Question:

What is this demonstration trying to show?

Answer:

Hydrostatic pressure increases with depth!

The card in the video experiences two forces. (1) The upward pressure force exerted by the water (F=P.A) and (2) the weigth of the brass weight (technically, it is the downward normal contact force exerted by the brass weight, which is numerically equal to the brass weight).

At sufficient depth, the pressure force is larger than the brass weight. The card is pressed upward against the tube, and the pig is safe.

However, as the tube is raised slowly towards the water surface, the pressure force decreases since hydro-static pressure is proportional to the depth of the fluid (P = hρg). When the depth is too shallow, the pressure force is less than the brass weight. That is when the card drops away from the tube, and the pig is doomed.

Delving Deeper

Let’s do a rough calculation. The mass of the brass weights is 300 g. The inner radius of the tube is 8.6 cm.

P.A = mg

hρg.πr² = mg

h(1000).(3.14)(0.043²) = 0.300

h = 5.17 cm

By equating the weight to the upward pressure force, we estimate the minimum depth to support the brass weights to be about 5 cm. Which looks about right in the video.

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