Transmission & Reflections. Consider a pulse moving through a medium - perhaps through a rope or a slinky. What happens to the pulse when it reaches the end of that medium? (Assume the end is fixed). `.
(Assume the end is fixed)
The pulse reflects off the free end and returns with the same direction of displacement. That is, a pulse with an upward displacement will reflect off the end and return with an upward displacement.
There is also no change in the speed of the pulse since the medium is the same.
(More dense to less dense Medium)
The fast medium acts like a free end and the reflected wave is not inverted. The transmitted wave is not inverted, travels with increased speed and wavelength and has a diminished amplitude.
Wave interference occurs when two waves act simultaneously on the particles of a medium.
There are two types of interference: constructive and destructive.
Destructive interference occurs when a crest meets a trough.
Constructive interference occurs when crests meet crests (supercrests) or troughs meet troughs (supertroughs).
Waves occupy same space
Waves occupy same space
Homework: Superposition worksheet
p. 222 1-3 (just sketch)
Resulting Wave Pattern
Remember this pattern only appears for an instant!
Resonance is the response of an object that is free to vibrate to a periodic force with the same frequency as the natural frequency of the object.
Therefore resonance is also a transfer of energy from one object to another having the same natural frequency.
If the two objects are touching, it is known as mechanical resonance.
A swing’s natural frequency will depend on the length of the chains.
A window rattles with its natural frequency.
Bridges, propellers, blades, turbines, glasses and many types of equipment all have a natural frequency.
Read p. 223-224 for examples.
When an object vibrates in resonance with another, it is called a sympathetic vibration.
The amplitude and the wavelength of interfering waves are often different.
However if the conditions are such that two waves have the same amplitude and wavelength and travel in opposite directions, then a special interference pattern known as a standing wave occurs.
Node (N): point that remains at rest
Antinode: point midway between nodes where maximum constructive interference occurs
f = 30 Hz
λ = ?
v = ?
Distance between successive nodes is ½ λ
½ λ = 10 cm
λ = 20 cm
v = f λ
v = (30 Hz)(0.20m)
v = 6.0 m/s