Why Do You Weigh Less in the Water
Answering the Questions Children Ask
while playing with my 6 year-old daughter in the swimming pool one day,
she asked the question "Mom, why can I lift you up in the pool but not
when we are in the house?" My answer was "Well, I'm not sure honey, it
has something to do with density and mass or something." Boy, that was
After a little research, I found out exactly why a person's body is so
much lighter underwater. It was quite logical really, but I needed to
put it in to terms that my 6 year-old daughter would understand.
Long Ago & Far Away
There was a man named Archimedes, he lived a very long time ago from
287-212 B.C. Archimedes wrote something called the Archimedes' Principal
and it says that when a person gets in to the water, that the person's
body moves the water out of the way. In turn, the water pushes back
upward as your body pushes downward. But where does that water go that
was there before?
Displacement - Huh?
This big word means "moved out of the way". When you get in to a
swimming pool, your body displaces the water that was there before you
got in. It moved it. So where did it go? When you got in to the pool,
the water level rose a bit. That is because the weight of your body is
forcing the water downward. So what makes you lighter?
Buoyancy - What's That?
Imagine the water that was there before you got in to the pool. Before
you moved it out of the way. That water has a weight. When you got in to
the pool, and the weight of your body pushed the water downward and out
of your body's way, the weight of the water that you displaced (moved
out of the way) pushed you back upward. Because you weigh more than the
water that you displaced, it will cause a force to push your body weight
upward. This is called buoyancy, or floating.
Density - Your Weight?
Not necessarily. Density is volume, not your weight. Density is what
allows you to sink or float when in the water. When you are in the
swimming pool, unless you swim, you will sink. If you fill your lungs
with air and hold your breath, you will rise. NEVER try this without a
parent with you! When you took that deep breath, you changed the volume
of your body, adding more air to it, allowing you to float.
You will need three half filled glasses of water, four ice cubes,
several small rocks, and a leaf.
In one glass of water, add an ice cube. See how the water rose? Add
another ice cube, and another, and finally the last one. The water rises
with each ice cube that you add. Did the ice cubes float or sink? They
float because the density of the ice cubes are less than the water. The
water rose because of the liquid that was displaced with the addition of
each ice cube.
In the second glass of water, add a small rock. Did the water rise? Did
the rock sink or float? Add each rock until all are in the water. These
rocks sank because their density was more than that of the water. The
water rose for the same reason as the ice cubes, the water was moved out
of the way, or displaced.
In the last glass of water, add the leaf. Did it sink or float? It
floats because the density of the leaf is less than that of the water.
Did the water rise? No! Why? Because the force of the leaf pushing
downward and the force of the water pushing upward are completely equal.
Next time you go swimming with Mom and Dad, try a couple of these fun
things that have to do with your new found knowledge!
- Bounce up and down in the water -- you are buoyant!
- With both of you in the pool, lift up your Mom just like Superman!
- If you know how to dive down for object, have Dad toss objects with
different densities in to the water to go after.
- Show your experiment to your friends, I'm sure they'll love it too.
© Amanda Formaro
Amanda Formaro is the mother of four children. She and her husband live
in southern Nevada. She is also the owner of familycorner.com magazine
at http://familycorner.com | email her at: WebMom@familycorner.com
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