Freefalling water tank

  • #1
tank with water is pushed from construction into freefall,
A is at 0.1m depth
B is at 10m depth (so manometer will show aprox 1bar of hydrostartic pressure when tank is at rest on construction)
from 0-5sec tank is accelrating,after 5sec tank is falling with constant speed..


1)is hydrostatic pressure during accelaration phase (0-5sec) greater at point A compare to point B?
2)after 5sec,tank is going with constant speed,so hydrostatic pressure at A and B is the same,but what value will show manometer?
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Answers and Replies

  • #2
A.T.
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View attachment 265126
tank with water is pushed from construction into freefall,
...
1)is hydrostatic pressure during accelaration phase (0-5sec) greater at point A compare to point B?
No

2)after 5sec,tank is going with constant speed,so hydrostatic pressure at A and B is the same
No
 
  • #3
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138
If the apparatus is accelerating due to gravity, the water falls with the vessel at the same speed, which implies that a thin layer of water does not apply any forces to the layer of water below it vertically, and therefore creates no pressure difference. When the speed becomes steady, the water layer begins to press the layer under it, which creates pressure on the bottom layer.
 
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  • #4
If the apparatus is accelerating due to gravity, the water falls with the vessel at the same speed, which implies that a thin layer of water does not apply any forces to the layer of water below it vertically, and therefore creates no pressure difference. When the speed becomes steady, the water layer begins to press the layer under it, which creates pressure at the bottom layer.
but where is normal force during freefaling at constant speed?air drag at bottom of tank?
 
  • #6
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but where is normal force during freefaling at constant speed?air drag at bottom of tank?
This situation is the same as stationary. Therefore, the bottom of the vessel holds the water.
 
  • #7
This situation is the same as stationary. Therefore, the bottom of the vessel holds the water.
what does "freefall" refers,falling with acceleration or when constant speed is reached?
 
  • #8
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what does "freefall" refers,falling with acceleration or when constant speed is reached?
You should have figured it out before asking. I think it means the acceleration of the equipment is g provided that there is no air resistance.
 
  • #9
This situation is the same as stationary. Therefore, the bottom of the vessel holds the water.
yes you are right when elevator moves down with constant speed in skyscraper i feel my weight..

why when you jump from high spot legs first you feel how stomach lift up,doesnt organs accelearte with same rate as body?
 
  • #10
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yes you are right when elevator moves down with constant speed in skyscraper i feel my weight..

why when you jump from high spot legs first you feel how stomach lift up,doesnt organs accelearte with same rate as body?
It does, but generally it is compressed due to gravity so as you jump it, you feel it stretched.
 
  • #11
If the apparatus is accelerating due to gravity, the water falls with the vessel at the same speed, which implies that a thin layer of water does not apply any forces to the layer of water below it vertically, and therefore creates no pressure difference. When the speed becomes steady, the water layer begins to press the layer under it, which creates pressure on the bottom layer.
in acceralting phase tank feel some air resitance when falling so his accelration is less than gravity=9.81,so does that mean that pressure B will be slightly bigger than A?
 
  • #12
jbriggs444
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in acceralting phase tank feel some air resitance when falling so his accelration is less than gravity=9.81,so does that mean that pressure B will be slightly bigger than A?
Yes.

Somewhat realistically, you will find that the supporting force from air resistance starts out negligible and then exponentially approaches a fixed limit with just enough to force to support the tank at constant velocity. There is no sudden cutoff from one condition to the other. It is a smooth transition.

The part where air resistance is negligible is "free fall".
The part where air resistance is nearly equal to gravity and speed has stablized is "terminal velocity".

[The transition from "tank of water supported by a rope" to "falling tank with snapped rope" is sudden, of course, but unlikely to immediately result in any dramatic splashes]
 
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  • #13
Now the question is.... What happens to the water pressure when it hits the ground at terminal velocity?
 
  • #14
jbriggs444
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Now the question is.... What happens to the water pressure when it hits the ground at terminal velocity?
[Sherman, let's go to the WayBack machine]

It splashes. Pressure is momentarily extreme.

If we assume a perfectly rigid container that lands upright and comes to an immediate stop, the water at the bottom of a tank comes to an immediate stop. Water is not perfectly incompressible, so it piles up at the bottom. But water is not very incompressible, so not much piles up before the pressure becomes extreme.

Exercise: Look up the bulk modulus for water (2 seconds on Google) and determine how much percentage reduction in volume is required to store the kinetic energy from water in a bucket that was falling at, say, 100 miles per hour. How much pressure does that take?

The rebound from this compression turns into a sound wave or perhaps even a shock wave in the water. There is plenty of energy in that shock wave -- most of the energy in the falling water is now embodied in the wave. When the wave reaches the surface, the surface water detaches as a splash. At terminal velocity, I'd expect wave energy to be adequate so that the entire contents of the bucket would splash out immediately.

Sounds like a fun experiment. Drop buckets of water from a tower and get slow motion video of the impacts.
 
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