# Homework Help: What do I need to learn in order to describe this?

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1. Feb 2, 2015

### Reflection

Hi!

I'm new here, so I apologize on beforehand for any mistakes I may make (grammar/wrong thread etc.).
Currently we are doing a project for our exam in my class, and in my group we chose to focus on the following scenario:

On a spinning "plate" (you know, like a merry-go-around... sorry, I'm not sure I can describe it better in English, but I think you get what I mean) we follow the movement of an object. Before the plate starts spinning, the object is placed on it in centrum of the plate (which has a shape like a circle). When the plate starts spinning fast enough, the objects should come off at some point.

We are now meant to describe the physics of this scenario and we are to come up with the experiments to validate the theories ourselves (why/how does it go off etc.). So any help to get us started would be appreciated (what should we describe/focus on? How can we validate it with an experiment?). Have someone done this before, maybe? Any sources would be much appreciated.
I suppose topics like friction, centripetal force, rotation -energy, -mechanics (Newton's laws) and -forces are relevant, and I guess I would also like to include or merely mention centrifugal force and relativity very shortly (just to show them I know about it =)).

Our teacher suggested describing the way the object comes off the spinning plate and why and how it goes that way. Also, I thought about calculating the time it would take it to go off or something like that, but I'm not sure how we should validate this with an experiment, so this is where we would like a little help too.

I hope I was clear enough, else I would gladly try to clarify.
I'm sorry but I wasn't really able to follow the template, because it is not a specific task... I hope it's okay, but please correct me if I'm doing this wrong.

2. Feb 2, 2015

### haruspex

OK, but practice on this forum. How would you discuss centrifugal force?
I feel it would be beyond the scope of the exercise to say anything of significance here (and get it right).
That could be a tough ask. But you could certainly try to develop the differential equations for the motion.

3. Feb 2, 2015

### BvU

Hello Reflection (noitcelfeR ?), welcome to PF :)

Excellent subject.
I have a suggestion: starting in the centre of the turntable is a bit awkward. You can imagine that the object takes off in a random direction, because it has no preference for a specific one. And then things go too fast to observe what happens.

As an alternative, you could set up some experiments with a newton-meter (i.e. a spring with a scale) to measure how hard objects have to be pulled inwards to stay in place, as a function of rotation speed, distance from the centre, object mass, temperature, humidity ;)

Have fun!

And you can bluff your way into special relativity by casually observing that "in view of the fact that $v << c$ we do not have to take relativistic effects into account" !

4. Feb 3, 2015

### Reflection

I would like to speak of the difference between centripetal and centrifugal force and how a person on ground is seeing things differently than the object on the plate would. I read about it on this site: http://staff.washington.edu/aganse/blog/files/centrip.html [Broken]

Heh, maybe you are right :-) But if I could just mention why people often talk about relativity when they talk about centrifugal force (like it does on the website), I think it would be great.

My teacher said so too, but if we could make it work it would certainly give os an advantage. Do you have any suggestions as to how we could calculate this (also validate it experimentally?), because I'm a bit unsure where to start, I just know we need to take friction (when the object is still and moving), centripetal force, the plates radius and the objects mass into consideration.

Last edited by a moderator: May 7, 2017
5. Feb 3, 2015

### Reflection

Thank you :)

It is a interesting subject, yes. I think you are right about that of the centre.

Thank you much for your suggestion, I have thought about that one too. However, I am a bit unsure of how I should place the newton-meter in order not to slow down the plate or something? Because I need to get the data to my computer while it is spinning, so it means there is a wire I need to take into consideration.

Thank you, I will definitely find a way to juust mention it somehow ;)