# Penny sits on top of a frictionless sphere, please

Penny sits on top of a frictionless sphere, urgent please

## 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?

LowlyPion
Homework Helper

Welcome to PF.

What considerations do you think need to be made?

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

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

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.

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?

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

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.

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

Last edited:

Am I getting closer to figuring this out? haha

LowlyPion
Homework Helper

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 ?

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

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.

Gotcha so it would only be

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

LowlyPion
Homework Helper

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 ...

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

LowlyPion
Homework Helper

... Cosx= 2h/r. But how is theta a function of h??

Even better then eh?

Looks like you can do a lot of substituting.

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

LowlyPion
Homework Helper

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.

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

anyone?

LowlyPion
Homework Helper

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 ?

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

LowlyPion
Homework Helper

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.

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