Physics circular motion help

[XanthePandy]

No idea where to put this. There should be a homework/study thread.

I have an exam coming up next week for my A-levels and am having difficulty with one question to do with circular motion.

I'd really appreciate some help, I have the answer but don't seem to be getting it.

1) A ceiling fan is turning at a rate of 100 revolutions per minute. A spider is clinging to a blade of the fan. If the spider experiences a centripetal acceleration greater than 0.3g, it will lose its grip on the blade and be flung off. How far from the centre of the fan can the spider safely go?

My process was:
(100 / 60) x 2pi to get ~10.5rad s^-1
rearranging a couple equations to get r = a / w^2
and that keeps getting me 0.027m instead of the answer of 1.05m that it has in the back of the book.

I'm sorry for making a whole new thread for one question but I'd love to figure it out before falling asleep and nobody I know is up for doing physics on a Friday night.

[Silversonic_11]

No idea where to put this. There should be a homework/study thread.

I have an exam coming up next week for my A-levels and am having difficulty with one question to do with circular motion.

I'd really appreciate some help, I have the answer but don't seem to be getting it.

1) A ceiling fan is turning at a rate of 100 revolutions per minute. A spider is clinging to a blade of the fan. If the spider experiences a centripetal acceleration greater than 0.3g, it will lose its grip on the blade and be flung off. How far from the centre of the fan can the spider safely go?

My process was:
(100 / 60) x 2pi to get ~10.5rad s^-1
rearranging a couple equations to get r = a / w^2
and that keeps getting me 0.027m instead of the answer of 1.05m that it has in the back of the book.

I'm sorry for making a whole new thread for one question but I'd love to figure it out before falling asleep and nobody I know is up for doing physics on a Friday night.

I'm doing this same exam next week, and I know the book you got the question from...all I can tell you is that the answer in the book is extremely wrong. I was also mystified by this question for a long time before I could go in to school and confirm it with a few of my teachers.

Edit: That book makes numerous mistakes, although I believe that's the most spectacular one.

[XanthePandy]

I'm doing this same exam next week, and I know the book you got the question from...all I can tell you is that the answer in the book is extremely wrong. I was also mystified by this question for a long time before I could go in to school and confirm it with a few of my teachers.

Edit: That book makes numerous mistakes, although I believe that's the most spectacular one.

Thank god. I was worried my mind was just not working for physics any more.

Is it possible you could tell me of any mistakes you remember? If not, don't worry and I'll just ignore any I'm having too hard a time answering.

[Silversonic_11]

Thank god. I was worried my mind was just not working for physics any more.

Is it possible you could tell me of any mistakes you remember? If not, don't worry and I'll just ignore any I'm having too hard a time answering.

I've only used it for half the course so far, so I'm not aware of the mistakes for Unit 5 (yet). I did note down where mistakes were, but not all of them.

These are only the few I marked;

Page 15 Question 1) I seem to get double the answer they have in the back (they have 2500N, I get 5000N). I'm pretty sure I confirmed that one to be wrong.

Page 15 Question 2c) Answer is 918N, but they round down to 910N in the back or something.

Page 18 Question 1) You'll get 650Ns, but they obviously have some dodgy rounding in the answers available.

Page 35, Q 2b) The answer they give in the back says to make it a conserved equation you have a electron neutrino, although I'm pretty sure you need an anti-electron neutrino.

Page 41, worked example part b) It says particle C is negatively charged. I'm certain it's positive.

Those are the only ones I've marked, but I'm certain there may have been a few others.


Do you visit http://www.thestudentroom.co.uk/forums? - It's a very useful (got nothing against BG) forum if you've got problems with A level questions or general queries, since there are dozens of people on there doing the same course.

Also, there's also a topic on there that'll link you to a document containing 200 past paper questions relating to unit 4 that some guy has put together. Since I've found that there's no previous material for physics with edexcel to test you on, that's a pretty good way to revise. Let me know if you need it.

[XanthePandy]

Thank you very much. I avoided TSR before but I'll have a look.

[Max™]

Also: http://www.physicsforums.com/forumdisplay.php?f=154

http://25.media.tumblr.com/tumblr_kz...xrh5o1_400.png

[Talus]

I think it's fairly common for there to be errors in student solutions manuals/instructor solution manuals. Whether they're intentional or not is debatable, the main reason to include them I assume is to protect students against themselves (i.e. just copying a complete solution to a physics problem instead of learning how to solve problems, aka cheating).