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Grade 11 Physics Unit 4: Waves and Sound Summary
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A detailed summary and test preparation aid of unit 4: Waves and Sound of Grade 11 Physics.
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Physics Unit 4: Waves and SoundOscillatory Motion: Motion in which the period of each cycle is constant
Simple Harmonic Motion (SHM)
● Motions that are repeated without variation, the restoring force is proportional to the
displacement of the mass
● For a mass-spring system to be in SHM, the net force must be equal to the restoring
force
● Three basic situations:
○ Oscillating springs
○ Swinging pendulums
○ Waves
Equilibrium Position: The place where the object/medium would be if it were at rest
Restoring Force: A force acting opposite to the displacement to move the object back to its
equilibrium position, points towards the equilibrium position for the medium
Hooke’s Law:
● Negative sign is present as a reminder that the direction of the restoring force is opposite
to the direction of the stretch/compression, the force “pushes/pulls” the spring back to its
equilibrium position
Why does the mass/bob swing through the equilibrium position even though the net
force is equal to zero at the equilibrium position:
● The net force being zero at the equilibrium position only affects the acceleration
(Fnet=ma), making it 0, but the mass/bob still has velocity so it will continue to swing
past the equilibrium position
Calculating the maximum speed of an object released from a mass-spring system:
● Eg: Slingshot, bow and arrow
● Use law of the conservation of energy, equate initial stored elastic potential energy
(Es=1/2kx^2) with the final kinetic energy (Ek=1/2mv^2) of the object being launched
Transverse Wave:
● The medium through which the wave is traveling moves perpendicular to the direction in
which the wave is traveling (Particles of the medium move perpendicular to the direction
of the flow of energy)
● Ex. Water waves moving up and down
● Crests: The highest point in a transverse wave
● Troughs: The lowest point in a transverse wave
,Longitudinal Wave:
● The medium through which the wave is traveling vibrates back and forth along the path
that the wave travels (Particles of the medium move parallel to the direction of the flow of
energy)
● Ex. Sound waves
● Compressions (Crests): The section of a longitudinal wave where the particles are
crowded together
● Rarefactions (Troughs): The section of a longitudinal wave where the particles are less
crowded than normal
How Changes in the Medium Affect a Wave’s Motion:
● Frequency will remain constant (frequency relates directly to the energy the wave has,
energy must be conserved so frequency must also be conserved)
● In order to maintain the constant frequency the speed and/or wavelength may need to
change (a change in one will cause a change in the other)
Changes in Depth:
● When a wave is traveling through water, it is causing the water molecules to rotate
upwards and downwards as the energy of the wave travels horizontally
● When waves travel from deeper water to shallower water, this forces the wave’s
wavelength to decrease as the waves will become closer together (if wavelength is
decreasing, speed must also decrease to maintain a constant frequency)
Changes of Medium:
● When a wave travels from a less dense material into a denser material, it will begin to
slow down (since speed decreases, so will the wavelength, to remain constant frequency
which causes the wave fronts to become closer together)
● The phenomenon of a wave changing its wavelength, speed and direction in a new
medium is called refraction
● Slower ⇨ Faster = Bends away from the normal
● Faster ⇨ Slower = Bends towards the normal
Wave:
● A series of pulses caused by repeated motion
● Any disturbance that transmits energy through matter and space
● They carry energy, NOT matter
Pulse: A single disturbance in a medium
Medium:
● The material that is being disturbed
● Ex: Air is the medium for a sound wave, water is the medium for a water wave
Amplitude: The maximum distance the wave vibrates from its rest point
, Period: Time taken for the motion to be completed once (T = time/cycle)
Frequency: The number of waves/cycles (completed motions) produced in a second
(f=cycle/time)
Ray: A line that indicates only the direction of motion of the wave front at any point where the
ray and wave intersect
● When a wave strikes a surface it will retain and reflect not only its energy (due to the law
of conservation of energy) but also its shape
“In Phase”: Crests and troughs of multiple waves align (In sync)
“Out of Phase”: Crests and troughs don’t align (Out of sync)
Extra Wave Information:
● The more rigid an object, the more effectively a wave travels through it
● With gasses, higher temperatures = waves travel faster (molecules move faster, so they
transfer their kinetic energy more efficiently)
● A string vibrating between two fixed points only has half a wavelength (To calculate the
total wavelength, multiply value by 2)
3 Categories of Sound Waves:
1. Audible Sound Waves:
● Within the range of sensitivity of the human ear
● Approximately 20 Hz - 20 KHz for healthy young adults
● The human ear is the most effective in detecting sound in the range of 1 KHz -
5.5 KHz
2. Infrasonic Waves:
● Have frequencies below the audible range
● Eg: Earthquake Waves
3. Ultrasonic Waves:
● Have frequencies above the audible range for humans
Speed of Sound:
● Depends on the density of air and its temperature
Mach Number:
● The ratio of the airspeed of an object to the local speed of sound
● No units
● Described using Mach 1, Mach 2 and so on…
Sound Intensity:
● Loudness describes how humans perceive sound energy and depends on sound
intensity
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