Why Do You Weigh Less in the Water?

Posted at January 3rd, 2011 by admin

Helping your child understand buoyancy

Summer is just around the corner. And school will soon be out. But that doesn’t mean that learning has to stop. The next time you take your little one for a dip in the pool, why not teach her something as well? Amanda Formaro shares what she learned after being stumped by one of her daughter’s many questions. Even better, she includes a few activities to help you test, teach and prove your newfound wisdom.

While I was playing with my 6-year-old daughter in the swimming pool one day, she suddenly asked me a question. “Mom, why can I lift you up in the pool but not when we are in the house?” My answer? “Well, I’m not sure honey. It has something to do with density and mass or something.” Boy, I thought to myself, that was well put.

After a little research, however, I found a much better way to explain 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 and Far Way

In the third century B.C. there lived a man named Archimedes, who wrote something called the Archimedes Principle. This states that when a person gets into the water, his body moves the water out of the way. In turn, the water pushes upward as his body pushes downward. But where does the water that was there before go? It gets displaced.

Displaced? As I explained to my daughter, this big word means “moved out of the way.” When you get into a swimming pool, your body displaces the water that was there before. It moves it. And where does it go? It moves upward. When you get into the pool, the water level rises a bit because the weight of your body is forcing the water underneath you downward. So it has nowhere to go
but up.

What Makes You Lighter?

Imagine the water that was there before you got into the pool, before you moved it out of the way. That water weighs something. And when you get into the pool, the weight of your body pushes the water downward and out of your body’s way. The water that you displace then pushes you back upward. This is called buoyancy, or floating, and this is what makes you feel lighter.

Is Density Your Weight?

What is density? It isn’t the same as weight. Density has to do with how much matter is contained in a given space. And density is what allows you to sink or float when in the water. When you are in the swimming pool, you will sink unless you swim. But if you fill your lungs with air and hold your breath, you will float. Why? Because when you take a deep breath, you change the density of your body, adding more air to it and allowing it to float.

Still Confused?

It might be easier to explain this to your child if you perform the following experiments.

Materials:

  • 3 half-filled glasses of water
  • 4 ice cubes
  • Several small rocks
  • A leaf

Experiment #1

In the first glass of water, add an ice cube. See how the water rises? Add another ice cube, and another, and finally the last one. The water will rise with each ice cube that you add.
Ask Yourself: Did the ice cubes float or sink?
Answer: They floated because the density of the ice cubes was less than that of the water. And the water rose because of the liquid that was displaced with the addition of each ice cube.

Experiment #2

In the second glass of water, add a small rock. Then add more rocks until all are in the water.
Ask Yourself: Did the water rise? And did the rocks sink or float?
Answer: These rocks sank because their density was greater than that of the water. The water rose for the same reason the ice cubes did. The water was moved out of the way, or displaced.

Experiment #3

In the last glass of water, add the leaf.
Ask Yourself: Did the water rise? Did the leaf sink?
Answer: Neither! Why? Because the force of the leaf pushing downward and the force of the water pushing upward are completely equal. The leaf floats because its density is less than that of the water.

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