Physics test prep

Question

Derive velocity of forward propagation = wavelength x frequency, probably deriving it from the formula for angular motion.

2 speakers, 1 behind the other, same hertz. Someone walking towards it, a distance of 12 meters walking for 6 seconds, find the points of constructive interference.

On an alien planet, figure out what acceleration from gravity is. String is hanging. Given length and density. There’s a mass hanging, weight given. Figure out the acceleration from gravity. Density is negligible compared to the mass of the string. The velocity of the wave through the string is given.

They are pretty vague because he didn't want to give us the exact questions but that's what I've got. Could someone help me understand how I should approach those problems on the test so when we have numbers or more specific questions I'll be able to get through them?

Thanks!

Answer 1

I'm not too sure about the first 2 (barely able to understand the scenario or what is being asked and may need more info), but I think I got the 3rd one, though there are 2 different set ups that come to mind.

For the most part, the easiest way is to determine the acceleration of gravity of the planet based on the question would be to set up a pendulum and find the period. Now, there are some things that may change the approach (depending on the information provided). For example, if the mass of the hanging weight is much greater than the strings mass, you should be able to just do the basic T = 2π√L/g. However, if the mass has noticeable weight (compared to the hanging weight), you may need to use the physical pendulum equation T = 2π√I_support/(mgL_cm).

Unfortunately, I'm not too sure what the line "density is negligible to the mass of the string"means. However, I'm not sure where the velocity of a wave would play into it. It's possible that what they want you to do instead is to set up a standing wave system (string placed horizontally above a table, held at two ends, with a mass hanging over the side to provide tension) and then creating a standing wave and use the equation v =√F_T/μ.