This Dam Acts Like a Water Cannon. Let’s Do Some Physics!

When folks construct dams—big partitions that maintain again whole lakes and rivers—they must construct an overflow channel known as a spillway, a mitigation in opposition to flooding.

A spillway could possibly be one thing so simple as a path for water to circulate excessive of the dam, or extra sophisticated, like a facet channel. Typically, there may be only a large gap on the backside of the dam (on the dry facet) in order that water can simply shoot out like a large water cannon. That is the way it works on the Funil Hydropower Plant in Brazil. There is a good video exhibiting the water popping out—it appears to be like like a river within the air, as a result of it principally is a river within the air.

However the actually cool physics of this spillway is that the pace of the water popping out of the outlet principally simply is determined by the depth of the water behind the dam. As soon as the water leaves the tube, it basically acts like a ball thrown at that very same pace. Sure, you realize what I’ll do: I’ll use the trajectory of the water leaving the spillway to estimate the depth of the water within the reservoir.

There’s truly a reputation for the connection between water circulate and depth—it is known as Torricelli’s legislation. Think about you will have a bucket stuffed with water and also you poke a gap within the facet close to the underside. We are able to use physics to search out the pace of the water because it flows out.

Let’s begin by contemplating the change in water stage throughout a really quick time interval because the water drains. Here’s a diagram:

Trying on the prime of the bucket, the water stage drops—even when just a bit bit. It does not actually matter how a lot the water stage decreases; what we’re excited about is the mass of this water, which I label as dm. In physics, we use “d” to symbolize a differential quantity of stuff, so this might simply be a tiny quantity of water. This lower in water stage on the prime implies that the water has to go someplace. On this case, it’s leaving by means of the outlet. The mass of the exiting water should even be dm. (You must preserve observe of all of the water.)

Now let’s consider this from an vitality perspective. The water is a closed system, so the whole vitality have to be fixed. There are two sorts of vitality to consider on this case. First, there may be gravitational potential vitality (Ug = mgy). That is the vitality related to the peak of an object above the floor of the Earth, and it is determined by the peak, the mass, and the gravitational discipline (g = 9.8 N/kg). The second sort of vitality is kinetic vitality (Okay = (1/2)mv2). That is an vitality that is determined by the mass and the pace (v) of an object.

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