WAVES AND SLINKY® LAB

 

 

 

Introduction  

One of the more useful toys in the Physics arsenal is the lowly Slinky®.  The Slinky®

can be used to demonstrate both kinds of waves: longitudinal and transverse.  The Slinky® can

also be used to investigate pulses, echoes, and interference.  Finally the Slinky® can be used to

investigate the effect of tension on pulse speed.  In this lab you will gather a lot of descriptive

data of wave phenomenon.

In this lab you should familiarize yourself (See chapter 12) with the following wave

terms:

 

Transverse wave

Longitudinal Wave

Pulse

Amplitude

Wavelength

Frequency

Period

Open-end reflection

Closed-end reflection

Phase

Wave speed

Superposition

Reflection

Refraction

Constructive Interference

Destructive Interference

Standing wave

Harmonic

Node

Anti-node

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Materials:

 Slinky® with a 3 m piece of string attached to one end.

 Long coil Physics Slinky® (you may have to share)

Stopwatch Meter Stick Tape

 

 

 There are two sections to this lab.

 

Both sections ask you to make

Pulse transverse, pulse longitudinal, continuous transverse, and continuous longitudinal waves.

 

You may use the same observations for some of the same questions.

 

You are asked also to investigate several waves together, with different strings/springs attached, reflection, interference, and the MEASURED  effects of factors on waves. No procedure is necessary, except if you decide to try something not listed (which is OK!).

 

Draw pictures, and make measurements as much as possible.

 

 BE CAREFUL with equipment!


A Stretch the Slinky® between two people.

Or, tie one end of the Slinky® to a chair or other solid object the same distance away.

 

 

To Do and observe

Move your hand up and down, record observations..

 

What is moving along the Slinky®? 
The up down motion moves along the Slinky®. This wave is a movement of motion! This wave is called a transverse wave because the motion of the Slinky® is sideways to the motion along the Slinky®. It provides a model for waves moving along strings, for light waves(in particular linearly polarized light waves, and for seismic waves called S waves. 
You may also move your hand side to side to send horizontally polarized waves.

 

Move your hand side to side and up and down at the same time so that it moves in a circle. A rotational wave moves down the Slinky®. This models circularly polarized light. There are two possible directions to move your hand in the circle, clockwise and counter-clockwise.

Move your hand toward and away from you. record observations..

 

This wave is a wave of motion back and forth along the Slinky® which travels along the Slinky®, because the back and forth motion is in the same line as the direction of motion this is called a longitudinal wave. The longitudinal wave is a model for sound waves in gases and liquids as well as for seismic P waves. 
This type of wave is also called a compression wave.

Move your hand toward and away from you at the same time that you move it up and down so that it makes a circular motion. record observations..

 

A circular motion wave moves along the Slinky®. This models ocean waves and a type of earthquake wave called a love wave. However, whereas the transverse and longitudinal waves described first moved through the bulk of a material, the ocean wave and the love wave are surface phenomena. Their motions are greatest at a surface and decrease with depth.

Move your hand side-to-side as well as toward and away to make a circle in a horizontal plane. A wave of this motion will not travel in liquids and gases but it will travel in solids like the earth and this Slinky®. This models a seismic wave known as a Rayleigh wave.

Rotate your wrist. record observations. This is a torsional wave.

 

When a bow is drawn across a violin string the string twists and a torsional waves travels down the string.

 

 

What’s Going On?

All waves involve restoring forces, forces which tend to restore a deformation to its original shape. Push a few atoms in a solid in any direction and forces will push them back towards their original position. Push some atoms in a gas or liquid to the side however and they will stay there. Without restoring forces waves cannot propagate, so transverse waves do not move through gasses and liquids.

Try to make a denser region in a gas or a liquid and forces will oppose you, When you make a longitudinal or compression wave, you make regions of high and low density. So that there are restoring forces and longitudinal waves can go through liquids and gases.

 

Etc

Transverse seismic waves cannot pass through the outer core of the earth while longitudinal seismic waves can, thus the outer core of the earth must be a liquid.

 

 

To Do and Notice

Hold the Slinky® between your hands. The Slinky® will be horizontal and sag. Move both of your hands up-and down together. Find the lowest frequency which produces the largest motion of the Slinky® for the smallest motion of your hands. (About one cycle per second.) One large hump, half-a-wave should appear moving up and down on the Slinky®.

Count the rhythm, 1,2,3,4,1,2,3,4,...

 

Move your hands in opposite directions, the right hand up when the left hand moves down and vice-versa. Move them in the same rhythm as above. Notice that your hands move a large distance. The center of the Slinky® does not move up-and-down at all.

 

 

What is Going On?

When you move your hands together you get a half-a-wave on the Slinky® the middle of the Slinky® is an antinode, the hand-held ends are nearly nodes.

When you move your hands opposite, a half-a-wave also fits on the Slinky®. However, this half wave has one node in the center and two antinodes near the hand-held ends.

The timing on both of these is the same. They both are one-half-wave resonances

 

ETC.
For the transverse motion of the Slinky®, pressure is modeled by the slope of the Slinky®. At places where the motion of the Slinky® passes through zero, a node of motion, the slope of the Slinky® changes the most, an antinode of slope.

 

 

Longitudinal Waves

 

To Do and Notice

Grab the ends of the Slinky® in your hands. Stretch the Slinky® to between 1 and 2 meters long.

Move your hands together and then apart, just as if you were clapping. Notice the motion of the Slinky®. Your hands move a lot while the center of the Slinky® moves very little. The center is a node.

 

Next, notice the spacing between the slinks (turns) of the Slinky®. When the slinks are jammed close together the Slinky® models high pressures in a gas where the atoms are closer together. When the slinks are far apart, the Slinky® models low pressure gas. Let’s call closely spaced slinks high pressure and widely spaced slinks low pressure. Notice that the pressure change is greatest at the center where the slinks alternately bunch-up and spread apart. Count the rhythm of this motion: 1,2,3,4,1,2,3,4,...

Move both hands in the same direction, if the Slinky® stretches right-left move both hands to the left then to the right. (One of our teachers described this as the buhdist clap, two one handed claps!)

 

Notice the motion of the Slinky® which is called longitudinal motion. Find the frequency of hand motion that produces the largest motion of the center of the Slinky® for the smallest motion of your hands. Count the rhythm of this motion: 1,2,3,4,1,2,3,4,...
Notice that the center of the Slinky® is an antinode, your hands are nearly nodes. Notice that in the center the Slinky® moves back and forth but the spacing between the slinks near the center does not change. The center is an antinode of motion but a node ( a place with no change) of pressure. At the nodes of motion near your hands, however the slinks bunch together and then spread apart: the pressure changes a lot. The hand-held ends are antinodes of pressure. Notice also that when one hand is at high pressure the other is low. The ends then swap the high pressure one becomes a low pressure and vice-versa. In other words, the slinks bunch up near one hand while they spread out at the other.

What is Going On?

When your hands move together one-half-wave of longitudinal motion fits on the Slinky®. This is the lowest frequency resonance of the Slinky® and it has what is called the fundamental frequency.

When your hands move opposite, one-half-wave of longitudinal motion also fits on the Slinky® but this time the node is in the middle while your hands are near antinodes.

A sound wave is a longitudinal wave. A sound wave can be viewed either as a wave of motion of atoms or as a wave of pressure. In a standing sound wave nodes of motion occur at the same place as antinodes of pressure.

To Do and Notice

Find a higher frequency resonance of the longitudinal wave in which you move both hands in the same direction. You should have to move your hands about twice as often as in the lowest frequency resonance you created before.

 

Count the frequency: 1,2,3,4,1,2,3,4 Notice the motion of the Slinky®, there are two nodes each about 1/4 of the way from each end. 
One full wave fits on the Slinky®. When there is a high pressure near one node there is low pressure near the other. The high pressure and low-pressure regions switch positions each cycle.

Move your hands opposite each other and find the next higher resonant frequency.

 

There will be three nodes on the Slinky®, one in the center and the other two1/6 of the Slinky® from each end.
3/2 of a wave fits on the Slinky®. Notice the pressure changes on the Slinky®, when one node is experiencing high pressure the adjacent one experiences low pressure. With time, each node oscillates from high pressure to low and back again.

 

What is Going On?

High pressure and low-pressure nodes alternate in time as well as in space. To create an odd number of nodes move your hands opposite each other, clap hands. To create an even number of nodes move your hands in the same direction.

 

 

 

 

 

 

Do pulses bounce off each other or pass through? Design, conduct, and  describe specifically the experiment you did to prove your conclusion.

 

 

 


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B Method:

1) Stretch the Slinky® out on the floor approximately 3 meters with one person sitting down

holding onto to each free end. One of the holders should transmit a transverse pulse by rapidly

moving the end of the Slinky® ON THE FLOOR from side to side. The pulse should be formed

by moving the Slinky® rapidly out to the right of the person forming the pulse and back to the

starting point. This will be called a ‘right’ pulse.  

 Describe what happens when the pulse reaches the far end (this is called a closed

reflection). Does it come back as a ‘right’ or ‘left’ pulse?

 

2) Stretch the Slinky® out on the floor approximately 3 meters with one person sitting down

holding onto to each free end. Have the person with the string end hold the Slinky® by pulling

lightly on the string.  The OTHER holder should transmit a transverse pulse by rapidly moving

the end of the Slinky® ON THE FLOOR from side to side.  The pulse should be formed by

moving the Slinky® rapidly out to the right of the person forming the pulse and back to the

starting point.  This will be called a ‘right’ pulse. 

 Describe what happens when the pulse reaches the far end (this is called an open

reflection).  Does it come back as a ‘right’ or ‘left’ pulse?

 Description:

 

3) Stretch the Slinky® out on the floor approximately 3 meters with one person sitting down

holding onto to each free end.   Both holders should transmit a transverse pulse by rapidly

moving the end of the Slinky® ON THE FLOOR from side to side.  The pulse should be formed

by moving the Slinky® rapidly out to the same side (one person’s right and the other’s left) and

back to the starting point.  

 Describe what happens when the two pulses meet in the middle (This is called

constructive interference).  Do they make a larger or smaller pulse at the moment of meeting? 

Note: it is some times easier to see by tying a small ribbon exactly in the middle an observing

how the ribbon moves (or doesn’t move) when the pulses meet.

 Description:

 

4) Stretch the Slinky® out on the floor approximately 3 meters with one person sitting down

holding onto to each free end.   Both holders should transmit a transverse pulse by rapidly

moving the end of the Slinky® ON THE FLOOR from side to side.  The pulse should be formed

by moving the Slinky® rapidly out to OPPOSITE sides (one person’s right and the other’s right)

and back to the starting point.  

 Describe what happens when the two pulses meet in the middle (This is called destructive

interference).    Do they make a larger or smaller pulse at the moment of meeting?   Note: it is

some times easier to see by tying a small ribbon exactly in the middle an observing how the

ribbon moves (or doesn’t move) when the pulses meet.

 Description:

 

5) Stretch the Slinky® out on the floor approximately 3 meters with one person sitting down

holding onto to each free end.  One of the holders should transmit a continuous set of transverse

pulses by rapidly moving the end of the Slinky® ON THE FLOOR from side to side and

continuing the movement.  The wave should be formed by moving the end of the Slinky® back

and forth rapidly.  The waveform desired will have a single “hump” which moves out to the right

and then out the left.  Some adjustment in timing will be necessary until the wave is formed. 

This is called a standing wave and also a fundamental.     

 Describe the number of points on the wave that move the maximum distance (antinodes). 

Describe the number of points on the wave that do not move at all (nodes).  Describe the

wavelength of the fundamental in terms of the distance between the two ends of the Slinky®.

 

6) Stretch the Slinky® out on the floor approximately 3 meters with one person sitting down

holding onto to each free end.  One of the holders should transmit a continuous set of transverse

pulses by rapidly moving the end of the Slinky® ON THE FLOOR from side to side and

continuing the movement.  The wave should be formed by moving the end of the Slinky® back

and forth rapidly.  The waveform desired will have two “humps” which move out to the right

and then out the left.  Some adjustment in timing will be necessary until the wave is formed. 

This is called a standing wave and also a second harmonic.     

 Describe the number of points on the wave that move the maximum distance (antinodes). 

Describe the number of points on the wave that do not move at all (nodes).  Describe the

wavelength of the second harmonic in terms of the distance between the two ends of the

Slinky®.

 Description:

 

7) Stretch the Slinky® out on the floor approximately 3 meters with one person sitting down

holding onto to each free end.  One of the holders should transmit a continuous set of transverse

pulses by rapidly moving the end of the Slinky® ON THE FLOOR from side to side and

continuing the movement.  The wave should be formed by moving the end of the Slinky® back

and forth rapidly.  The waveform desired will have three “humps” which move out to the right

and then out the left.  Some adjustment in timing will be necessary until the wave is formed. 

This is called a standing wave and also a third harmonic.     

 Describe the number of points on the wave that move the maximum distance (antinodes). 

Describe the number of points on the wave that do not move at all (nodes).  Describe the

wavelength of the third harmonic in terms of the distance between the two ends of the Slinky®.

 Description:

 

8)  Given the pattern for the above three activities, describe the 

a)  number of nodes for the nth harmonic

b)  number of antinodes for the nth harmonic

c)  wavelength of the nth harmonic in terms of L (the distance between the two ends

of the Slinky®)

Description:

 

 

9) Tie the Slinky® with the other long coil of wire.  Stretch the combined Slinky® out on the

floor approximately 6 meters with one person sitting down holding onto to each free end.  One of

the holders should transmit a transverse pulse by rapidly moving the end of the Slinky® ON

THE FLOOR from side to side.  The pulse should be formed by moving the Slinky® rapidly out

to the right and back to the starting point.  This will be called a ‘right’ pulse.  

Describe what happens when the pulse encounters the boundary between the two coils

(this is called refraction and reflection).  Note: Two pulses should be observed!

  Description:

 

10) Tie the Slinky® with the other long coil of wire.  Stretch the combined Slinky® out on the

floor approximately 6 meters with one person sitting down holding onto to each free end.  One of

the holders should transmit a longitudinal pulse of the Slinky® ON THE FLOOR.  The pulse

should be formed by gathering some Slinky® coils at one and then suddenly releasing them.

 

 

This will be called a longitudinal pulse.

Describe what happens when the pulse encounters the boundary between the two coils

(this is called refraction and reflection).  Note: Two pulses should be observed!

  Description:

 

11) Stretch the Slinky® out on the floor approximately 3 meters with one person sitting down

holding onto to each free end.  One of the holders should transmit a transverse pulse by rapidly

moving the end of the Slinky® ON THE FLOOR from side to side.  The pulse should be formed

by moving the Slinky® rapidly out to the right of the person forming the pulse and back to the

starting point.  Using the stopwatch, determine the time for a single pulse to travel from the

originating end, down to the other end and return back to the originating end.  Repeat this twice

more and get an average time.  Measure the length of stretch and determine the velocity.  Repeat

this experiment for 3.5 m, 4 m, 4.5 m and 5 m.

 

Graph the velocity versus the length.  Determine a relationship between the length of Slinky®

and the velocity of the wave.

 

TAKE DATA and observations  ON THE FOLLOWING FOR A TRANSVERSE WAVE WITH A COILED SPRING (one or two pulses is preferable)

 

You need stopwatches, metersticks and good eyes.

 

 

The effect of amplitude on the speed of the wave.

 

 

 

The effect of amplitude on the frequency and wavelength of the wave.

 

 

The effect of amplitude on one point on the wave (mark with tape or paper).

 

 

The effect of tension on the speed, frequency, wavelength and one point on the wave