to make room for your body. So, part of the story is that you displace more water than a rock. A boat displaces more water than an anchor. Then what? Gravity tries to pull things down into the water, but fluids have a force that pushes upward, against gravity. ItБs called buoyancy (BOY-an-cy). This is what helps you or the boat float. A Greek mathematician named Archimedes (Ark-ih-MEED-eeze) discovered the relationship between waterБs displacement and its buoyancy more than 2,000 years ago, while taking a bath.

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LetБs do a simple experiment to see how buoyancy works. YouБll need some aluminum foil, 10 pennies and a sink or large bowl half-filled with water. 1. Cut the foil into two pieces, each six inches square. Put five pennies in the middle of one piece of foil. Then crumple the foil around the pennies very tightly, squeezing out all the air and making a ball. 2. Take the other piece of foil and bend the edges up tightly, forming a boat or raft shape with a flat bottom. Put five pennies in your boat. 3. Gently place both foil-penny combinations in the water. ThatБs the rest of the story. Objects react differently in water because of their density. Your foil ball, crumpled into a tight clump, has more density because its pennies are crowded into a smaller space than those in the foil boat.

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The foil boat has less density because it is spread out and filled with air. Things float when they have less density than water, but sink when they have more. Can I keep my foil ball from sinking? Yes. Gently put the foil ball on top of your foil boat. What happens? ItБs the same thing that happens when an anchor is placed in a boat. Because of buoyancy, boats can carry objects that wonБt float on their own. And, a hand-size rock wonБt sink if itБs resting on your tummy while you are floating! БWhich floats: a marble or a tennis ball? An orange with the peel on or an orange with the peel off? БWill an egg float better in plain water or saltwater? БWe know rocks sink. What happens if you put the rock in a paper cup and then put the cup in water? Liquids have different densities, and objects will sink if they are of a greater density than the substance they are placed in; dense objects, including rocks and pieces of metal, will sink if placed in water.

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Lighter objects, including most types of wood, will float. Dense material can be made to float if it contains enough air, which has a density much lower than that of water. A metal ball that is hollowed out and appropriately sealed can have a density less than water, which will cause it to float. Heavier metals need more air, but any material can be made to float. Boats float by taking advantage of this principle, and they can be made of woods that are denser than water. The large hull of a boat holds a significant amount of air, which lowers its total density. If the boat is loaded too heavily or if water enters the hull, the boat can sink if its density gets too high.

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