How Things Work - Spring 2018

https://youtu.be/HSLxNHPVtxY

Consider the musical note G, 392 Hz.  Find the following: 1.  The frequencies of the next two G's, one and two octaves above. 2.  The frequency of the G one octave lower than 392 Hz. 3.  The frequency of G#, one semi-tone (piano key or guitar fret) above this G. 4.  The frequency of A#, 3 semi-tones above G. 5.  The wavelength of the 392 Hz sound wave, assuming that the speed of sound is 340 m/s. > 6.  Describe the Doppler Effect. 7.  An ambulance (emitting a 1000 Hz tone) approaches you and then passes you.  Describe the tone heard in these situations.  Give answers as 'greater than 1000 Hz', 'equal to 1000 Hz', or 'less than 1000 Hz.' a.  What tone does the ambulance driver hear as they approach you? b.  What tone do you hear as the ambulance approaches you? c.  What tone does the ambulance driver hear after they pass you? d.  What tone do you hear after they pass you? Note...

The Doppler Effect First, some animation: http://www.lon-capa.org/~mmp/applist/doppler/d.htm http://falstad.com/ripple/ So above, the blue dot is emitting sound and moving to the right.  Since it is moving AND emitting sound at the same time, the waves are getting closer on the right - resulting in a shorter wavelength (or higher frequency).  And it is the complete opposite on the left. http://falstad.com/ripple/ Play with this and choose the "Doppler Effect 1" example. And for some more visuals: https://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites/dl/free/0072482621/78778/Doppler_Nav.swf::Doppler%20Shift%20Interactive The key in the Doppler effect is that motion makes the "detected" or "perceived" frequencies higher or lower.  This is summarized very nicely in an equation: It's worth noting that the effect also works in reverse. If you (the detector) move toward a sound-emitter...

Tonight we will chat about the most well-known of mechanical waves:  sound. Sound is a mechanical wave, meaning that it REQUIRES a medium through which to travel.  Whereas light (or other EM waves) can travel anywhere (more or less), and travel fastest (at the speed of light) through a vacuum, sound is restricted greatly.  It can only travel through a medium, which itself carries the vibrations that are sound waves. The same characteristics previously discussed still apply:  frequency, wavelength, speed, amplitude, crest, trough.  However, sound itself is a longitudinal wave (jiggling "back and forth") rather than a transverse wave (like EM waves, which vibrate "up and down"). Animation of sound as a mechanical wave: http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html Music In western music, we use an "equal tempered (or well tempered) scale."  It has a few noteworthy characteristics; The octave is defined as a doubling (o...

Wave work Try wave problems in the set below. And if time permits, play around with this: https://phet.colorado.edu/en/simulation/wave-on-a-string Answers below. Wave questions I 1.  Differentiate between mechanical and electromagnetic waves.  Give examples. 2.  Draw a wave and identify the primary parts (wavelength, crest, trough, amplitude). 3.  Find the speed of a 500 Hz wave with a wavelength of 0.25 m. 4.  What is the frequency of a wave that travels at 24 m/s, if 3 full waves fit in a 12-m space?  (Hint:  find the wavelength first.) 5.  Show how to compute the wavelength of WTMD's signal (89.7 MHz).  Note that MHz means 'million Hz."  Recall that radio waves travel at the speed of light. - Answers: 1/2.  See notes 3.  v = f l = 500(0.25) = 125 m/s 4.  wavelength (l) = 12/3 = 4 m v = f l 24 = f (4) f = 6 Hz 5.  v = f l Speed of light = frequency x wavelength  300,000,000 = (89,700,000) l l = 300,000,000/89,7...

How things wave Introduction to WAVES. So - Waves.....   We spoke about energy.  Energy can, as it turns out, travel in waves.  In fact, you can think of a wave as a traveling disturbance, capable of carrying energy with it.  For example, light "waves" can have energy - like solar energy.  Ocean waves can certainly carry energy.   There are several wave characteristics (applicable to most conventional waves) that are useful to know: amplitude  - the "height" of the wave, from equilibrium (or direction axis of travel) to maximum position above or below crest  - peak (or highest point) of a wave trough  - valley (or lowest point) of a wave wavelength (lambda - see picture 2 above)  - the length of a complete wave, measured from crest to crest or trough to trough (or distance between any two points that are in phase - see picture 2 above).  Measured in meters (or any units of length). frequency (f...

What is energy? Energy??? I stole my energy story from the famous American physicist Richard Feynman. Here is a version adapted from his original energy story. He used the character, "Dennis the Menace." The story below is paraphrased from the original Feynman lecture on physics (in the early 1960s). Dennis the Menace Adapted from Richard Feynman Imagine Dennis has 28 blocks, which are all the same. They are absolutely indestructible and cannot be divided into pieces. His mother puts him and his 28 blocks into a room at the beginning of the day. At the end of each day, being curious, she counts them and discovers a phenomenal law. No matter what he does with the blocks, there are always 28 remaining. This continues for some time until one day she only counts 27, but with a little searching she discovers one under a rug. She realizes she must be careful to look everywhere. One day later she can only find 26. She looks everywhere in the room, but cannot find them. Then...

FYI - exams are not yet graded.  Sorry! A very useful concept in physics is Center of Gravity (AKA CM, Center of Mass - they are usually the same point).   Recall the demo with the mass on a stick.  Same mass, held at a further distance from the "fulcrum", is harder to support.  It twists your wrist more - it requires a greater "torque". So, what is torque? Torque - a "rotating" force T = F L For an object to be "in equilibrium," not only must the forces be balanced, but the torques must also be balanced. Consider a basic see-saw, initially balanced at the fulcrum:  See image below. You can have two people of different weight balanced, if their distances are adjusted accordingly:  the heavier person is closer to the fulcrum.   Mathematically, this requires that the torques be equal on both sides. Consider two people, 100 lb and 200 lb.  The 100 lb person is 3 feet from the fulcrum.  How far from the fu...