So if gravity pulls everything down, why is it that some things float in water and don’t sink?
Archimedes' define buoyant force as:
The buoyant force or up thrust experienced by and object immersed in a liquid equal to the weight of a the liquid displaced.
Buoyancy is another force which acts on objects as well as Gravity.
Buoyancy acts in liquids and gases. Its the same force that keeps helium balloons floating.
As an object enters water it moves, or displaces, the water around it. The denser the item the more water displaced.
For example, although a beach ball and a bowling ball may be the same size, the bowling ball is denser meaning it will displace more water.
You’ll notice that when you place a cork in water, it floats. We can push down on the cork, but when we let go, it floats back to the top.
That means that there is some force pushing back up on the cork. This upward force experienced is buoyancy.
So observe the animation to the right. We have a rock tied to a scale.
It weighs 20 kg
Notice what happens to the weight of the rock when we drop it in the water. It drops by 5 kg.
Also notice how the water becomes displaced and pours into the small beaker. Notice how much water is displaced. 5 ml
This means the buoyant force (or upward force) exerted on the rock by the water is 5kg.
This is why if we drop this rock in water, it would sink to the bottom.
Using the same rock and glass above, we can see why the rock would sink. When the force of gravity (15kg) is greater than the force of buoyancy (5kg) then the object will sink.
If the buoyant force is greater than the object will float.
So now we know why objects float and sink in water, but how can they float in the air?
Rising in air, and floating on water work in much the same way.
When you fill a balloon with helium it rises. This is because the combined weight of the balloon and helium is less than the amount of air it displaces.
Meaning the force of gravity pushing down on the balloon, is less than the buoyant force pushing up.
This is why we launch hot air balloons in the morning when the air is colder (denser) and the air inside the balloon is heated (thinner).
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