Feel the Pressure

Material

A sink or a deep pail full of water (the deeper the better)

A "Playtex living glove" (you may also substitute a bread bag or the bag that the morning paper comes in.)

Optional, a stack of 6 nearly identical books such as an encyclopedia.

To Do and Notice

Put the glove on one hand, or the bag.

Point your fingers straight down and insert your gloved hand into the water.

Do not insert your hand so deep that the glove or bag fills with water.

Notice that the deeper you push your hand the greater the squeeze-force exerted by the water.

Repeat the experiment without using the glove or bag, i.e. insert your other hand into the water.

Optional experiment, make a stack of 6 nearly identical books. Place your hand underneath the top book, feel the force.

Place your hand underneath the bottom book, feel the greater force.

What's Going On?

The water at the bottom of the pail is supporting all the water above. When you place your gloved hand deep into the water you feel the pressure generated by the weight of the overlying water.

When you placed your hand under the entire stack of books you could feel a large force on your hand because it was holding up all of the books.

Why do you need the glove or the bag?

Human sensors detect differences or changes in a signal. When you stick your ungloved hand into the water the water exerts a uniform force on your hand. It flows around every hair and every wrinkle in your skin. Now a single hair is bent to the side. When this happens you cannot feel the pressure exerted by the water.

However when you wear the bag or the glove they will bend down the hairs on your hand, and the glove and the bag may have folds that exert uneven forces on your skin. So that you can "feel" the force exerted by the water.

Math Root

Water is an almost incompressible fluid. So under pressure its density remains the same r = 1000 Kg/m^3. The pressure, P in pascals i.e. N/m^2, at a depth, h measured in meters under water on earth where the acceleration of gravity is g = 9.8 m/s^2 is

P = rgh

For example at a depth of 10 m the pressure is approximately

P = 1000*10*10 where we have approximated g = 10 m/^

P = 10^5 Pa = 100 kPa

The pressure at the bottom of the atmosphere on earth is 101 kPa on the average.

So 10 m or 30 ft of water exert 1 atmosphere of pressure.

So What?

When you are swimming and dive down underwater you can feel the pressure on your eardrums. You do not feel the pressure on your ungloved hand. This is because there is an air cavity behind your eardrum. Under pressure the air is compressed and the eardrum moves creating a sensation of pressure or pain. The hand is full of solids and liquids which are nearly incompressible so there is no change in shape and no perception of pressure is generated.

 Scientific Explorations with Paul Doherty © 2006 26 April 2006