Physics 1005 Week 3 (1/31)
Summary - Vibrations, Waves
Before we began discussing waves we talked about the related phenomenon of vibrations, or oscillations. A vibration is simply some back and forth motion, like a pendulum, or a mass on a spring. The following points were made about a mass/spring oscillating system:
The period, T, is defined as the time it takes to complete 1 cycle, where 1 cycle involves the mass returning to the same position (for example the time to start at the bottom, go up, and then all the way back down again. It is NOT the time to go from the bottom to the top. That is ½ cycle)
The frequency, f, is the number of cycles the mass makes per second.
The relationship between the frequency and the period is given by T = 1/f) which is the same
mathematically as f = 1/T
Amplitude is the distance the mass travels between its maximum (or minimum) position and its equilibrium position.
I demonstrated that the frequency of a particular mass/spring system did not depend on how much you displaced the mass from its equilibrium position before letting go (the amplitude). If you pulled the mass down a little or a lot, the frequency was still the same. This points to the fact that all objects have a natural frequency.
All objects have a "natural frequency". Most things are complicated systems that have a combination of natural frequencies, but for a simple object like a pendulum or a mass-spring system there is just one natural frequency.
We discussed resonance: If you apply a periodic force (push) to an oscillating system, in phase, at the natural frequency, the amplitude will get larger and larger. This condition is known as resonance. To illustrate this, we talked about a child on a swing. The child must pump his/her/their legs at the right frequency to make the amplitude increase - it doesn't work to just pump legs randomly.
When as system is resonating, the amplitude of the vibrations can become extremely large and systems can go out of control - like the bridge we saw destroyed in the video.
Waves are vibrations that move. Most waves begin with the disturbance of some material.
A wave is something that transports energy without any accompanying matter.
Sometimes a wave, or pulse, is a one-time disturbance, like the clap of hands or a single pulse traveling on a rope. Other times waves keep going from multiple disturbances. In this case a wave train, or a continuous wave, is formed.
The image below shows a wave pulse traveling on a rope. The rope itself only moves up and down, but the wave is moving to the right, so the medium moves, but not along with the wave. The wave shown is called a transverse wave because the medium vibrates perpendicular to the direction of the wave. That is, the rope
only moves up and down while the direction of the wave pulse is to the right.
Pulse
•
In a longitudinal wave the medium vibrates back and forth in the same direction as the wave. In class I demonstrated longitudinal waves on the long spring, like in the image below. The alternating compressions and rarefactions travel along the spring creating the wave. But again, although the metal of the spring moves back and forth locally, the material itself does not actually travel along with the wave. A metal segment of the spring that was near the middle doesn't end up at the end of the spring!
It's the wave that moves to the right and carries energy, not the medium or material that the wave is traveling
Knowt
Note
Studied by 2819 people
