# Centripetal Acceleration of two masses when one mass is half a radius away

1. Oct 31, 2012

### anomalocaris

1. The problem statement, all variables and given/known data

Two objects, m1 and m2, both of mass m, are place on a horizontal platform which is rotating at a constant angular velocity. m1 is located at a distance R from the axis of rotation and m2 is located at a R. The centripetal acceleration of mass m1 ____ to the centripetal acceleration of m2.

2. Relevant equations

aci=rω2

3. The attempt at a solution
Okay,the correct answer (according to my homework) should be "less than," but I do not understand why. Here's what I did:

Mass 1: aci=(1/2)Rω2
so (2aci)=Rω2
Mass 2: aci=Rω2

So shouldn't mass 1 have more centripetal acceleration? Or should I not have brought the (1/2) to the other side of the equation. My professor gives a lot of questions like this, and I seem to always get them wrong because I make them more complicated than they should be. But it makes sense that an object farther away from the center of rotation should have more acceleration since it would be traveling a greater distance in a larger circle.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 31, 2012

### tiny-tim

hi anomalocaris!
(i take it you mean m2 is located at 2 R ?)

i honestly don't see how you got the opposite result, even from those equations

but anyway i strongly recommend that you don't use the same letter for two different things …

in this case, call the accelerations a1 and a2 (and not both aci), and then you can put them both into the same equation, and compare them!

3. Oct 31, 2012

### anomalocaris

Okay! Thanks tiny-tim! So Should I live a1=(1/2)ω^2? Not 2a1=a1=(1/2)ω^2 ? Because that would make a2 the smaller one?

Thank you!

4. Oct 31, 2012

### anomalocaris

5. Oct 31, 2012

### tiny-tim

hi anomalocaris!
it really doesn't matter, so long as you end up with an equation with a1 on the LHS and a2 on the RHS

6. Oct 31, 2012

### anomalocaris

Wow embarrassing typographical error! Sometimes I type faster than I think!

Why should they be on opposite sides? How would this look? Sorry for all these questions, I'm probably making a simple concept more complicated, but I just want to understand this since this kind of problem is helpful for solving all kinds of other problems.

7. Oct 31, 2012

### tiny-tim

you're aiming for an equation 2a1 = a2

it doesn't matter how you get there (there's more than one way)

anyway, try it and see

8. Oct 31, 2012

### anomalocaris

Okay so then a1=(a2/2)? But either way, a1 is greater than a2. The "correct" answer states that a1<a2. Sometimes the HW keys are wrong though.

9. Oct 31, 2012

### tiny-tim

no, a1=(a2/2) means a1 is less than a2

10. Oct 31, 2012

### anomalocaris

OH!!! Epiphany moment! Okay I finally get it! a2 is greater because half of it would be equal to one whole a1. Thanks for sticking with me, tiny-tim! I really really appreciate it!