Download - Ch 14 web - Rock Creek USD 323
Ways to transport Energy
• Particles– Throw a ball, it is a particle that moved and
carried energy with it
• What about a wave through the slinky? Does it matter physically move to a new spot?
Wave Properties
• Waves carry energy, but not matter– Throwing a ball carries energy and matter
– Waves carry the energy of vibration, but no matter
• Like sound waves
• Wave –– A rhythmic disturbance that carries energy
through matter or space
Mechanical Waves• Require a medium (something to travel
through)– Water, air, ropes, and springs
• Wave Pulse –– A single bump or disturbance that travels through
a medium
• Continuous Wave –– Multiple disturbances
Longitudinal Waves
• A wave that vibrates in the same direction or parallel to the direction of motion– Sound waves
– Fluids such as liquids and gases
Surface Waves
• A.k.a. – water waves
• Have characteristics of both transverse and longitudinal waves– Move parallel and perpendicular to the motion
Measuring a Wave
• Speed– Can be found just like any other moving object
• Distance traveled over time it took to travel
• Must use same part of wave to measure displacement
Measuring a Wave
• Wavelength (λ)– Shortest distance between two points where a
wave repeats itself
– Trough – low points
– Crest – high points
Measuring a Wave
• Period and frequency can only be measured for continuous waves
• Period (T) –– Time it takes to complete one wavelength
Measuring a Wave
• Frequency (f) –– Number of complete oscillations in each second
• Measured in Hertz (Hz)
1fT
Example
• A sound wave has a frequency of 262 Hz and a wavelength measured at 1.29m.– What is the speed of the wave?
– How long will it take the wave to travel the length of a football field, 91.4 m?
– What is the period of the wave?
Activity for Properties of a Wave
• Stretch out the coiled spring 2 or 3 meters• Send a longitudinal pulse down the spring
– What time did it take the pulse to reach the end?
• Increase the amplitude by pulling back more coils– What time did it take the pulse to reach the end?
• Stretch the spring to a different length and send a pulse– What time did it take the pulse to reach the end?
• What can you conclude?
Waves at Boundaries
• Speed depends only on properties of the medium
• Incident wave –– Wave that strikes a boundary
Waves at Boundaries
• Reflected wave –– Wave that is reflected back or returns
• Continuing wave is erect (upwards)
• Reflected wave can be– Erect – if beginning material is “thicker”
– Inverted – if beginning material is “thinner”
Nodes and Antinodes
• Node – location where displacement is zero
• Antinode –location where displacement is the greatest
Waves in Two Dimensions
• Law of reflection –– Angle of incidence is equal to the angle of
reflection
• Refraction –– The change in the direction of waves at the
boundary between two media
– Or the wave that goes through to the new boundary