125 Physics Projects for the Evil Genius (39 page)

Figure 65-1
Ripple tank showing shadows of wave patterns on white board
.

1. Insert the concave reflector. This is where the sides curve
   toward
   the source of the ripples. Observe how the waves are reflected. Do the waves converge or diverge?
   
2. Generate ripples that originate at that focal point. How do the ripples move?
   
3. Insert (or reshape) the reflector, so it is convex. This is where the sides curve away from the source of the ripples. Do the waves converge or diverge?
   

1. Place a thick plate in the tank.
   
2. What happens to the speed of the waves as the waves cross over the plate? What happens to the wavelength? Does this make sense given that the frequency doesn’t change?
   
3. Direct waves to the plate at an angle. What happens when the waves cross from the deep water to the shallower water?
   

1. Generate ripples and observe what happens when they encounter a pencil held vertically in their path.
   
2. What happens when a larger barrier, such as a glass or beaker, is held in the path of the ripples?
   

1. Generate ripples from two different locations. The ripples should be synchronized in such a way that each ripple maker goes up at the same time and down at the same time. (This means the sources of the waves are in phase.)
   
2. Observe what happens to the pattern as the waves from the two sources overlap and interact with each other.
   

Straight barrier: the incoming angle equals the outgoing angle.

Concave barrier: the reflected waves converge at a focal point.

Concave barrier: ripples generated at the focus regroup and emerge as a single wave.

Convex barrier: waves diverge from any location.

Plate: the waves slow as they cross over the plate; the
wavelength increases
.

Plate: the waves coming toward the plate at an angle are bent to a
less-severe
angle.

Figure 65-2
Ripple patterns cross over the convexshaped barrier, resulting in the convergence of the wave pattern. Courtesy PASCO
.

Figure 65-3
This ripple tank (with vertical display) shows the diffraction pattern produced by two separate sources of wave generation. Courtesy PASCO
.

Diffraction: the wave fronts regroup around a small barrier, but not a larger one.

Interference: two ripple locations result in a fixed pattern of high and low waves.

Water waves exhibit basic wave properties, including:

• Reflection from straight surface: Angle of incidence equals angle of reflection (with all angles defined with respect to the perpendicular or
  normal
  line that can be drawn to the reflecting surface).
  
• Reflection from a concave surface: Waves are reflected from a curved surface with the law of reflection applying to the tangent line of the curve at that point. For approximately parabolic reflectors that include semicircular reflectors, this results in waves passing through a focal point. If the waves are generated at that focal point, they become focused and propagate in a single direction.
  
• Reflection from a convex surface: Waves diverge and propagate over a wider range of angles than when they started. There is no focal point when waves reflect from a convex surface.
  
• Refraction: Waves bend toward the perpendicular line (called the normal line) when they enter a region where the light waves move more slowly.
  
• Diffraction: Waves bend around a barrier in their path if the diameter of that barrier is small compared with the wavelength.
  
• Interference: Crests and troughs of waves combine to form an overall pattern based on constructive and destructive interference.
  

A large stationary body of water can serve as a large ripple tank. In this case, traveling waves can be observed without the complication of reflections from the side of the ripple tank. Pictured in
Figure 65-4
is an interference pattern formed by two rocks thrown into a lake.

Waves exhibit certain characteristic behavior, including reflection, refraction, diffraction, and interference. These properties are common to all types of waves.

Figure 65-4
Two rocks form an interference pattern as the ripples they produce spread across the surface of a lake
.

A pendulum undergoes a type of motion that is predictable. The consistency of pendulum motion has allowed it to be used to drive the timing mechanism of clocks. In this experiment, you investigate what causes a pendulum to swing faster or slower. At least for a pendulum on the surface of the Earth, only one variable determines the time it takes for a pendulum to swing back and forth one time.

• several masses that can be attached to a string (such as 20 g, 50 g, 100 g, 200 g)
  
• several strings of varying lengths from 0.1 to 1.0 m (strong enough to support the masses)
  
• support for each pendulum
  
• stopwatch
  
• meterstick
  

1. Set up a basic pendulum with a measured length and mass free to swing.
   
2. Pull the pendulum back to the side through a small (less than 15 degrees) angle and get the stopwatch ready.
   
3. Release the pendulum and start the stopwatch as the pendulum is released.
   
4. Count ten cycles back and forth. Cycle number one is when the pendulum returns to its original position. Be careful not to count “one” when the pendulum is released.
   
5. The length of the pendulum is the distance from the point where the string is supported to the center of the mass.
   
6. Record the time (in seconds) for the pendulum to complete ten complete cycles.
   
7. Divide the time for ten cycles by ten to get the time for one cycle or the period of the pendulum for the conditions you are testing.
   
8. You can proceed in several ways at this point, with many opportunities to develop your own plan. Here are a few suggestions:
   

– What variable matters: Mass? Length? Angle? Test the selected variable while holding the others constant. For instance, test light, medium, and heavy mass, and then determine whether the period of the pendulum is dependent on mass. This can be done by measuring the period of a pendulum constructed with each of the three masses. It can also be done qualitatively by setting up three pendula and observing how fast they swing compared to each other.

– Once you determine which variable(s) affects how fast the pendulum swings, you can set up an experiment to measure how the period changes over a range of the variables you selected. The other variables should be kept constant.

Figure 66-1
Simple swinging pendulum
.

The
only
variable that affects the period of a pendulum is length. The mass does not matter at all. For angles smaller than 15 degrees, angle is insignificant. Insignificant means less than 1 percent.

The longer the string, the longer the period (
period
is the time to go back and forth one time).

The dependence of period on length is not linear.

Figure 66-2
Period versus length for a pendulum
.

A graph of period versus length is shown in
Figure 66-2
. The model for the graph shows the period is dependent on the square root of the length.

The period of a pendulum is the time it takes for the pendulum to move from one position and return to the same position. The period of a pendulum (in seconds) is given by: