Physics Friday 101

If one punctures a small hole in the side of a large reservoir of liquid, at a point below the surface level of the liquid, one will have a jet of liquid from the hole. The lower the hole, the faster this jet will be. If the liquid in the reservoir has a depth of H, at what height h above the bottom of the reservoir should one place the hole so that the jet gets the most range?

In this situation, we can use Torricelli’s law, which says that the speed of efflux from the hole is , where z is the distance of the hole below the liquid surface; this is equal to the velocity an object gains from falling freely through a height z.
Here, z=Hh, so our fluid is launched horizontally from a height h with speed .

Now, when an object is launched with horizontal speed v from a height h, the kinematic equations for position as a function of time are
.
Eliminating time between the equations, one finds
.
Solving for y=0, we get a range of
.
Now, plugging in our velocity, we get
.
This is at its maximum when is maximal, which occurs at . The maximum range is thus , the total depth of the liquid in the reservoir.

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One Response to “Physics Friday 101”

  1. Physics Friday 102 « Twisted One 151's Weblog Says:

    […] Friday 102 By twistedone151 Torricelli’s law, mentioned in the previous Physics Friday, is derived from Bernoulli’s Principle, via a few assumptions. Let us have a small hole at […]

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