# Going Fishing

Discussion in 'Physics' started by dthx, Jun 3, 2013.

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1. ### dthx Thread Starter Member

May 2, 2013
194
14
This is not a hypothecal question....I dont know the answer and would like to hear a rational discussion on the subject.
I buy minnows at my local bait shop by the pound...
They're \$1.00 per pound....so you still can get SOMETHING for a buck....anyway....I digress...
They put a small galvanized bucket on the scale and fill it about 1/2 way up.
Then they zero out the scale.
Then they start throwing in the minnows....until the scale reads 1 Lb....
The minnows start swimming around in their new home...not knowing what is to befell them shortly....
The minnows are suspended in the water....they have a little air sac in their little bodies that gives them neutral bouyancy while they're swimming...
Question ....If I took the minnows out and weighed them by themselves...would I have 16 oz. of minnows....
Or more....
Or less.....
What say all of you?

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Yes, they have neutral buoyancy in water, not in air. If it were lighter than air it would be a different story.

3. ### Metalmann Active Member

Dec 8, 2012
700
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Their air bladders do not weigh much, so........

4. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
Correct. Or to put it another way, if the scale was in the water, then the air in the blatter might skew the measurement. But since the scale is weighing the little fishes in air, then the measurement is accurate. So, don't worry and go fishing!

5. ### dthx Thread Starter Member

May 2, 2013
194
14
Wait a minute....
So the attendant places the bucket with water in it on the scale and zeros the scale , and puts enough minnows in the bucket with water to tip the scale to 1 lb...if I dipped the fish out of the water and weighed them in air.....
Would the group of minnows (in air) weigh 1 lb. ?
Am I buying exactly 1lb of minnows....or more....or less?
OR
If the minnows all died instantly and sank to the bottom of the bucket, I can see how the 1lb would be accurate....but do they weigh a pound when swimming....?

6. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
Wether swimming or laying dead, they weigh 1 pound.

Eureka!

7. ### dthx Thread Starter Member

May 2, 2013
194
14
yeah...but why?
Could you please use your physics background to explain the forces acting on the scale.

While I was typing that I may have figured it out myself.
The fish have got to be pushing down on the water and hence the scale to be able to stay suspended, correct?
but then again....the air keeps them afloat.
I dont get it....

What if you had a glass bottle that weighed 1 lb. empty.
And you put another pound of flies in the bottle ....
And they were all flapping their wings and were flying around.
The bottle would weigh 2 lbs..?

8. ### dthx Thread Starter Member

May 2, 2013
194
14
Alright....theyre pushing down on the air to stay in flight and thay gives weight to the bottle....
But what if they are gliding and not flapping their wings..
The diff. in air pressure would give them lift...I understand that....
But would they weigh anything...
Of course they would....
Or else they would never tumble to the ground.....
NeverMind.
Same as the fish....
Thanks...just needed to talk it out...
Ive got to explain this to a 9th grader tomorrow ....

9. ### LDC3 Active Member

Apr 27, 2013
920
160
Reminds me of a joke about the trucker and a load of birds. If all the birds are perching, then it would be over his weight limit. So if he could keep half the bird flying around, he'd be fine... or would he?

10. ### dthx Thread Starter Member

May 2, 2013
194
14
Well.....lets say that the back of one of those semi tractor trailers and truck is open and its rolling down the road at some speed......
And 3 birds weighing 1 lb each ...fly into the back of the open trailer being pulled by the truck.
But they dont perch...they continue to fly around...
Are you guys telling me that the trailer becomes 3 lbs heavier at that point?

11. ### Georacer Moderator

Nov 25, 2009
5,151
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12. ### #12 Expert

Nov 30, 2010
16,705
7,355
The concept is called, "The boundaries of the system".

Perfect example was in a movie where Jimmy Stewart pretends to be Charles Lindberg flying across the Atlantic ocean. There is a fly in the cockpit. Jimmy Stewart wonders, "If the fly is flying, does he contribute to the weight of the airplane?"

Yes. If the fly is sitting on the seat, he obviously adds to the weight in the seat. If the fly is flying one inch above the seat, the downward force of his flapping wings adds to the weight pushing down on the seat.

Now, expand that thought. As long as the fly is ANYWHERE within the boundaries of the system, his weight counts. Seated, flapping, inspecting the altimeter, going up Jimmy Stewart's nose, he is still within the boundaries of the system. Consider the idea that Jimmy Stewart is in a space ship with a fly. They are surrounded by vacuum. The fly is still inside the space ship. Define the boundaries of the system and you will know the answer.

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13. ### SplitInfinity Member

Mar 3, 2013
369
9

You have a problem and it has nothing to do with the fish in the water or air in the fish...none of that matters as the weight of the bucket and water subtracted from the weight of the bucket, water and fish will give you the proper weight of fish.

The PROBLEM is resetting the scale to ZERO!

When a scale is reset by using either a dial or lever to move the measurement dial to zero from either a few pounds or ounces more or less than zero...it is setting the scale to read weight upon the specific surface the scale is upon.

When a person dials out the entire weight of the water bucket...well the reading you will get now will not be accurate at all.

Split Infinity

14. ### LDC3 Active Member

Apr 27, 2013
920
160
It is as accurate as without the bucket and water since the force on the spring is the same. F = k x d -- The force is equal to the spring constant times the distance compressed (or stretched).
The same is true with piezoelectric pressure sensors.
There is no problem with resetting the zero point (also called taring the balance).

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15. ### dthx Thread Starter Member

May 2, 2013
194
14
Thank you #12 and Georacer....

To SplitInfinity...I know your trying to tell me something...but, withall due respect, Im not sure what it is....
Wouldnt that be one way to get the "net" weight?
Thanks to all
D.

shortbus likes this.

Apr 27, 2013
920
160
Yes 5678910

17. ### tracecom AAC Fanatic!

Apr 16, 2010
3,879
1,396
Where is this place? At \$1.00 per pound, I may start eating them myself.

18. ### SplitInfinity Member

Mar 3, 2013
369
9
You are looking at this as if this scale was calibrated by NASA to weigh an Astronauts Jock Strap to calculate weight to force ratio's for liftoff.

What he is talking about is a NON-DIGITAL spring based scale used at a BAIT SHOP with a dial or lever that adds or subtracts tension upon that spring.

He should count himself lucky that the owner of the place isn't using a rope slung over a beam tied to two buckets and using rocks as a way to calculate weight! LOL!

dthx...TRUST ME...spinning that dial or pulling a lever to zero out a weight as much as a bucket of water...is going to throw that scale so out of whack that you are going to be paying for something you didn't get.

Split Infinity

19. ### LDC3 Active Member

Apr 27, 2013
920
160
That's is what I referred to with a spring scale. The force is equal to the spring constant times the distance. F = k X D

For example:
The distance the scale moves is the same for the fish when the spring is compressed at 40% (bucket and water) or at 10% (without bucket and water).
The same inaccuracies are present for both conditions.

20. ### SplitInfinity Member

Mar 3, 2013
369
9
All things being equal...I would agree with you...but I am fairly certain all things are not equal in this case.

Spring loaded scales are not even LEGAL for use in my state for use for the purpose of retail and wholesale sales based upon weight. They are just too unreliable and almost impossible to calibrate so as to weigh anything upon multiple surfaces so it always comes up the same weight upon such a scale.

Your statement is only as accurate as is the springs ability to remain and give a constant tension upon it's flex.

Trust me...this thing is not going to be accurate dialing out as much as a bucket of water.

Split Infinity