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Explore the generation, motion, and reflection of pulses on a frictionless slinky. Measure pulse speed, observe pulse behavior, and investigate superposition of pulses.
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Transverse & Reflected Pulses Created for CVCA Physics By Dick Heckathorn 28 April 2K + 5
02 Transmitted and Reflected Pulses 16Pulses on a “Frictionless” Slinky 28 Reflection and Refraction
Place the slinky on the floor and extend it so that it is under tension. Before doing this, ask instructor for some advice. • Be careful not to over-extend the slinky, since this causes permanent damage.
Generate a pulse by giving the slinky a flick at right angles to the length of the slinky. • Can you make different pulse shapes? • Describe how you make different pulse shapes. Figure 1
Figure 1 The generation and motion of a pulse along a spring shown by a series of pictures taken with a movie camera.
How fast do the pulses travel? • Estimate the speed of the pulse. • To do so, make the necessary measurements and calculations. • Record your results.
Change the speed of the wave? • Describe what you did to change the speed. • Record them. • Calculate the new speed.
Observe carefully what a specific point on a slinky is doing as a pulse goes by. • You may wish to put a paper clip over one of the rings of the slinky to mark a point. Remove when done. • Describe what you observe. Figure 2
Figure 2 The motion of a pulse from right to left along a spring with a ribbon around one point. The ribbon moves up and down as the pulse goes by but does not move in the direction of motion of the pulse.
What is happening to the shape of the leading half of the pulse? • To the trailing half of the pulse? • Describe what you observe.
Make a pulse. • Observe its propagation and then make a sketch of the pulse at one instant of time. • Then make a sketch of the pulse a very, very short time later.
You have surely noticed that the pulse shape changes as it moves along the slinky. • What do you think is happening here?
How would you expect the pulse to behave if the slinky were suspended so that it didn't touch the floor? • What if it was suspended in water?
Make further observations of a single point or particle of the slinky as a pulse goes by from right to left. • Sketch the left and right deflection of a particle as a function of time.
While observing the propagation of a pulse on the slinky, you may have noticed that the pulse does not vanish when it arrives at the far end. • Describe what happens to the pulse as it is reflected.
Figure 6 Reflection of a pulse from a fixed end. The reflected pulse is upside down.
Tie a two meter piece of string onto one end of the slinky and repeat #9. • What do you observe?
Figure 9 The blurring of the thread in the middle frames of the sequence indicates that the particles of the thread are moving at a high speed as the pulse passes. Can you determine the direction of this motion in each of the frames? A pulse passing on a spring reflected from a junction with a very light thread. The whole pulse returns right-side up.
Pulses on a “Frictionless” Slinky • For this part of the experimentation, you will be watching two people investigate pulse phenomena on a slinky with very little friction.
Send a pulse from one end. • Describe what happens to the pulse as it reflects from a held end?
2. Determine the speed of the pulse by first finding the time for the wave to travel from one end to the other and back to the original end.
The slinky is stretched to a greater length. • Time the pulse from one end to the other and return.
How does the speed of the pulse in the further stretched slinky compare to the speed in the original length? • Record measurements and results.
4. Identify and record the property(s) of a slinky which changed as it was stretched?
A pulse is sent from one end. • Describe what happens to the pulse as it reflects from an end held by a long string?
Two similar pulses (same amplitude) are sent down the slinky, one from each end, oriented in the same direction, at the same time. • Describe what one observes?
Figure 5 The superposition of two equal pulses on a coil spring, each of which is symmetrical about its center line. In F, we see the absence of blur indicating no motion at that instant.
Does one pulse pass through or reflect from the other pulse? • What might one do to find out? • Describe what one can do and the result.
Figure 3 Two Pulses crossing each other. Notice that the two pulses have different shapes. Thus we can see that the one which was on the left at the beginning is on the right after the crossing, and vice versa.
Set a pop can half way from both ends and a distance away from the slinky of between 1 and 2 amplitudes of the pulse. • Send one pulse from each end with the orientation in the same direction. • Describe what happens to the pop can. Explain why.
With the pop can in the same position, send one pulse from each end with the orientations in opposite directions. • Describe the result.
Figure 4 The superposition of two equal and opposite pulses on a coil spring. In the fifth picture they almost cancel each other.
Directions • Tie a metal slinky and a plastic slinky together with some masking tape.
A. Metal Into Plastic • With the system stretched, send a pulse from the metal slinky end. • Describe its shape of the pulse as it moves down the metal slinky.
Figure 8 A pulse passing from a heavy spring (left) to a light spring. At the junction the pulse is partially transmitted and partially reflected. The reflected pulse is right-side up.
A. Metal Into Plastic • 1. The wave reaches the point where the metal slinky and the plastic slinky are joined.
A. Metal Into Plastic • Does any of the pulse continue into the plastic slinky? • If yes, describe all the properties of the pulse.
A. Metal Into Plastic • Does any of the pulse reflect back from the junction between the two slinkies? • If yes, describe all the properties of the pulse.
A. Metal Into Plastic • c. Describe the pulse in the plastic slinky after it reflects from the held end.
