MCAT Periodic Motion and Waves

Question 1

Oscillation

Answer 1

When an elastic material is stretched from equilibrium it oscillates (back and forth movement).

Question 2

What is the period ( T) of a wave?

Answer 2

The time it takes for the wave to complete one full cycle.

Or in other words the time it takes to cross a fixed point.

Question 3

How to graph the oscillation of a wave?

Answer 3

The y- axis contains the oscillating variable while x- axis represents time.

Question 4

Crest v. Trough

Answer 4

The crest is the maximum point of the wave from the equilibrium position while the trough is the minimum point from the equilibrium position.

Question 5

Amplitude v. Wavelength

Answer 5

Amplitude is the distance from the equilibrium position to either the crest or trough. Whereas the wavelength is the distance from crest to crest or trough to trough.

Question 6

Frequency

Answer 6

The number of wavelengths that passes a fixed point.

You can calculate it by take the inverse of the period (T).
Frequency is measured in hertz where 1S^-2 = 1Hz.

Question 7

Transverse waves v. Longitudinal waves

Answer 7

Transverse waves are waves in which their movement happens perpendicular to their propagation.

Longitudinal waves are waves that moves in the parallel direction of the wave.

Question 8

Rarefaction

Answer 8

The repetitive compression and decompression of longitudinal waves as it moves through a medium.

We can measure wavelength by measuring the distance from compression to compression or expansion to expansion.

Question 9

What determines a waves speed (V) in a medium?

Answer 9

NOT the properties of the wave but only by the properties of the medium, the tension and density.

Wave speed increases with increased tension of the medium.

Wave speed decreases with increased density.

Question 10

How do we calculate a wave’s speed in a medium?

Answer 10

V = (wavelength)/ (period)

Question 11

Law of superposition

Answer 11

States that the interference of waves creates a resultant amplitude that’s the sum of each of the individual wave’s amplitude.

Question 12

Instructive v. Destructive interference

Answer 12

Instructive interference is when crest matches to crest or trough matches to trough and the resultant amplitude is added cumulative.

Destructive interference is when crest matches to trough and the resulting amplitude diminishes.

Question 13

Phase difference. What are the two-phase differences?

Answer 13

The degree by which two waves overlap.

(wavelength) / 4 = 90 degrees
(wavelength) /2 = 180

Waves continue propagating the same regardless of the interference.

Question 14

Standing waves

Answer 14

Waves that are fixed at two ends. We can change frequency by changing the length, tension, or density.

Question 15

Describe how ultrasound imaging uses sound?

Answer 15

A probe creates sound that’s much higher than human hearing. This sound travels through the body and is reflected off body tissue creating an echo, the ultrasound machine uses this echo to create the image.

The elapsed time is the total time it takes for sound to travel to target tissue and back to ultrasound. Can be calculated via the following equation:

distance =(elapsed time) X (V, speed of sound in body) / 2

Question 16

Shock wave lithotroscopy

Answer 16

Technique that’s used to break kidney stones into smaller pieces so it can be passed in the urine.

Ultrasound waves converge at a target point creating a shock wave. The shock wave creates a pressure differential that breaks apart the stones.

Ultrasound waves are constructive interference, so they increase the energy, and the frequency is matched to the resonant frequency of the kidney stone.

Question 17

Sound waves

Answer 17

Longitudinal waves that displace the medium in the direction that’s parallel to the propagation of the wave.
Sound requires a medium and so it cannot move in a vacuum.

Question 18

What is the tone (pitch) determined by?

Answer 18

The frequency of the wave

Question 19

What variable effects sound wave’s speed?

Answer 19

The temperature. Waves travel faster in warmer temperatures and so has a greater frequency.

Question 20

How does the type of medium we have affect the propagation of waves?

Answer 20

They travel the fastest in solids and the slowest in gases.

Question 21

What happens when a sound wave hits the interface between two media?

Answer 21

Some of the sound waves are reflected and some enters the new media.

When a wave changes media it’s frequency doesn’t change but it’s wavelength does. The reflected wavelength has all the same properties as the original wave. The new wave’s speed is directly proportional to the wavelength and so it changes as well.

Question 22

Attenuation

Answer 22

When the energy of a wave decreases as it enters a new media.

Associated with a decrease in the intensity of the wave.

Question 23

Mechanical energy of a wave

Answer 23

Sound waves carry energy as it moves through the medium, the larger the displacement the greater the energy.

To find the mechanical energy of a wave take the square root of the amplitude.

Question 24

What is the SI unit for the intensity of a sound wave?

Answer 24

Watts/meter^2

Question 25

What is loudness

Answer 25

The logarithm of the intensity. Because it’s a logarithm it’s a 10- fold increase.

Loudness is measured in decibels.

Question 26

What happens a when a wave is fixed at two points?

Answer 26

You get a standing wave in which the wave doesn’t seem to be moving. Caused by constructive.

Creates nodes which are points of zero displacement and antinodes which are points of maximum displacement.

Question 26

Fundamental frequency v. Fundamental wavelength

Answer 26

The fundamental frequency is the lowest resonant frequency a wave can have. n =1 with subsequent frequencies having multiples of the lowest (resonant frequency).

The fundamental wavelength is the longest resonance wavelength. n=1 with subsequent wavelengths being divided by the resonant wavelength.

Question 27

What’s the relationship between the intensity of a sound wave and the distance of the sound’s source?

Answer 27

The intensity of a sound wave is inversely proportional to the inverse of the distance squared.

Question 28

How to find the speed of a wave on a string?

Answer 28

We take the square root of tension force over the density of the string.

Question 29

The open end of a pipe corresponds to what? What does the closed end of the pipe correspond to?

Answer 29

Open end corresponds to antinodes while closed end corresponds to nodes.

Question 30

What is the relationship between the wave speed and the density of the gas in a tube?

Answer 30

Wave speed is inversely proportional to the square root of the density of the gas in a tube.

Question 31

What is the doppler effect?

Answer 31

Change in frequency between the source and the observer based on whether they’re moving toward each other or away from each other.

The perceived frequency is greater than the source frequency when moving toward each other.

The perceived frequency is less than the source frequency when moving away from each other.

Question 32

When do we not experience the doppler effect?

Answer 32

When there is no relative motion between the observer and source.

We can view this quantitively when we solve for the perceived frequency when using the doppler effect equation and it equals the source frequency.

Question 33

What properties of waves change and doesn’t change when the sound wave enters a new medium?

Answer 33

Frequency doesn’t change but the wavelength, speed, and amplitude.

All the properties remain the same when the wave is reflected back into the same media.

Question 34

Which wave property is associated with the intensity of a wave?

Answer 34

The amplitude of a wave. The intensity of amplitude is associated with the energy of the wave.