Centripetal Force - trying to find time

AI Thread Summary
The discussion focuses on solving a centripetal force problem involving a rotating platform. The physicist observes a weight hanging at a 10° angle from vertical while standing 10 feet from the center. Participants emphasize the importance of drawing a free-body diagram to analyze the forces acting on the weight, particularly the tension in the string and the gravitational force. By breaking down the components of these forces, they derive equations that allow them to eliminate mass and tension, ultimately leading to a calculation of the velocity. The final calculated velocity is 7.51 ft/s, confirming the correctness of the approach taken.
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Homework Statement


A physicist is standing 10 feet from the center of a platform that can rotate about a vertical axis - the platform is horizontal. She is holding a weight that is hanging freely from a string. The platform starts rotating and eventually she notices that the weight is no longer hanging straight down but is making an angle of 10° with the vertical. How fast is she moving at that time?


Homework Equations


Normally, for centripetal equations I would use a=v2/R
I've also used the equation Fcentripetal=4∏2Mf2R


The Attempt at a Solution


I've never had to solve a centripetal force equation with knowing the mass of either the person or the weight or without a velocity. I'm honestly not quite sure what route to go to solve this problem.
 
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Draw a free-body diagram for the weight.
Once you take x- and y-components, you should be able to cancel the mass out.
 
So I'm not sure if I drew this correctly or not, but drawing out the free body diagram for the weight, would those components cancel out with the tension of the string?
 
It should be clear from your diagram that the y-component of the tension cancels out the weight.
The x-component of the tension will be the only horizontal force on the object - this provides the centripetal force.

Have you written out the components? They should simplify down to a simple expression.
 
For components, I would have:

Ftension; the y-component pointing upwards canceled out with the weight of the mass pointing downwards. There is no Ffriction since the object is in the air. Otherwise, I'm not sure what other forces.

I could be going about this completely wrong, but from other examples I had, I used force equations such as

\SigmaFx = Fnet
Ftension = ma

Problem I run into with this, is that I don't have a way of calculating the mass or the acceleration. All I know is that the angle of the mass is 280° from the +x axis.
 
So, for the y-components, you have
Tcos(10°) - mg = 0
→ Tcos(10°) = mg

and for the x-components,
Tsin(10°) = ma
where a is the centripetal acceleration

(your components may be slightly different depending on your axes)

If you insert the expression for centripetal acceleration and combine these 2 equations, you can get rid of T and m.
 
Sneaky approach:

If you consider the string and weight to be stationary in a rotating frame of reference (think of taking a snapshot of the rotating platform at some instant of time when you have a profile view of the happenings) then it becomes fair to speak about centrifugal forces and accelerations in that frame of reference. It turns out that these centrifugal effects have the same magnitude but opposite direction to our usual centripetal ones. Then, using Newton's second law f = ma, we can drop the constant m's and label a diagram with accelerations rather than forces:

attachment.php?attachmentid=54009&stc=1&d=1355762164.gif


Does this give you any ideas about solving for ac?
 

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ap123 said:
So, for the y-components, you have
Tcos(10°) - mg = 0
→ Tcos(10°) = mg

and for the x-components,
Tsin(10°) = ma
where a is the centripetal acceleration

(your components may be slightly different depending on your axes)

If you insert the expression for centripetal acceleration and combine these 2 equations, you can get rid of T and m.

So I went with this and worked with the axes a bit to make it make more sense to me. I ended up with this as a result:

\SigmaFx=max
Tx+(mg)x = max
T(cos100°) +mg(cos270°) = ma(cos180°)
T(cos100°) = -ma
T(cos100°) = -mv2/R

T(sin100°) - mg = 0
T(sin100°)/g = m

T(cos100°) = -T(sin100°)/g * v2/R

My final equation was v2 = (T(cos100°))gR/-T(sin100°)
v2= -55.56t/-0.98t = 56.47 which I then took the square root of and so the velocity ended up being 7.51ft/s.

Did I go about this correctly?
 
Your final answer agrees with mine :)
 
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