Physics 19 Questions

How many vibrations per second are represented in a radio wave of 101.7 MHz?

101,700,000 vibrations 1 vibration per second = 1Hz, 1 million vibrations per second = 1MHz

A bow wave on the surface of water is two-dimensional. What about a shock wave in air?

A shock wave in air is three-dimensional.

How does a sine curve describe a wave?

A sine curve is a pictorial representation of a wave, produced by simple harmonic motion.

What kinds of waves can show interference?

All waves show interference, whether water, light, or sound. The overlapping of waves forms an interference pattern. Crest over Crest = constructive interference with increased amplitude. Crest over Trough = destructive interference with reduced amplitude.

What is meant by a blue shift and a red shift for light?

An approaching source has increased light frequency towards the high-frequency end of the color spectrum - a blue shift. A receding source has a decreasing frequency towards the low-frequency spectrum end - a red shift.

Why is there a Doppler effect when the source of sound is stationary and the listener is in motion? In which direction should the listener move to hear a higher frequency? A lower frequency?

Approaching the source and close the the source, the listener will hear higher frequencies. As the listener pulls away and further from the source, the frequency will decrease with distance, as the waves will take longer to reach the listener.

How fast does a supersonic aircraft fly compared with the speed of sound?

As fast as or faster than the speed of sound.

How fast must a bug swim to keep up with the waves it produces? How fast must it move to produce a bow wave?

As fast as the wave speed; as fast as or faster than the wave speed. When the bug swims as fast as the speed of the waves it produces, the waves pile in front of the source. When it outruns the waves it produces, it will always be entering water with a smooth surface.

In what direction are the vibrations relative to the direction of wave travel in a transverse wave?

At right angles to the direction of wave travel.

Can the Doppler effect be observed with longitudinal waves or with transverse waves?

Both. This includes light and sound.

Distinguish between constructive interference and destructive interference.

Constructive interference occurs when the crests of two waves add together. Destructive interference occurs when a crest of one wave is reduced by the trough of another.

True or false: In order for an object to produce a sonic boom, it must be "noisy."

False. A whip and a bullet move through the air without making sounds other than the shock wave. Once an object is moving faster than the speed of sound, it will *make* sound. Neither are sound sources, but when traveling at supersonic speeds, will produce their own sounds as they generate shock waves.

True or false: A sonic boom occurs only when an aircraft is breaking through the sound barrier. Defend your answer.

False. An aircraft creates a shock wave continuously as it is supersonic. This is heard as a sonic boom by an observer. The "barrier" is not real; an aircraft with sufficient power easily travels faster than the speed of sound. It reaches the ground to listeners below, who hear the sharp crack.

In what direction are the vibrations relative to the direction of wave travel in a longitudinal wave?

Parallel to the direction of wave travel. "Motion is along the direction of the wave rather than at right angles to it."

Distinguish between these different aspects of a wave: period, amplitude, wavelength, and frequency.

Period is the time from one crest to the next, amplitude is the distance from the midpoint to a crest, wavelength is the distance from one crest to the next, and frequency is the number of crests that pass per second.

Which has the longer period, a short or a long pendulum?

The long pendulum has the longer period. The period is proportional to the length.

What is the source of all waves?

The source of all waves, whether mechanical or electromagnetic, is something that is vibrating.

What is a wiggle in time called? What do you call a wiggle in space and time?

a periodic wiggle in time is called a *vibration*, while a wiggle in space and time is a *wave*. Think of a single guitar string vibrating, while a large swell of ocean water is a wave!

The wavelength of a transverse wave is the distance between successive crests (or troughs). What is the wavelength of a longitudinal wave?

The wavelength is the distance between successive maximum compressions (or rarefractions).

Imagine a superfast fish that is able to swim faster than the speed of sound in water. Would such a fish produce a "sonic boom"?

This could be considered a "water boom", as the fish speeds through the water, entering water with an undisturbed area.

Astronomers find that light emitted by a particular element at one edge of the Sun has a slightly higher frequency than light from that element at the opposite edge. What do these measurements tell us about the Sun's motion?

This is characteristic of the Doppler effect, meaning the Sun's motion leads to a change in frequency on its edges.

If a pendulum is shortened, does its *period* increase or decrease?

decrease. A short pendulum has a shorter period time of to and fro swing than a long pendulum.

wavelength

distance from one crest to the next crest, or between any two identical points of the wave.

Amplitude

distance from the midpoint to a crest. Think visual amplitude on sound board from flat bottom screen to its possible height.

In one word, what is it that moves from source to receiver in wave motion?

disturbance. Energy can be transferred from source to receiver without transfer of matter. What is propagated is the disturbance, not the medium through which is transferred. The medium returns to its initial condition after the disturbance has passed.

If a pendulum is shortened, does its frequency increase or decrease?

increase. A shorter pendulum swings to and fro more frequently than a longer one.

frequency

number of crests that pass per second.

What kind of motion should you impart to a stretched coiled spring (or Slinky) to provide a transverse wave? To provide a longitudinal wave?

To provide a transverse wave to a Slinky, impart a continuous up and down motion. The motion of the Slinky is transverse to the direction the wave travels. For a longitudinal wave, push and pull the end of the Slinky. Motion will be along the direction of the wave.

What is the relationship among frequency, wavelength, and wave speed?

Wave speed is frequency multiplied by wavelength. w speed = frequency X wavelength

What is meant by the superposition principle?

When two or more waves occupy the same space, wave displacements add at every point.

Can standing waves be formed with transverse waves or longitudinal waves?

standing waves can be produced with either transverse or longitudinal vibrations.

What is meant by the period of a pendulum?

the time a pendulum takes to swing to-and-fro. This is only dependent on the length of the pendulum, not the mass or size or arc.

How are frequency and period related to each other?

Frequency and period are inversely proportional. Frequency = 1/period Period = 1/Frequency

What is a node? What is an antinode?

In a standing wave, a node always has zero displacement and an antinode always has maximum displacement. The incident and reflected waves will form a standing wave, where *nodes* appear to be standing still, and *antinodes* are regions of maximum displacement and maximum energy.

When you blow your horn while driving toward a stationary listener, the listener hears an increase in frequency of the horn. Would the listener hear an increase in horn frequency if he or she were also in a car traveling at the same speed in the same direction as you are? Explain.

In the same continuous vicinity as your car, a same speed traveling car would not hear an increased frequency, but a constant frequency. If you were to pull away from the second car, the listener would hear decreased frequency with distance.

Does the medium in which a wave travels move with the wave?

No. The medium returns to its initial condition after the disturbance has passed.

In the Doppler effect, does frequency change? Does wave speed change?

Only frequency changes. Both the frequency and wavelength undergo change when the source is moving, but the wave speed does not.

How does the V shape of a bow wave depend on the speed of the source?

The angle of the V gets smaller as the speed of the source gets faster.

Period

time from one crest to the next.