Let’s start talking about water. Nobody really thinks much about water unless there is too much of it or not enough. Let’s look at too much water first. We’ve all seen the pictures and videos of tsunamis washing over an island or of floods rushing down a valley, relentlessly smashing and washing away everything in the way. If you’ve been reading the previous articles, you already know why water can be so destructive. Water is dense—it has a lot of mass per volume. A gallon of water weighs a little more than 8 pounds (8.3). There are about 7 ½ gallons in one cubic foot of water (a cubic foot isn’t very big—it’s a cube 1 foot by 1 foot by 1 foot), so one cubic foot of water weighs about 62 pounds. Your car probably weighs somewhere around 3000 to 3500 pounds. Let’s go with 3500. So 56 cubic feet of water weighs roughly the same as your car. 56 cubic feet isn’t much. Picture a box about 3.8 feet wide and deep. It’s about the size of washing machine. Keep that washing machine-sized volume of water in mind.
Tsunami, Yamada, Japan, 2011.
Source: NOAA, Earthquake Memorial Museum, Yamada, Japan
Now, let’s go back and talk about energy a little more. We’ve touched on the energy in chemical bonds (chemical energy) and the energy in sunlight (radiant energy, mostly heat). Those are specific types of energy. In the article on inertia and mass, we also hit a little bit on the energy of movement (kinetic energy). A quick review is in order. If you have a brick, and the brick is lying on the ground, where will the brick be tomorrow? It will be right there on the ground where you left it, because it’s on the ground and the only force that is acting on it (gravity) can’t pull it any further, because it’s already on the ground. This is called the “ground state”, which is the lowest-energy state for a system. The system in this case is the Earth and the brick, which are exerting gravitational forces on each other. They are in the lowest-energy state because they can’t move, relative to each other. The brick can fall no further, because it’s already on the ground.
Now, pick the brick up and put it on a table. Now, what can you say about the system? It’s no longer at its lowest-energy state, because the brick could now fall from the table to the ground (if you were to push it off the table, for instance). The brick now has what is called “potential energy”, because the energy is there and could potentially be released (when it fell to the ground). The amount of potential energy in the brick is exactly equal to the amount of energy your muscles had to use when you bent over, picked the brick up and placed it on the table. You, in effect, spent your chemical energy (energy from the food you eat, used by your muscles) to pick the brick up (fighting the pull of gravity between the Earth and the brick) and put it on the table, giving the brick potential energy.
Okay, so you gave the brick potential energy when you raised it above the ground and put it on the table. So now what? Is the brick going to fall off the table? Probably not, unless someone pushes it or the table collapses. Let’s go with pushing it off. If you push the brick off the table, what will happen? Yep, it will fall to the ground. Nobody said Physics had to be complicated. What happens to the potential energy of the brick as it falls? The answer is that the potential energy of the brick gets converted into what is called “kinetic energy”, or the energy of a mass in motion. It this case, the kinetic energy comes from gravity pulling the brick toward the ground, and it can be quite substantial. If you want to measure the kinetic energy in this system, just put your foot on the ground under the brick. You will discover that a 5-pound brick falling three feet generates considerable force. Because there are both stupid people and lawyers in the world, I should probably mention that you really shouldn’t do this experiment by dropping a brick on your foot. It would be bad.
We now get what potential and kinetic energy are. We used a brick as our example, but the principle is the same, regardless of the object. Take water, for instance. A washing machine-sized volume of water moving 20 miles an hour has the same energy that your car moving 20 miles an hour has, which is a lot. Think of what would happen if your car hit something going 20 miles a hour. Now think about all that water rushing down the river valley. It’s probably tens of thousands of cubic feet, if not millions. That flood water is going to hit with thousands of times more energy than a freight train. That is why floods and tidal waves, or even small amounts of water trickling down a hill can cause so much damage. Water is dense and when it moves, it carries a lot of energy. If you’ve ever wondered why gravel from gravel driveways washes out in the road every time it rains, it’s because it doesn’t take very much water moving very fast to have enough energy to push that gravel. It’s why it’s so hard to prevent erosion on creek banks or on fresh dirt before the grass grows on it. Another example of the power of water that most of us may be familiar with is in a river. You don’t have to wade very far out into a river, even one that seems to just be lazily rolling along, to feel the push of the current. This is also why you NEVER should drive your vehicle into a flooded place in the road. It doesn’t take very much moving water to sweep you and your car away. The Salmon River in Idaho is a small river that runs into the Snake and then the Columbia Rivers. Lots of people raft and kayak on it, because it’s beautiful and has lots of white water. For most of it’s length, its probably 50-75 feet wide–which isn’t much by Ohio or Mississippi River standards. However, the Salmon river flows down a steep canyon, so it flows pretty fast. Right now, the river is fairly low as the spring snowmelt surge has passed. In most places, it’s probably not 20 feet deep. Even now, when it’s fairly tame, about 8000 cubic feet PER SECOND is flowing down the river. That’s fast enough the the river was given the name “The River of No Return” back in the pioneer days, because you could go downstream in a boat (if you were skilled enough to get through the rapids), but you couldn’t take a boat upstream. Today, it can only be done with powerful jet boats. When the flow of the river goes up during the spring melt, the river doesn’t get much wider, because of the canyon. It just gets deeper and faster. At it’s peak, the river can flow at over 100,000 cubic feet per second. Think of 560,000 gallons of water (over 4 1/2 million pounds), moving at about 4 or 5 miles an hour, passing by every second. That’s really an awe-inspiring amount of energy. This power of moving water is also why hydroelectric power plants work.
In the next article, we will talk some more about water and why it is so important for us and other living things. Here’s a preview: if anyone ever asks you what the secret to life is, a pretty good answer would be “ice floats”.