Circular Accelerometer

A bead is positioned on a friction-less circular wire with a radius of 1. There is a uniform downward gravitational acceleration g g .

The horizontal acceleration of the wire is a ( = 2 g ) a\, (= 2g) . Determine the location on the wire at which the bead could be placed, such that it would remain at rest relative to the ring. Give your answer as the horizontal distance from this location to the wire's vertical axis of symmetry.


The answer is 0.894.

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3 solutions

Oliver Piattella
Dec 20, 2016

Referring to the figure in the text of the problem, let θ \theta be the angle formed between the directions 1) bead-centre of the wire and 2) centre of the wire-down along the wire's vertical axis of symmetry (sorry, I am too lazy to draw).

The bead's weight generates a tangential acceleration g sin θ g\sin\theta in the anti-clockwise sense. The acceleration of the wire generates a tangential acceleration a cos θ a\cos\theta in the clockwise sense. Since the bead is in equilibrium, we have:

a cos θ = g sin θ a\cos\theta = g\sin\theta

that is

tan θ = 2 \tan\theta = 2

Since the radius of the wire is 1, the distance of the bead equilibrium position to the vertical axis is sin θ \sin\theta , which expressed through the tangent gives:

sin θ = tan θ 1 + tan 2 θ = 2 5 = 0.894427... \sin\theta = \frac{\tan\theta}{\sqrt{1 + \tan^2\theta}} = \frac{2}{\sqrt{5}} = 0.894427...

Steven Chase
Dec 19, 2016

I had a doubt. Please help, if you don't mind. Can we say that the bead here experiences centripetal (or as a consequence centrifugal) force? The particle here is at equilibrium, so I don't think so, but I wanted opinions.

Swapnil Das - 4 years, 3 months ago

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I'm not inclined to describe it that way, since there isn't really any rotation. But if you can make a case, then maybe so.

Steven Chase - 4 years, 3 months ago
Md Zuhair
Dec 19, 2016

This can be very easily done. First of all , the bead is expriencing , two accelartions, 1 horizotal, 1 verital. Now By Vector Law, the resultant of the two perpendicular acceleration vectors are ( 2 g ) + ( g ) 2 \sqrt{(2g)^+(g)^2} = a r a_r = g g 5 \sqrt{5} .

Hence now, this acceleration is equal to the centrepetal accelaration a c a_c = v 2 / r v^2/r where r=1m

Hence a r a_r = a c a_c or g g 5 \sqrt{5} = v 2 v^2

Hence 1/2mv^2 = mgh, now h = 5 \sqrt{5} /2 from the uppermost part of the axis.

Now it will look something like this

Now by pythagoras, we get Distance = 0.89.. (approx)

Interesting. You applied rotational concepts to a non-rotational problem and got the right answer. What would your answer have been if a and g were the same?

Steven Chase - 4 years, 5 months ago

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Thanks let me see

Md Zuhair - 4 years, 5 months ago

Are you in fb? It is easy to discuss there

Md Zuhair - 4 years, 5 months ago

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No, I'm not. But for the case where a and g are the same, I get x = 1/sqrt(2). My solution is also posted above.

Steven Chase - 4 years, 5 months ago

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@Steven Chase ok. so let me check if it is coming..

Md Zuhair - 4 years, 5 months ago

@Steven Chase is my solution wrong anywhere? I dont think so

Md Zuhair - 4 years, 5 months ago

@Steven Chase Yes, same, i am also getting 1 / 2 1/\sqrt{2}

Md Zuhair - 4 years, 5 months ago

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@Md Zuhair We agree in both cases. I was just surprised, since it seemed like we solved it very different ways.

Steven Chase - 4 years, 5 months ago

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