School boy experiment with water

[vineet_kiran]

 
School boy experiment with water and tube

[Low-Q]

School boy experiment with water and tube

I have tried it and you are wrong in your conclusion. The air bubbles is forced to push the water out of the tube first before air can be pushed out. You will feel a counterforce by the buoyancy of the immersed air column. The weight of the tube will probably be the same as the counterforce, but when you take the tube out of the water to repeate the cycle, you must lift both the tube and the water inside the tube above the water level in the two tanks before water is released.


Vidar

[vineet_kiran]

@Vidar,

1)      The  counter force  on the air column   acts  only on the right side portion of  the tube  ie.,  on the end from which air comes out,   which can be absorbed  by  fixing that portion  of the  tube  to an external  support  attached to container or some other place.  Since tube is flexible, the movement of  left portion of the  tube will not be affected.

2)      I have mentioned that  you have to provide a valve (or plug) on the tube  preferably  on the portion which is not immersed in water   to repeat the cycle again.    When  you open the valve the pressure inside the tube will become atmospheric  and then if you lift the tube (left portion only)  keeping the valve open,  water  column will not come along with the tube.   Once you lift the tube  you have to close the valve again and repeat  the cycle. 
  Kindly confirm the results and  tell  me if I am wrong.

Regards,
Vineet.K.

[Low-Q]

No matter how you put it, there is a given amount of force you have to move. If there isn't any force on the "bubble side", you have ALL force on the other side. Try with 4 inch tubes instead so more water can be displaced. Then you can actually feel what forces you're dealing with.


Vidar.

[TinselKoala]

Vineet, your diagram misrepresents the true situation. Please consider my corrected and annotated version of your diagram which I have attached below.
What your diagram omits is the displaced water. When you push one end of the tube into the water, keeping the other end at the same level as before, you are decreasing the volume _above the waterline_ occupied by the air. It must escape or become pressurized. It pushes the water out of the other end of the tube, RAISING the level of the water in the container by an equivalent volume. The force required to raise this water and produce the bubbles is coming from your hand pushing down the other end of the tube.
If you push both ends down simultaneously of course you will get no bubbling and the level of water in both tubs will rise as you submerge more and more of the tube (and your hands). This lifting up of the weight of the water IS the buoyancy force that tries to push your tube and your hands back up out of the water.

(Actually the level in the tube to the left will also be pressed downward and the level in its tub will rise as well, and should be the same as in the other tub. The difference being that the air can escape in the right tube end but the left tube end is pushed down farther so the air cannot escape there.)