Real Life Physics Puzzles

[onlinebiker]

If you have one - post it here.

Here's mine.

I have a 500 gallon tank in my barn. ( used for making maple syrup)

This tank is an upright cylinder. There is a tap on the side of the tank which has a shutoff valve - then a 3/4 inch line that runs out the side of the barn, then down to the ground and runs in a buried line 75 feet to the west - then comes back out of the ground up 5 feet to a feedline and shutoff. (At the shed where we boil sap to make syrup) This shutoff valve is more than 10 feet below the level of where the line comes out of the tank.

If I put 100 gallons in the tank - and open both valves - the water ( or sap) DOES NOT FLOW. You can wait for an hour - and it does not flow. If you apply vacuum to the line - you can get it to flow and it will empty the tank.

If you put 250 gallons in and open the valves - it will flow a good stream all on it's own.

How is this possible?

[BrianSoddingBoru4]

It's probably quantum.

Boru

[Fireball]

If I read this right, there is a 5 foot difference in the heads at either end. I don't know the viscosity of maple sap, but it may be just high enough to slow the flow to some small value (combined with pressure drop from friction of the fluid with the pipe). Have you tried it with plain water? That would answer the question about viscosity, because water would flow, regardless, though it might be slow. Next part- once you put a suction on the line, the flow, once started, will continue due to the siphoning effect of the fluid flowing in the line. More fluid on the source side overcomes the viscous flow and/or pipe friction. What kind of pipe are you using?

[Anomalocaris]

1. How far is the tap on the side of the tank from the bottom of the tank?

2. Is there any air holes on the top of the tank preventing vacuum build up inside the tank?

3. Is there any air trap in the line?

[polymath257]

If there is air in the hose, you won't get a flow. The suction bring the water or sap through to the point that such gaps are eliminated, allowing flow.

[chimp3]

Pancakes!

[onlinebiker]

If I read this right, there is a 5 foot difference in the heads at either end. I don't know the viscosity of maple sap, but it may be just high enough to slow the flow to some small value (combined with pressure drop from friction of the fluid with the pipe). Have you tried it with plain water? That would answer the question about viscosity, because water would flow, regardless, though it might be slow. Next part- once you put a suction on the line, the flow, once started, will continue due to the siphoning effect of the fluid flowing in the line. More fluid on the source side overcomes the viscous flow and/or pipe friction. What kind of pipe are you using?

Maple sap is usually about 2 % sugar -  but it does the same with water.

The pipe is a Silicon based semi rigid tubing made specifically for syrup use. It will withstand 100 psi and does not collapse with 25 inches of mercury vacuum

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1.   How far is the tap on the side of the tank from the bottom of the tank?

Less than 2 inches

2.   Is there any air holes on the top of the tank preventing vacuum build up inside the tank?

There is an access hatch - it can be removed and does not affect the situation. 

3.   Is there any air trap in the line?

Not sure what you mean. 

---

If there is air in the hose, you won't get a flow. The suction bring the water or sap through to the point that such gaps are eliminated, allowing flow.

How would air in the line prevent flow?

If that were the case a sink would not drain when you pulled the plug.....

[Anomalocaris]

If I read this right, there is a 5 foot difference in the heads at either end. I don't know the viscosity of maple sap, but it may be just high enough to slow the flow to some small value (combined with pressure drop from friction of the fluid with the pipe). Have you tried it with plain water? That would answer the question about viscosity, because water would flow, regardless, though it might be slow. Next part- once you put a suction on the line, the flow, once started, will continue due to the siphoning effect of the fluid flowing in the line. More fluid on the source side overcomes the viscous flow and/or pipe friction. What kind of pipe are you using?

Maple sap is usually about 2 % sugar -  but it does the same with water.

The pipe is a Silicon based semi rigid tubing made specifically for syrup use. It will withstand 100 psi and does not collapse with 25 inches of mercury vacuum

---

1.   How far is the tap on the side of the tank from the bottom of the tank?

Less than 2 inches

2.   Is there any air holes on the top of the tank preventing vacuum build up inside the tank?

There is an access hatch - it can be removed and does not affect the situation. 

3.   Is there any air trap in the line?

Not sure what you mean. 

---

If there is air in the hose, you won't get a flow. The suction bring the water or sap through to the point that such gaps are eliminated, allowing flow.

How would air in the line prevent flow?

If that were the case a sink would not drain when you pulled the plug.....

Not just air in the hose, but an actual air trap.   That’s where somewhere along it’s path the hose bows upwards to form a hump.   If the top of the hump is higher than the top of the liquid level in the tank, You have an air trap.   If you apply suction to the end of the hose, you drop the pressure of the air in the air trap.   This imbalances the pressure of the air in the tank and pressure of the air in the hose, and the air pressure in the tank will force the liquid up over the hump, then siphoning effect will take hold and the tank will drain.

If you top up the tank until the liquid level in the tank is higher than the hump in the hose, then the hydrostatic pressure of the water will overcome the air in tthe hump and the tank will drain naturally.

[onlinebiker]

Maple sap is usually about 2 % sugar -  but it does the same with water.

The pipe is a Silicon based semi rigid tubing made specifically for syrup use. It will withstand 100 psi and does not collapse with 25 inches of mercury vacuum

---

---

How would air in the line prevent flow?

If that were the case a sink would not drain when you pulled the plug.....

Not just air in the hose, but an actual air trap.   That’s where somewhere along it’s path the hose bows upwards to form a hump.   If the top of the hump is higher than the top of the liquid level in the tank, You have an air trap.   If you apply suction to the end of the hose, you drop the pressure of the air in the air trap.   This imbalances the pressure of the air in the tank and pressure of the air in the hose, and the air pressure in the tank will force the liquid up over the hump, then siphoning effect will take hold and the tank will drain.

If you top up the tank until the liquid level in the tank is higher than the hump in the hose, then the hydrostatic pressure of the water will overcome the air in tthe hump and the tank will drain naturally.

I see what you mean.

But no - the line out of the tank is level with the tank exit - till it goes through the exterior of the barn, then goes down. At no point is the line higher than the exit of the tank.

[ignoramus]

a simple explanation would be: a gravity malfunction