Hi, for the first part of question 21 I made the equation 2piR/T where T is the time period of the orbit. Obviously this is really simple but I haven't covered the equations you used in physics yet so I didn't know what else to put. What marks would I get for putting this answer and if it is incorrect why is it wrong? Thanks
An interesting question. I don't know what marks you would be given but, if I was marking it, I'd give you both marks. Each of our answers includes something not mentioned in the question (M or T). It's not like T would be difficult to measure, so I don't see it being a problem. My answer is mostly a reflex response to seeing a circular motion question and the word "derive", which usually means doing this! Just to add that I don't particularly like this question - it gives an initial impression of being a real-world situation (which provokes various intuitions about how the world works) but it is really deceiving us.
Hi, for Q19 i thought that when sound waves reflect off a surface they are phase shifted by 180 degrees. Wouldnt this mean that the reflected waves would have to travel another half a wavelength (in addition to a wavelength) in order to be in phase again?
My recollection is that sound waves experience a phase shift when going from solid to air but not air to solid. It's easy to lose track of when phase changes happen, so I could be wrong. In this question, I assumed no phase shift because when there is a phase shift PAT questions usually say to ignore it. The fact they haven't said to ignore it made me think there isn't anything to ignore, if that makes sense. It's worth checking, though, because I may have got it wrong.
