8.1: What is a Vibration?

Recall: The Particle Theory of Matter All matter is made of _______________ Particles are constantly ______________ As temperature increases, particles move __________ As temperature increases, distance between particles _______________ What state of matter does each of these images represent?

Vibration Imagine a pendulum: Point #2 represents the equilibrium position. Points #1 and #3 are equal distances away from equilibrium. vibration: the cyclical motion of an object about an equilibrium point (one back and forth motion of the pendulum)

What’s happening to the energy of the bob as it goes through each vibration(i.e. goes through a cycle)? What if you were looking at the motion of particles instead?

Mechanical Wave Transfer of energy through a material due to vibration The material is called a medium Can be solid, liquid, or gas How long a vibration can continue in a medium depends on how much energy is gained/lost by the medium in the vibration. (usually very little)

Net Motion The displacement of a particle over a certain time interval. The difference between the particle’s initial and final positions Net motion of particles causes the vibration Ideally, net motion is zero once vibration has stopped. Therefore no work is done and no energy is lost by the vibration

Particle Behaviour in Different Media All vibrations need a medium to transfer the waves. A medium’s effectiveness at transmitting vibrations depends on its molecular and mechanical structure density temperature Speed and distance a wave can travel depends on the composition of the medium. more rigid = faster waves that travel farther less rigid = disperse more energy thus reducing speed and distance

Particle Behaviour in Different Media Solids Atoms held together in crystal formation so they can only vibrate slightly Elastic: medium returns to its original shape after being disturbed Most solids have this property Rigid materials transfer energy more efficiently than non-rigid materials

Particle Behaviour in Different Media Fluids Liquid molecules are in contact, so they are still very effective transmitters of waves Gases have the lowest particle density, so they are the least effective wave transmitters Gases rely on translational motion: The straight-line motion of a molecule

Types of Mechanical Waves Transverse Waves: particles vibrate perpendicular to the direction of energy flow. Eg. Guitar string: energy flows down the string but motion is back and forth from the finger/pick plucking the string

Longitudinal Wave: particles vibrate parallel to the direction of energy flow. Eg. slinky

Parts of a Longitudinal Wave Compressions: regions where particles in a wave are close together (high pressure in gasses) Rarefactions: regions where particles in a wave are further apart (low pressure in gasses)

Sound Longitudinal waves produce sound: Energy produced by rapidly vibrating objects that are detectable by the ear Sound (and other forms of energy, too) is transmitted ONLY as a longitudinal wave in fluids, but can be either transverse or longitudinal in solids.

What Type of Wave? A complex wave – combination of both types due to wind effects near the surface

Wave Characteristics f = 1/T and T = 1/f Time-based characteristics: Cycle One complete vibration = one cycle Frequency (f) Number of cycles per second Units: 1/s = hertz (Hz) Period (T) Time required for one cycle f = 1/T and T = 1/f

Wave Characteristics Amplitude Maximum distance from equilibrium Wavelength (λ) Distance between midpoints of two crests, or midpoints of two troughs

Wave Characteristics Phase The x-coordinate of a particle When the waveform has been shifted along the x-axis, this is a phase shift Particles are considered to be in phase when: They have the same amplitude They are vibrating in the same direction They have the same phase shift SHOW WAVE INTERACTION IN MY NELSON

Homework From textbook: 8.2 Practice Questions # 1-6, 8