Water in a conical flask

Classical Mechanics Level 2

A conical flask is filled with $\SI{100}{\gram}$ of water. How much force does the base of the flask exert on the water?

Take the acceleration due to gravity as $g = \SI[per-mode=symbol]{10}{\meter\per\second}^2.$ Neglect the atmospheric pressure.

Less than

\SI{1}{\newton}

Equal to

\SI{1}{\newton}

Greater than

\SI{1}{\newton}

1 solution

Rohit Gupta
Sep 16, 2017

Consider the equilibrium of the water. Three forces are acting on it, 1) the weight 1N, 2) the walls of the container pushing the water vertically downwards, 3) the base of the container supporting the water in the upward direction.

For the equilibrium, the upward force equals the downward force. Thus, the force on the water by the base is greater than the weight that is 1N.

The base of the cylinder has a top surface AND a bottom surface . The Normal reaction force from the top surface is equivalent to the Weight of the water . 1 N. The normal reaction force from the table on the bottom surface of the base and indeed the entire flask will be greater than 1N since the weight of the entire flask and contents is greater than 1 N. Never the less the Weight of the water is 1N. The answer given is incorrect.

Steven Sinclair - 3 years, 8 months ago

How does the normal reaction from the top surface equivalent to the weight of the water?

Rohit Gupta - 3 years, 8 months ago

Agree with you

Ioan Calapar - 3 years, 8 months ago

Where do you find the flaw in my solution? The walls of the conical flask are pushing the water down and increasing the normal reaction at the top surface of the conical flask.

Rohit Gupta - 3 years, 8 months ago

It's hard for me too comprehend how changing shape of flask changes downward force...So your saying, if put on a scale, changing flask to conical shape would weigh more?

Reed Chaber - 3 years, 8 months ago

You are correct , changing the shape of the flask will not change the weight of the water. It will however change the pressure this force exerts. That is not what we have been asked to consider.

Steven Sinclair - 3 years, 8 months ago

No, we are not weighing the conical flask, we are measuring the force of water only on the base of the conical flask which will be greater than 1N.

If we put the conical flask on a weighing machine and weigh it the reading will be equal to total mass irrespective of the shape of the container. If we assume the conical flask massless, then, the weighing machine would read 100 gm.

Rohit Gupta - 3 years, 8 months ago

No, no, we cant increase a given force by just modifying the shape of the enclosure. Where is Sir Newton to hear this ?

Ioan Calapar - 3 years, 8 months ago

the push by the conical flask to the water is not contributed to the force acting on the bottom of the flash, it just equate the force acted by the water to the wall of the flask.

Jeriel Villa - 3 years, 8 months ago

The question is asking about the water only.

Ali Kwj - 3 years, 8 months ago

Water in a conical flask

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