Solve Physics Exam Confusion: Sound Waves & Calorimetry

[NutriGrainKiller]

My physics professor attempted to teach two chapters of new material within two class periods, but unfortunately this is not enough time to cover the material as in depth as we will be tested on. Exam is this mon and I am very confused about the things he did not explain - I feel like he left some stuff out. The chapters are sound waves and calorimetry. I am going to list the homework problems I don't understand how to do and how far I got.

1

At a temperature of 27.0C, what is the speed of longitudinal waves in hydrogen (molar mass 2.02 g/mol)? The ratio of heat capacities for hydrogen is 1.41.

I'm guessing I should use Q=MC(Tf-Ti), but Q is the change in thermal energy, how does that relate?

2

Two sinusoidal sound waves with frequencies 108 Hz and 114 Hz arrive at your ear simultaneously. Each wave has an amplitude of 1.70×10−8 m when it reaches your ear.
a)What is the maximum amplitude of the total sound wave?
b)What is the minimum amplitude?

here I know I've got to add the amplitudes. But here is where I am confused - when it says arrives simultaneously, does it imply in phase? Waves with different frequencies can't travel in phase but can overlap temporarily (forming beat frequency)

3

A swimming duck paddles the water with its feet once per time interval of 1.30 s, producing surface waves with this period. The duck is moving at constant speed in a pond where the speed of surface waves is 0.380 m/s, and the crests of the waves ahead of the duck have a spacing of 0.150 m.
a)What is the duck's speed?
b)How far apart are the crests behind the duck?

In part a the unknown is Vemit. I should use the following equation: Fdet=Femit[(Vwave +- Vdet)/(Vwave -+ Vemit)]...right? How can I view this problem from the perspective of a detector?

Any and all pointers are greatly appreciated. Thanks guys

 

[Tide]

1. The speed of sound depends on the ratio of specific heats. I'm sure the relation is shown in your textbook.

 

[quicknote]

Hello,

I understand your frustration with the limited time to cover these two chapters in depth. It can be difficult to fully grasp and apply concepts when they are not thoroughly explained. Here are some tips that may help you with these problems:

1. For the first problem, you are correct in thinking that the equation Q=MC(Tf-Ti) is relevant. However, in this case, we are not dealing with a change in thermal energy (Q), but rather the speed of sound waves (V). The equation you need to use is V = √(γRT/M), where γ is the ratio of heat capacities (1.41 in this case), R is the gas constant, and T is the temperature in Kelvin. This equation relates the speed of sound to the temperature and molar mass of the gas. You can then solve for V using the given information.

2. For the second problem, you are correct in thinking that you need to add the amplitudes of the two waves. Since both waves have the same amplitude (1.70×10−8 m), the maximum amplitude of the total sound wave will be 2(1.70×10−8 m), or 3.40×10−8 m. The minimum amplitude will be 0 m, since the waves will either cancel each other out or add constructively, depending on their relative phase. The phrase "arrives simultaneously" does imply that the waves are in phase.

3. For the third problem, you are on the right track in using the equation Fdet=Femit[(Vwave +- Vdet)/(Vwave -+ Vemit)]. However, you do not need to solve for Vemit. Instead, you can use the given information to solve for the duck's speed (Vdet) and the wavelength (λ) of the waves. Once you have those values, you can use the equation V = fλ to find the speed of the waves (Vwave) and then use that in the original equation to solve for Vemit.

I hope these tips help you with your confusion and give you a better understanding of these concepts. Remember, it's always a good idea to ask your professor for clarification if you are still unsure about any of the material. Good luck on your exam!