Water hammer (Rob Vondrak)

Author

Rob Vondrak

Principles

Inertia

Sound & Vibrations

Air resistance

Vacuums

Fluid flow

Standards

Crosscutting Concepts: Energy and Matter

HS-PS3.B Conservation of energy and energy transfer

HS-PS4.A Wave Proporties

ETS1.B Developing Possible Solutions

Materials needed

A water hammer (can be purchased form science supply vendors)

An alternative demo using the same principle requires an empty glass bottle.

Procedure

Hold the glass tube vertically upright with the large bulb on top. Gently, but quickly lower the tube. The tube needs to be kept vertical during the movement.

Alternatively, you can also quickly move the tube up in the air to "toss" the water up, then let the water land.

If done successfully, you should hear a thunk. It will sound like a hammer hitting a nail.

CAUTION: It is possible to break the glass if there is enough force.

Explanation

The sound is created when the water falls and slams into the glass tube. The collision creates a pressure wave that vibrates the tube and creates the hammer noise.

When the tube is moved down, its acceleration is greater than the acceleration due to gravity. The water on the other hand will fall at the acceleration due to gravity, so it will lag behind giving it space to fall down. The water "delaying it's fall" can also be explained using inertia.

The tube it self has been evacuated to minimize air resistance and allow the water to move faster and as one discrete mass. Normally when a stream of water falls through air, the air molecules cause the stream of water to break apart into drops that disperse in different directions.

Questions

1. When the water moves it has kinetic energy, but when it slams into the bottom of the tube it stops. Where does the energy go? (The energy is

2. transferred to the glass tube. The glass tube now has vibrational energy - also kinetic. That energy is transferred to the surrounding air in the form of sound.)What ways can a water hammer effect be eliminated in a plumbing system? (Water hammer arrestors -pictured to the right- can be installed. It is basically a vertical pipe with a piston that will allow the pressure wave to be dampened)

3. Sound requires a medium to travel, but the tube has had all the air sucked out, so why are we able to hear the hammer sound? (The glass tube is vibrating which then causes the air outside of the tube to vibrate. That is the sound we hear.)

Everyday examples of the principles illustrated

Water hammer is a common problem in plumbing. When a valve connected to a water pipe is suddenly closed, a pressure wave is created that travels through the pipe. This is also referred to as hydraulic shock. Not only is the hammering noise quite annoying it can also cause damage to the plumbing system.

Photos