WAVES

INTRODUCTION

A wave is a disturbance that propagates through space and time, usually with the transference of energy. A mechanical wave is a wave that propagates or travels through a medium due to the restoring forces it produces upon deformation. Waves travel and transfer energy from one point to another, often with no permanent displacement of the particles of the medium (that is, with little or no associated mass transport.

Periodic waves are characterized by crests (highs) and troughs (lows), and may usually be categorized as either longitudinal or transverse. Transverse waves are those with vibrations perpendicular to the direction of the propagation of the wave; examples include waves on a string, and electromagnetic waves. Longitudinal waves are those with vibrations parallel to the direction of the propagation of the wave; examples include most sound waves. When an object bobs up and down on a ripple in a pond, it experiences an orbital trajectory because ripples are not simple transverse sinusoidal waves.

All waves have common behavior under a number of standard situations. All waves can experience the following: reflection, refraction, diffraction and interference. In this experiment, you will investigate wave movement through different mediums that encompass the three phases of matter, sound wave movement, and surface wave movement. Please see pages 125-126, 129, and 131 for calculations. The entire chapter contains information pertinent to the lab exercise and should be thoroughly reviewed before completing the experiment.

MATERIALS

2 balloons

1 pencil with an eraser 1 wood block

1 2m length of string

1 rubber band

1 fork (metal)

1 spoon (metal)

A surface that can be smacked with the spoon and fork (metal surfaces work best) - table, oven, counter, car door, etc.

1 very large shallow container

2 coins of a single denomination (pennies, dimes, nickels, or quarters) 2 coins of a denomination unlike the first set of 2

1-meter stick

1 stopwatch or cellular phone with timing capabilities to at least a tenth of a second and plenty of water

PART#1: Sound Waves Through Mediums

Procedure:

• Blow into a balloon and then seal it.
• Place the inflated balloon next to your ear and gently tap it with the eraser part of the pencil.

• Next fill a balloon with water and then seal it.

• Repeat step #2 by tapping on the balloon with the pencil.
• Next get the block of wood.

Repeat step #2 by tapping on the wood with the pencil. Results: Observations: (note quality of sound, intensity of sound, etc.) Question 1.

Air-filled	Water-filled	Solid block

Questions:

• Through which medium did the sound travel best?
• What were the most noticeable differences between the mediums?

PART#2: Sound Waves Through String

Procedure:

• Measure 2 meters of string.
• Using a rubber band, attach a metal fork to the midpoint of the string.
• Wrap the ends of the string around your fingers and rest your fingers in your ears (don't try to poke your brain).
• Standing next to a table (or other metal surface), rock your body back and forth so that the fork taps against the side of the table. You may need to stand on a step stool or chair with the longest length of string.
• Record your observations.
• Repeat steps #1-#5 with a 1m length of string and fork.
• Repeat steps #1-#5 with a 0.5m length of string and fork.
• Repeat steps #1-#5 with the 1m length of string and a metal spoon.

Results:

Observations: (note quality of sound, intensity of sound, etc.) (Question #4)

Fork w/ 2 m string	Fork w/ 1 m string

Fork w/ 0.5 m string	Spoon w/ 1 m string

Question:

• How is it possible to hear a sound through a string?

PART#3: Surface Waves Procedure:

• Fill the tub with a layer of water at least 2 inches deep.
• Measure the length (meters) of the wave path from the center of the container (tub) to the side farthest from the center point. If you are unable to find a large container (greater than 0.5 m from center, measure from a point close to one side and drop the coin there. This drop point will create some interference but you should still be able to determine the experimental answers.
• Let the water settle (no waves).
• Have the timer ready. Drop a coin in the center while starting the timer. Stop the timer when the first wave reaches the side of the tub farthest from the center point.
• Let the water settle again
• Drop another coin of the same denomination in the same place, this time counting how many waves occur between the center point and the side.
• Repeat steps #3-#6 with the other two coins of the same denomination (completing the data collection in duplicate).

Results: (Question #6)

Coin	Length (m) same for all drops	Time (s) to the hundredth place	Number of Waves

Questions:

• What is the average wavelength (length/#of waves) of the first coin denomination?
• What is the average speed (length/time) of the wave for the first coin denomination?
• What is the average frequency (# of waves/time) of the first coin denomination?
• What is the average wavelength of the second coin denomination?
• What is the average speed of the wave for the second coin denomination?
• What is the average frequency of the second coin denomination?
• Is there any difference in magnitude (perceived height) of the wave between the two different coins?