1. 8.1: What is a Vibration?

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. 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)

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 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)

6. 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

7. 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

8. 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

9. 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

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 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.

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:
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

16. Wave Characteristics Amplitude Maximum distance from equilibrium
Wavelength (λ)
Distance between midpoints of
two 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 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

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