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# AP Physics 1 review of Waves and Harmonic motion

In this video David quickly explains each concept for waves and simple harmonic motion and does an example question for each one. Created by David SantoPietro.

## Want to join the conversation?

• Any tips for the AP Physics 1 exam?
• Sure, time management is just as important as knowing the content. I have found this out first hand when I ran out of time on both the AP Chemistry and AP Physics exams I wrote this year. I would highly recommend finding a practice AP exam, and timing yourself over and over until you can get well within the time allotted. Another thing that caught me off guard (which may have only been the case with my school) was when I found out I was not allowed to bring water into the AP exam room. Throughout my studying, I had constant access to water and found it quite hard to focus when having to go 1.5 hours straight without water. I would advise finding out if this is the case for your school as well, and if so, training yourself to not rely too much on water throughout your studying. Also, check to see if there is a non calculator section on the exam you are writing, as relying too much on your calculator during studying could significantly impede your abilities during a non calculator section on an AP exam. Finally, don't expect a whole lot of numerical based questions. Usually AP exams focus on practical stuff, like designing an experiment (especially written questions). Otherwise, it's mostly algebraic manipulation and comparison based questions.
• what's the difference between F=kx and F=-kx
• The difference is about direction.

the minus sign says that the RESTORING FORCE ( ie the force pulling the mass back towards the rest position) is in the OPPOSITE DIRECTION to the displacement.

This is one of the essential features of a simple harmonic oscillator
ok??
• Are transverse waves and longitudinal waves drawn in the same manner?
• They can be drawn in the same manner if we choose the Y and X variable carefully. In transverse waves, the typical sinusoidal pattern can be seen when at a fixed time, displacement of the particle from their mean is plotted on Y and the distance from the origin of the particle on X. Similarly, for an individual particle, time is plotted on X and displacement from the mean on Y. For longitudinal waves, when, say for example, the change in density from the resting density is plotted on the Y axis (with the X axis same as the distance from the origin at a fixed time) we get the same sinusoidal pattern. Or if displacement of a single particle is plotted against time. All these are just different manifestations of a motion called the Simple Harmonic Motion.
• What does it mean for a spring to be an ideal spring?
• An ideal spring will have no dampening or energy lost through non-conservative forces such as friction. An ideal spring will oscillate forever.
• Wait... I thought sound waves are considered transverse waves because sound waves are mechanical waves. Where as, In this video, the teacher considers sound waves as longitudinal waves. Am I wrong?
• Mechanical waves are waves that require a medium to travel. They can be of two types: transverse and longitudinal. Sounds waves are longitudinal waves because the medium (air, water, etc.) oscillates back and forth, parallel to the direction in which the waves travels.
Whereas making wave on a tight rope is an example of transverse wave because the medium (rope) oscillates up and down, perpendicular to the direction in which the wave travels.
• At , you say that the force exerted by an ideal spring is proportional to the amount the spring is stretched or compressed. This refers to the restorative force, right? In that case, wouldn't the equation for spring force be F=-kx?
(1 vote)
• Yes, that is Hooke's law where F is the restorative force. But when all you want to find is the force YOU need to exert a to displace the spring a certain distance, F = kx is the equation.
• Can a displacement-distance graph be made for a SINGLE particle in SHM?
(1 vote)
• if your particle is moving in one dimension, then I would say no. or, at least, it would not be useful. (You would be just plotting displacement against the positive value of the magnitude of that displacement)

If the object was moving in a circular path, for example, then you might draw a graph of vertical dispalcement against horizontal distance for example. but, again I wonder how useful this would be.

I think think most likely answer to your quesiton is 'it can but not usefully...'

:)
• This might be dumb, but I was thinking about the equation f_B = 1/ T_B at in the video.

If f_B = 0, what is T_B?

Logically seen, T_B would equal 0 as well, right? But when I tried to algebraically solve for T_B, I would get 1=0 or 1/0 and other non-nonsensical answers.
(1 vote)
• Mathematically, T = 1 / 0 or infinity or any other equivalent. You can think of zero frequency as taking an infinitely long time (so never) to reach the next cycle, but there's not a meaningful physical interpretation for infinity.
(1 vote)
• At , How do we know that the air will follow the first harmonic? How come it can't be a second or third harmonic? Because wouldn't then you get L= 3(lambda)/2 or L = 5(lambda)/2 and then get a different answer?
(1 vote)
• It can be any harmonic or many of them at the same time.
(1 vote)
• At , David notes that amplitude causes the sound to seem softer. Why is this?
(1 vote)
• It appears softer because there is less movement and therefore less displacement of the air around the wave so it is less loud. Also a higher amplitude makes it louder.
(1 vote)