1. Sep 14, 2014

### qetup

I have a big confusion. There is a question in my book which basically says that a ball is tied to a string and rotated. and it asks me to tell whether the following statement is true of false. Direction of radial acceleration MAY remain the same. This statement is true. Please explain to me a case where this is possible. I think that this is only when an object moves straight and in circular motion i think radial acceleration is always towards the centre. Is it that the frame of reference is different or is it something else?

2. Sep 14, 2014

### rcgldr

3. Sep 14, 2014

### qetup

4. Sep 14, 2014

### Staff: Mentor

Which book and where? It's going to be tough explaining what it's saying if we don't know ourselves what it's saying.

5. Sep 14, 2014

### qetup

Naming the book wont help. it is in the form of a question which says-
a ball is tied to a string and rotated.the following statement is true of false- Direction of radial acceleration MAY remain the same?
And according to the book it is true.

6. Sep 14, 2014

### sophiecentaur

So that implies there is more than just a question. it's reasonable to ask for an actual quote / extract or the name of the book.
Could the question be asking you to consider the frame of reference?

7. Sep 14, 2014

### qetup

no there is only one question -a ball is tied to a string and rotated.the following statement is true or false- Direction of radial acceleration MAY remain the same?
yes it may ask me to do that but according to you is this ever possible? because the question says it "may" be possible.
this book is for iit jee so any wild idea would do.

8. Sep 14, 2014

### sophiecentaur

The question does not say it May be Possible, at all. It asks IFFFFFF, which is an entirely different thing. What is your thought on the matter? How does 'the statement' square with the idea of rotation? There is no point in my just telling you my opinion - that isn't the PF way. We make people (and each other) work for our answers. Can you think of a situation when you have circular motion without the direction of the string changing?

Sorry; I have no idea what iit jee means.

9. Sep 14, 2014

### qetup

Thanks for the hint. is it when i revolve a ball attached to a string round and round in 1-d by spining the string?
also can this be possible when i rotate a ball in 2-d attached to a string and supposing a butterfly is facing the object which rotates always so that there is no relative acceleration. so the radial acceleration will be at one direction only?
iit jee is just a prestigious exam in india for getting through the best engineering colleges.

10. Sep 14, 2014

### sophiecentaur

Basically and imo the question is not a good one. You always wonder about the author of such smartypants questions in an elementary book.
You seem a bit confused with your terminology about 1D and 2D but I think you are in fact considering the reference (or observation) frame, as I suggested. You cannot have rotation in just 1D. There is only one direction involved - but that's obvious, ain't it.
Must go out and make the dinner now.

11. Sep 14, 2014

### qetup

please give me another hint. isnt the example i gave you about the butterfly...correct?
relative to the butterfly the direction of radial acceleration doesnt change

12. Sep 14, 2014

### qetup

if the string is at rest and nothing is moving then there is no radial acceleration but that is a stupid answer according to me. what if we pull the string with the object towards ourselves. isnt that only one direction? and the radial acceleration is also towards us?

13. Sep 14, 2014

### voko

Have you ever been on a carousel?

14. Sep 14, 2014

### qetup

lol yes i have and the radial acceleration changes wrt the ground and not wrt me like my earlier example.so are all these examples discussed in this thread correct?

Last edited: Sep 14, 2014
15. Sep 14, 2014

### voko

Exactly.

There is only one, which is about a butterfly. It is not very clearly stated, but it probably also means that the butterfly is in a rotating reference frame.

16. Sep 14, 2014

### qetup

yes i meant that only. thanks a lot.

17. Sep 14, 2014

### qetup

also what if we pull the string with the object towards ourselves. isnt that only one direction? and the radial acceleration is also towards us? is this correct also?

18. Sep 14, 2014

### voko

"Towards ourselves" is not specific enough. You need to think in terms of a reference frame, not just a single point. Take a bug sitting on a slowly rotating wheel. The bug's reference frame is naturally the wheel itself. In that reference frame, the velocity and the acceleration of the bug is zero. For you, however, that reference frame is not natural; your reference frame is the Earth, and for you the bug's velocity and acceleration are not zero. And even if you can say that the bug's acceleration is "towards the center of the wheel", that direction is not constant.

19. Sep 14, 2014

### qetup

ok let me try again. if i am sitting on a table and pulling a string at the end of which is a ball. now another person sitting next to me will see that the radial acceleration is towards me wrt him right ? so in this case the radial acceleration doesnt change?

20. Sep 14, 2014

### voko

I do not think it is a good explanation. The crucial point is that we always use reference frames to specify directions. A person next to you is not a reference frame; the person is a point, not a frame.