2 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?
3 VibrationImagine 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)
4 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?
5 Mechanical Wave Transfer of energy through a material due to vibration The material is called a mediumCan be solid, liquid, or gasHow 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)
6 Net MotionThe displacement of a particle over a certain time interval. The difference between the particle’s initial and final positionsNet motion of particles causes the vibrationIdeally, net motion is zero once vibration has stopped. Therefore no work is done and no energy is lost by the vibration
7 Particle Behaviour in Different Media All vibrations need a medium to transfer the waves.A medium’s effectiveness at transmitting vibrations depends on itsmolecular and mechanical structuredensitytemperatureSpeed and distance a wave can travel depends on the composition of the medium.more rigid = faster waves that travel fartherless rigid = disperse more energy thus reducing speed and distance
8 Particle Behaviour in Different Media SolidsAtoms held together in crystal formation so they can only vibrate slightlyElastic: medium returns to its original shape after being disturbedMost solids have this propertyRigid materials transfer energy moreefficiently than non-rigid materials
9 Particle Behaviour in Different Media FluidsLiquid molecules are in contact, so they are still very effective transmitters of wavesGases have the lowest particle density, so they are the least effective wave transmittersGases rely on translational motion:The straight-line motion of a molecule
10 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
11 Longitudinal Wave: particles vibrate parallel to the direction of energy flow. Eg. slinky
12 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)
13 Sound Longitudinal waves produce sound: Energy produced by rapidly vibrating objects that are detectable by the earSound (and other forms of energy, too) is transmitted ONLY as a longitudinal wave in fluids, but can be either transverse or longitudinal in solids.
14 What Type of Wave?A complex wave – combination of both types due to wind effects near the surface
15 Wave Characteristics f = 1/T and T = 1/f Time-based characteristics: CycleOne complete vibration = one cycleFrequency (f)Number of cycles per secondUnits: 1/s = hertz (Hz)Period (T)Time required for one cyclef = 1/T and T = 1/f
16 Wave Characteristics Amplitude Maximum distance from equilibrium Wavelength (λ)Distance between midpoints oftwo crests, or midpoints of two troughs
17 Wave Characteristics Phase The x-coordinate of a particle When the waveform has been shifted along the x-axis, this is a phase shiftParticles are considered to be in phase when:They have the same amplitudeThey are vibrating in the same directionThey have the same phase shiftSHOW WAVE INTERACTION IN MY NELSON
18 Homework From textbook: 8.2 Practice Questions # 1-6, 8