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

When individuals construct dams—big partitions that maintain again whole lakes and rivers—they should construct an overflow channel known as a spillway, a mitigation towards flooding.

A spillway could possibly be one thing so simple as a path for water to move excessive of the dam, or extra sophisticated, like a facet channel. Typically, there may be only a massive gap on the backside of the dam (on the dry facet) in order that water can simply shoot out like an enormous water cannon. That is the way it works on the Funil Hydropower Plant in Brazil. There is a good video displaying the water popping out—it appears 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 largely simply depends upon the depth of the water behind the dam. As soon as the water leaves the tube, it primarily 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 really a reputation for the connection between water move and depth—it is known as Torricelli’s legislation. Think about you will have a bucket filled with water and also you poke a gap within the facet close to the underside. We will use physics to seek out the pace of the water because it flows out.

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

Illustration: Rhett Allain

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

Now let’s consider this from an vitality perspective. The water is a closed system, so the entire 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 depends upon the peak, the mass, and the gravitational area (g = 9.eight N/kg). The second sort of vitality is kinetic vitality (Okay = (1/2)mv2). That is an vitality that depends upon the mass and the pace (v) of an object.

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