1. 12.4 Wave Interactions pp. 459 - 465 Mr. Richter

2. Agenda  Warm Up  Check and Review HW  Schedule Update:  Today- 12.4.1 and Finish Lab  Tomorrow- 12.4.2  Thursday- Problem Solving Practice  Friday- Review  Monday- Test  Notes:  Interference  Superposition  Reflections  Finish Collecting Data  Start Reading

3. Objectives: We Will Be Able To…  Apply the superposition principle.  Differentiate between constructive and destructive interference.  Predict when a reflected wave will be inverted.  Predict whether specific traveling waves will produce a standing wave.  Identify nodes and anti nodes of a standing wave.

4. Warm-Up:  Why do you think some frequencies make standing waves in the vibrating string, and other frequencies do not?

5. Wave Interference

6.  When waves collide, they occupy the same place at the same time. Think about two sounds happening at once.  This can happen because mechanical waves are not actually matter, they just displace matter.  In addition, waves pass unaffected by each other after they collide.

7. Wave Interference  The combination of two overlapping waves is called superposition.  The superposition principle: When two wave pulses collide, their amplitudes are added together to form a resultant pulse  http://www.youtube.com/watch?v=YviTr5tH8jw

8. Constructive Interference  If the displacement of the two pulses is on the same side of equilibrium, they have the same sign.  The resultant displacement (sum of the pulses) is larger than either of the original pulses. http://www.youtube.com/watch?v =YviTr5tH8jw

9. Destructive Interference  If the displacement of the two pulses are on opposite sides of equilibrium, they have opposite signs.  The resultant displacement (sum of the pulses) is smaller than both of the original pulses. http://www.youtube.com/watc h?v=URRe-hOKuMs

10. Reflections

11.  When waves reach a boundary, they are often reflected back (or in some other direction).  Like echoes. And mirrors.

12. Reflections at “Free” Boundaries  If the end of the medium is free to move, it behaves as if it is whipped back the other way.  The return wave is identical to the original pulse.  Like sound waves in a trumpet. http://www.youtube.com/watch?v=aVCq q5AkePI

13. Reflection at “Fixed” Boundaries  If the end of the medium is fixed (cannot move), then the wall exerts an opposite downward force on the medium.  The pulse wave is inverted after the reflection.  Like a plucked guitar string. http://www.youtube.com/watch?v=LTWH xZ6Jvjs

14. Wrap-Up: Did we meet our objectives?

15. Homework