# Penny sits on top of a frictionless sphere, please

#### malta

Penny sits on top of a frictionless sphere, urgent please

1. Homework Statement

At the top of a frictionless sphere of radius R a penny is given a push to speed x. At what angle, measured from the vertical does the penny leave the surface?

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#### LowlyPion

Homework Helper
Re: Penny sits on top of a frictionless sphere, urgent please

Welcome to PF.

What considerations do you think need to be made?

#### sArGe99

Re: Penny sits on top of a frictionless sphere, urgent please

Normal reaction = 0 when the block has left the surface. Did you try that out?

#### malta

Re: Penny sits on top of a frictionless sphere, urgent please

I have no clue how to start this problem, I look at a similar problem here (https://www.physicsforums.com/showthread.php?t=260338) but i could not figure out what the last post tried to say. This is going to be a problem on my test tomorrow and I need to figure it out. Do any of you guys have aim or msn?

#### LowlyPion

Homework Helper
Re: Penny sits on top of a frictionless sphere, urgent please

Well admittedly that was a brilliantly constructed suggested direction to go in solving the problem.

Maybe you should consider using some of that to figure it out?

Doing homework through other venues is not something encouraged here. And if that is too subtle, it's just not permitted, even through PM.

#### malta

Re: Penny sits on top of a frictionless sphere, urgent please

The thing is that i dont care much for the answer, I want help on starting the problem so I can work it on my own. Would you mind pointing me in the right direction please?

#### sArGe99

Re: Penny sits on top of a frictionless sphere, urgent please

You have to work on it. If you don't, then what is the point in doing homework? You should at least try.

#### LowlyPion

Homework Helper
Re: Penny sits on top of a frictionless sphere, urgent please

The thing is that i dont care much for the answer, I want help on starting the problem so I can work it on my own. Would you mind pointing me in the right direction please?
Well ... I'd start with that even if I did suggest it myself.

#### malta

Re: Penny sits on top of a frictionless sphere, urgent please

I have been working on it, I've been stuck on the damn problem for 3 days now, I know that energy and momentum are conserved, but I cant seem to translate that into actual equations : /

Edit: ok i think im getting somewhere.

I know that the point where the particle leaves is = 0, so mgcosx-ma=0 right?

So Cosx = a/g

Also Etot = Ekin + Epot= .5mv^2 - mgh= 0
=====> v^2= 2gh

But im stuck in a loop now haha

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#### malta

Re: Penny sits on top of a frictionless sphere, urgent please

Am I getting closer to figuring this out? haha

#### LowlyPion

Homework Helper
Re: Penny sits on top of a frictionless sphere, urgent please

I have been working on it, I've been stuck on the damn problem for 3 days now, I know that energy and momentum are conserved, but I cant seem to translate that into actual equations : /

Edit: ok i think im getting somewhere.

I know that the point where the particle leaves is = 0, so mgcosx-ma=0 right?

So Cosx = a/g

Also Etot = Ekin + Epot= .5mv^2 - mgh= 0
=====> v^2= 2gh

But im stuck in a loop now haha
OK it's good knowing that momentum is conserved and perhaps you will need that elsewhere on your exam. But ... not on this problem.

Now the mgCosθ term is the weight component of gravity. But isn't what you are interested in balancing the outward centripetal acceleration? mv2/R ?

#### malta

Re: Penny sits on top of a frictionless sphere, urgent please

Ok so this is independent of the mass of the penny and of g. So would i have

mgcosx= mgh + mv^2/R??

#### LowlyPion

Homework Helper
Re: Penny sits on top of a frictionless sphere, urgent please

Ok so this is independent of the mass of the penny and of g. So would i have

mgcosx= mgh + mv^2/R??
Part of the problem with that equation is that 2 of those terms are Force, the other energy.

#### malta

Re: Penny sits on top of a frictionless sphere, urgent please

Gotcha so it would only be

mgcosx= mv^2/R? But what would i solve for?

#### LowlyPion

Homework Helper
Re: Penny sits on top of a frictionless sphere, urgent please

Gotcha so it would only be

mgcosx= mv^2/R? But what would i solve for?
Since V is, as you found, a function of h, and so is θ ...

#### malta

Re: Penny sits on top of a frictionless sphere, urgent please

Alright, so since v^2 = 2gh you substitute and get Cosx= 2h/r. But how is theta a function of h??

#### LowlyPion

Homework Helper
Re: Penny sits on top of a frictionless sphere, urgent please

... Cosx= 2h/r. But how is theta a function of h??
Even better then eh?

Looks like you can do a lot of substituting.

#### malta

Re: Penny sits on top of a frictionless sphere, urgent please

Yeah but with what? what can i substitute cosx with?

#### LowlyPion

Homework Helper
Re: Penny sits on top of a frictionless sphere, urgent please

Yeah but with what? what can i substitute cosx with?
For one thing it's not 2h/r.

But if you make a drawing you can figure out what Cosθ is in terms of R and h.

#### malta

Re: Penny sits on top of a frictionless sphere, urgent please

Ok lets see now, so Cosx= a/g

a= v^2/R

So Cosx= a/g = v^2/ Rg = 2h/R

Ok and Cosx=R-h/R

So R-h/R = 2h/R

thats what I could figure out, but i need to figure out the angle when it drops off the sphere

#### malta

Re: Penny sits on top of a frictionless sphere, urgent please

anyone?

#### LowlyPion

Homework Helper
Re: Penny sits on top of a frictionless sphere, urgent please

Ok lets see now, so Cosx= a/g

a= v^2/R

So Cosx= a/g = v^2/ Rg = 2h/R

Ok and Cosx=R-h/R

So R-h/R = 2h/R

thats what I could figure out, but i need to figure out the angle when it drops off the sphere
That's almost right.

Cosθ is what you're solving for right? So don't you need to be eliminating h and not Cosθ?

And isn't Cosθ really = (R-h)/R ?

#### malta

Re: Penny sits on top of a frictionless sphere, urgent please

Oh ok yeah im solving for Cosx=(R-h)/R
So wouldnt the angle be arcCos of (R-h)/R?

#### LowlyPion

Homework Helper
Re: Penny sits on top of a frictionless sphere, urgent please

Oh ok yeah im solving for Cosx=(R-h)/R
So wouldnt the angle be arcCos of (R-h)/R?
Yes. But you need to eliminate h. Or eliminating R works too.

Cosθ is just a dimensionless ratio.

#### malta

Re: Penny sits on top of a frictionless sphere, urgent please

How would I go by eliminating either h or R? Also would you mind looking at my other post about static friction, its driving me nuts trying to figure it out

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