## Tuesday, April 1, 2014

### A wave is a disturbance that moves along a medium from one end to the other. If one watches an ocean wave moving along the medium (the ocean water), one can observe that the crest of the wave is moving from one location to another over a given interval of time. The crest is observed to cover distance.  The speed of an object refers to how fast an object is moving and is usually expressed as the distance traveled per time of travel. In the case of a wave, the speed is the distance traveled by a given point on the wave (such as a crest) in a given interval of time. In equation form,

• If the crest of an ocean wave moves a distance of 20 meters in 10 seconds,then the speed of the ocean wave is ???
• 2.0 m/s
• On the other hand, if the crest of an ocean wave moves a distance of 20 meters in 10 seconds (the same amount of time), then the speed of this ocean wave is ???
• 2.5 m/s.
The faster wave travels a greater distance in the same amount of time.

### ***Sometimes a wave encounters the end of a medium and the presence of a different medium.

• For example, a wave introduced by a person into one end of a slinky will travel through the slinky and eventually reach the end of the slinky and the presence of the hand of a second person.
One behavior that waves undergo at the end of a medium is reflection. The wave will reflect or bounce off the person's hand. When a wave undergoes reflection, it remains within the medium and merely reverses its direction of travel.
• In the case of a slinky wave, the disturbance can be seen traveling back to the original end. A slinky wave that travels to the end of a slinky and back has doubled its distance. That is, by reflecting back to the original location, the wave has traveled a distance that is equal to twice the length of the slinky.

### Reflection phenomena are commonly observed with sound waves.

• When you let out a holler within a canyon, you often hear the echo of the holler.

The sound wave travels through the medium (air in this case), reflects off the canyon wall and returns to its origin (you). The result is that you hear the echo (the reflected sound wave) of your holler. A classic physics problem goes like this:

### Noah stands 170 meters away from a steep canyon wall. He shouts and hears the echo of his voice one second later. What is the speed of the wave?

• In this instance, the sound wave travels 340 meters in 1 second, so the speed of the wave is 340 m/s.

Remember, when there is a reflection, the wave doubles its distance. In other words, the distance traveled by the sound wave in 1 second is equivalent to the 170 meters down to the canyon wall plus the 170 meters back from the canyon wall.

### Variables Affecting Wave Speed - Think!!!!

What variables affect the speed at which a wave travels through a medium?

Does the frequency or wavelength of the wave affect its speed?
Does the amplitude of the wave affect its speed?
Or are other variables such as the mass density of the medium or the elasticity of the medium responsible for affecting the speed of the wave?

The speed of sound depends on the elasticity, density & temperature of medium

Elasticity - ability of a material to bounce back after being disturbed.
• rubber band is an elastic substance
sound travels more quickly in mediums that have a high degree of elasticity b/c when the particles are compressed, they quickly spread out again.
Densityof a medium is how much matter, or mass, there is in a gvine amount of space, or volume.

• ·       Sound travel more slowly in dense metals.

Temperature – room temperature of about 20*C, sound travels at about 340 m/s.

·         In a given medium, sound travel more slowly at lower temperatures & faster @ higher temperatures.

•   Factor Change in Factor Effect on Speed of Sound Elasticity Increase in elasticity Density Increase in density temperature Decrease in temperature

### SPEED OF A WAVE - Check Your Understanding

1. A teacher attaches a slinky to the wall and begins introducing pulses with different amplitudes. Which of the two pulses (A or B) below will travel from the hand to the wall in the least amount of time? Justify your answer.

2. The teacher then begins introducing pulses with a different wavelength. Which of the two pulses (C or D) will travel from the hand to the wall in the least amount of time ? Justify your answer.

3. The time required for the sound waves (v = 340 m/s) to travel from the tuning fork to point A is ____ .
 a. 0.020 second b. 0.059 second c. 0.59 second d. 2.9 second

4. Two waves are traveling through the same container of nitrogen gas. Wave A has a wavelength of 1.5 m. Wave B has a wavelength of 4.5 m. The speed of wave B must be ________ the speed of wave A.
 a. one-ninth b. one-third c. the same as d. three times larger than
5. An automatic focus camera is able to focus on objects by use of an ultrasonic sound wave. The camera sends out sound waves that reflect off distant objects and return to the camera. A sensor detects the time it takes for the waves to return and then determines the distance an object is from the camera. The camera lens then focuses at that distance. Now that's a smart camera! In a subsequent life, you might have to be a camera; so try this problem for practice:
If a sound wave (speed = 340 m/s) returns to the camera 0.150 seconds after leaving the camera, then how far away is the object?
6. TRUE or FALSE:
Doubling the frequency of a wave source doubles the speed of the waves.
7. While hiking through a canyon, Noah Formula lets out a scream. An echo (reflection of the scream off a nearby canyon wall) is heard 0.82 seconds after the scream. The speed of the sound wave in air is 342 m/s. Calculate the distance from Noah to the nearby canyon wall.

8. Mac and Tosh are resting on top of the water near the end of the pool when Mac creates a surface wave. The wave travels the length of the pool and back in 25 seconds. The pool is 25 meters long. Determine the speed of the wave.

9. The water waves below are traveling along the surface of the ocean at a speed of 2.5 m/s and splashing periodically against Wilbert's perch. Each adjacent crest is 5 meters apart. The crests splash Wilbert's feet upon reaching his perch. How much time passes between each successive drenching? Answer and explain using complete sentences.