PASCO WA-9857 User Manual
Instructions
Demonstrations
Experiments
Sample Data
Instruction Manual
No. 012-08798A
String Vibrator
WA-9857
String Vibr a tor Model No. WA-9 85 7
Contents
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Equipment Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Introductory Activity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Demonstration 1:
String Density and Wavelength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Demonstration 2:
Closed Tube Analogy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Experiment 1:
Wave Speed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Experiment 2:
Standing Waves In Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Experiment 1:
Teachers’ Notes–Wave Speed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Experiment 2:
Teachers’ Notes–Standing Waves In Strings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Technical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Copyright and Warranty Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Model No. WA-9857 String Vibra t or
®
Power Supply
String Vibrator
String Vibrator
Model No. WA-9857
Included Equipment Replacement Part Number
String V ibrator
Power Supply
Wave Cord (3 meters, not pictured)
The demonstrations and experiments described in this manual call for additional equipment. For details,
see the equipment list for each activity.
WA-9857
540-050
SE-9409 (90 m roll)
Introduction
The PASCO scientific WA-9857 String Vibrator drives a string or elastic cord to produce a
standing wave. W ith it, you can study frequency, wavelength, and resonance, as well as the factors
that affect those properties. It is well-suited for classroom demonstrations and hands-on
experiments.
The String Vibrator uses a coil-and-magnet design to vibrate a stainless steel blade, to which you
attach a string or elastic cord. The driving signal and power come from the included power
supply, or (for a variable signal) from an optional signal generator, such as the WA-9867 Sine
Wave Generator.
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String Vibr a tor Equipment Setup
®
St a cki n g Pin
(one per corner)
Clamping
Surfaces
Power Input
Vibrating
Built-in Rod
Clamp
Equipment Setup
Power
The AC Power supply plugs into the Power Input of the String Vibrator. It drives the String
Vibrator with a constant-frequency, constant-amplitude sine wave. The driving frequency equals
the frequency of the mains power supply (50 or 60 Hz in most countries).
If you would like to dr ive the String Vibrator with a variable signal, you can use any function
generator capable of producing a 10 V amplitude sine wave at up to 1 A, including the following:
• Sine Wave Generator (WA-9867)
• Digital Function Generator (PI-9587)
• 750 or 700 Interface with Power Amplifier II (CI-6552A)
Mounting Options
The String Vibr ator can be fastened to a table in a variety of ways.
C-Clamp
Two recessed Clamping Surfaces on the
String V ibrator allow it to be secured to a
table with a C-clamp. You must use a Cclamp wide enough to accommodate the
thickness of the tabletop plus 3 cm (1¼
inch).
The PASCO Small C-Clamp (SE-7286,
6-pack) can clamp the String Vibrator to
tables up to 5 cm (2¾ inches) thick; the
Large C-Clamp (SE-7285, 6-pack)
accommodates tables up to 7 cm (2¾
inches) thick.
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Model No. WA-9857 Equipment Setup
®
Rod Clamp
The case of the String Vibrator has a
built-in rod clamp for mounting it either
horizontally or vertically on a rod with
a diameter up to 12.7 mm (1/2 inch).
Slide the rod through the case in the
preferred orientation and tighten the
thumb screw.
The Universal Table Clamp (ME-9472)
and 45 cm Rod (ME-8736) work well in
this application because you can clamp
the rod vertically to the edge of a table.
Permanent Mounting
Two through-holes in the clamping
surfaces allow the String Vibrator to be
mounted permanently on a flat surface.
Place a washer under each screw head
to protect the plastic case.
String Setup
String Selection
The included elastic wave cord works we ll for demonstrations and many la b activities. T he elastic
cord is easy to see, produces good amplitude, and it does not require a hanging mas s to provide
tension, but it does not have constant linear density under tension. For experiments exploring the
relationship between wave speed and string density, it is better to use an inelastic string such as
Braided String (SE-8050) or Yellow Cord (ME-9876).
Attaching the String
If you are using the included elastic
cord, or any other thick cord, pass it
through the grommet, then tie a fr eestanding knot that cannot pass through
the hole when you pull back on the
cord. If the end of the cord is frayed,
trim it to make it easier to thread
through the grommet.
If you are using thin string, thread it
through the grommet at the end of the
blade and tie it in a loop.
5
String Vibr a tor Equipment Setup
®
V ert ical Str in g
The vertical arrangement with
the elastic cord makes a good
classroom or lecture
demonstration. It requires a
vertical rod and a horizontal
component at the top of the
rod, such as a Pendulum Clamp
(SE-9443), to attach the elas tic
cord. To adjust the length and
tension, move the top mount
vertically on the rod.
Horizontal String
The pictures below show the horizontal arrangement in two ways. Since the standing waves
produced sometimes vibrate in a plane, it may be necessary to rotate the case for the wave to be
visible.
In the orientation pictured on the left, the wave is visible from above, but
not as easily seen by a student sitti ng out in the c lassroom . As shown on t he
right, the wave is visible from the side, which is most useful for
demonstrations.
Applying Tension to Inelastic String
When you use inelastic string, it is necessary to apply tension. This can be
accomplished with the string oriented horizontally, and with a hanging
mass, a pulley and a table clamp as shown here. The tension on the string is
equal to the weight of the hanging mass.
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Model No. WA-9857 Equipment Setup
®
Bad Node
Good Node
Good Nodes Versus Bad Nodes
Most demonstrations and
experiments involve
adjusting the length, tension
or frequency to produce a
standing wave pattern. It is
tempting to look only at the
amplitude of the wave and
concentrate on making it as
large as possible; but it is
also important to check that
the nodes are “clean” and
well defined, especially the
node at the vibrating blade.
Check the end of the
vibrating blade. There
should be a node at the point
where the cord attaches, as
shown in the first picture to
the right.
An example of a bad node is
shown in the second picture.
The blade rattling against
the plastic case indicates a
bad node.
The method for correcting a bad node depends on the type of experimental setup. With the elastic
cord, the adjustment is usually made to the length and tension simultaneously by moving one of
the end points. With an inelastic string set up with a pulley and a hanging mass, you can adjust the
length of the string by moving String Vibrator, or adjus t the tension by changing the hanging
mass. With either type of string, if you are using a variable-frequency signal generator you can
adjust the driving frequency.
Storage
Pins on the top corners and matching
holes on the bottom corners of the String
Vibrator allow you to stack two or more
units for stora g e.
7
String Vibr a tor Introductory Activity
®
Introd uctory Acti vity
Equipme nt R equired Part Nu m ber
String V ibrator
Power Supply
Elastic Wave Cord (1 meter)
Clamp or other device for securing the String Vibrator
WA-9857
Part of WA-9857
Part of WA-9857 (or SE -9409)
SE-7286 or similar
This activity works best with two or more people.
1. Attach the String Vibrator to the table. You’ll be stretching the cord to about 2 m, so leave
enough space.
2. Cut 1 m of elastic cord and attach one end to the vibrating bla d e.
3. Connect the AC power supply to the String V ibrator.
4. Hold the free end of the cord as shown, and slowly increase the tension by pulling it away
from the String Vibrator.
5. Observe the standing wave patterns that occur as you stretch the cord. Note what happens to
the number of segments as you increase the tension. Does increasing the tension cause the
number of segments to increase or decrease?
6. Adjust the tension until the cord vibrates in 4 segments. Then adjust the tension slightly so
that there is a good node at the blade. Maintain that tension for the rest of the activity.
7. Measure the wavelength. (How is the wavelength related to the length of one segment?)
8. Touch the cord at one of the antinodes (the points of maximum vibration). What happens?
9. Touch the cord at one of the nodes. What happens? How is touching the c ord at a node
different from touching it at an antinode?
10. Have a lab partner pinch the cord at the mi ddle node without changing the tension. What
happens to the wavelength?
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