Geek Challenge: A Balancing Act

Geek Challenge

Ken Brey

02/05/2014

8 Comments

An apparatus is constructed as shown below comprising equivalent buckets of water. The buckets and water were placed on the scale first, and it balanced. Then two suspended balls of equal diameter are added in the configuration shown. One is wooden, and floats. It is suspended from the bottom of the bucket. The other is a pool ball that sinks. It is suspended externally.

Assuming the weight and displacement of the strings is not significant, what happens to the scale?

A. Scale tips Left
B. Scale tips Right
C. Scale remains balanced.
D. The scale is in unstable equilibrium, and could tip either way.

As always, the winner will go to the correct answer with the best engineering content. Submit your responses to geekchallenge@dmcinfo.com.

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Comments

# Anonymous User

Thursday, March 6, 2014 12:21 PM

Geek Challenge Results: A Balancing Act

# John Schaefer

Saturday, February 8, 2014 10:28 AM

Hello,
It's the first time I've looked at your geekchallenge, having only recently been added to the DMC email list. I mention this only because I do not know the protocol in submitting an answer. As an adjunct professor to various colleges over the years, I like these type of challenges primarily for the dialogue during the solution process. This creates a transparent thinking process shared among the participants - rather than an answer without showing the work used in reaching it (remember your math instructors lowering your score for not showing the work?)
Anyway, what reaction will the scale have to the conditions provided?

The pool ball is suspended from above. Though it may weigh more that the wood ball it isn't important to me, because it is suspended from above.

The wooden ball is suspended from below. This is important to me because the wooden ball floats and is being held in place by the string attached to bottom of the glass.

Hmmmmmmm.

Both spheres are displacing the same amount of water.
All other conditions ( glass size-amount of water etc.) appear to be the same from the problem conditions information.

So the first question to me becomes this. The force of the floatation from the wooden ball is being transferred to the bottom of the glass and would that change the force transferred to the scale platform?

It seems to me that it would.

So, my answer might be that the scale would tip to the right. However, as soon as it does that (if I'm right in the first place) then the conditions of the right glass and suspended pool ball immediately begins to change because the ball is suspended from outside the glass. It would then, I think, cause the left glass to exert more pressure on it's side of the scale. It would start going down until ..... well you know the rest of my thinking there so.... I think the correct answer might be D - with a caveat - though it would be unstable I think it would start by tipping to the right.

Right or wrong - that was fun.
Please let me know what the correct submittal is.
John Schaefer

# Jeff Butler

Friday, February 7, 2014 8:14 PM

I think "B" is the correct answer

# Ed

Thursday, February 6, 2014 3:26 PM

# Chris Bramlage

Thursday, February 6, 2014 3:24 PM

The scale will tip to the left due to the added weight of the wooden ball. Because the * ball is suspended from above, it adds no weight to the right side

1 (original weight of bucket and water) + WB (wooden ball) > 1 (original weight of bucket and water) + 0

# Bob Reynolds

Thursday, February 6, 2014 3:07 PM

I'm no engineer, but I believe that the scale will tilt left due to the added weight of the wood ball. The fact that it is tethered to the bottom is irrelevant.

# David Zapata

Thursday, February 6, 2014 2:54 PM

Scale tips left.

# Dave Dubey

Thursday, February 6, 2014 2:49 PM

A. The scale will tip left.

The scale will tip left because the left side of the scale has the weight of the water plus the weight of the wooden ball. The right side of the scale is only supporting the weight of the water.

The scale will stop tipping left when either the pool ball touches the bottom of the pail or the scale bottoms out on the left side, whichever occurs first.

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