Did You Know?
You, babies, are probably wondering, "What does propulsion even mean?". Well, little ones, propulsion is when something is pushed or driven forward. What people usually think of when they hear "propulsion" is actually what is referred to as a "propulsion system". Propulsion systems are machines or mechanisms that produce "thrust(s)" that push an object forward. "Thrust" is just a fancy way of saying there was a sudden push in a specific direction. So when you get upset it's nap time and throw your stuffie in a fit of rage, you are thrusting it towards the ground. This goes without saying but babies, don't be throwing stuffies. This was just an example (no stuffies were harmed in the making of this experiment :P).
Now that we know all of that, we can get to the actual "did you know?' section! If you've read the other two Science Boonie posts you'll know how much we love Newton and his laws. Just like with practically everything else fundamental in physics, Newton had a role in discovering and figuring out how to use propulsion to our advantage. Newton made a machine that is commonly referred to as the "Hero's engine". The official name is "Aeolipile" but that's hard to pronounce and type so I'm sticking with Hero's engine. Now, I'm not sure if he was the first guy to make this or if he made one after it had already been discovered but either way, his second and third laws were vital to the creation and understanding of how this steam engine works!
Listen babies, this thing is really cool! He basically made a giant metal ball with two spouts facing opposite directions. He would fill the ball with a tiny bit of water and heat the bottom. The water would boil and as a result, the pressure builds. The only two exit points are the spouts so high-pressure steam shoots out of them and since they're facing opposite directions on the sphere, the ball rotates, rapidly. My explanation does not do it justice, please, watch this video to see how cool it is (https://www.youtube.com/watch?v=3FyhNpHeHMM&ab_channel=TSGPhysics). You now know about Hero's engine! You're going to be the smartest little one on the playground! :)
A straw - if you can, get multiple straws with different diameters to test if your ship will go faster or slower.
Duct tape or an elastic band
Any other materials you want to craft a ship (paper plates, popsicle sticks, foam balls, etc.)
A bathtub or basin of water
A friendly tip - make sure to test your ship regularly to ensure it can float so you don't spend a bunch of time trying to fix it, at the end!
Step one - get your balloon, straw, and rubber band/duct tape and attach your balloon to the straw.
Step two - craft your ship! You can make it out of whatever you want. Maybe you want to include a little sea monster or captain on your ship!
Step three - attach your balloon and straw to your ship.
Step four - blow up your balloon and place it in the water. Does your ship move? If not, it might be too heavy! Try taking some stuff off. Don't get discouraged if this happens, it's all a part of the scientific process!
Big Boon Brain Explanation:
It's my favorite time, babies! The science behind propulsion is so interesting. There are so many different types of propulsion and while they all rely on the same fundamental principles, their applications, and how they actually work, vary so much! Since I could go on practically indefinitely about this, I'm going to only talk about how the way we did it (using a balloon) works.
From an energy perspective, the balloon is a bit different than the other means of propulsion. Usually, propulsion is chemically powered, or at least, chemical energy gets converted into kinetic energy. This isn't the case with the hero's engine (because having water go from a liquid to steam is a physical change, not chemical) but it is with rockets, for example. Rockets use mini explosions (which are often referred to as "combustions" or, if you're KatieBoon, "boon-splotions") to propel them. These combustions are chemically induced.
With the balloon, we have elastic energy getting converted to kinetic energy. Elastic energy is when you temporarily "strain" an object. Strain is a fancy way of saying an object has been squished or stretched outside of its normal shape. An example of this would be a stress ball. When it's just resting on the table, it's a normal sphere. When you grab it and squeeze, it compresses. The ball is experiencing strain. What's crazy is when you let go, it returns back to its original shape. What in the boon is happening here?! Well, children, this is where things get spicy!!
To explain, I'm going to use a different example, rubber bands. I'm sure we've all played with an elastic before. Sometimes, you get a little too into it and it snaps. Why does this happen? Well, babies, the elastic (as its name implies) has elastic properties. That means when it's underloading and or, experiences strain, it will deform. Once the loading/strain stops, the object will return back to its original state, just like the stress ball. If you apply too much force to the object, it'll break or at least be permanently damaged (this is commonly referred to as "plastic deformation"!). In engineering, we have a whole system to test different materials to find the point at which materials will stop being elastic and when they will fail. This system is called the stress-strain curve. I've included a labeled example, below. Please note, that this is a generic diagram, the one for highly elastic materials looks a bit different but this one is more informative :). If you're interested in learning more you can google the terms I included on the diagram to find out more about them. It's truly a fascinating field of research!
Okay, back to the main point, the balloon. When we blow it up, we cause the balloon to undergo elastic deformation. This results in elastic energy. That elastic energy then gets converted to kinetic energy. Kinetic energy is energy that an object gains as it moves. We can see this in the kinetic energy equation [kinetic energy = 0.5*(mass*(velocity)^2)]. With this equation, we can see that as the velocity increases, the kinetic energy increases, rapidly. An increase in mass would also increase the kinetic energy but not nearly as much as an increase in velocity since the mass term is not squared.
So, with the balloon, as the air escapes, the balloon gets smaller and smaller and as a result, the elastic energy decreases. We know from Newton's Laws, that the energy in a system is constant. If we ignore losses (because who the heck wants to account for that, boring!), all of the elastic energy should then be converted to kinetic energy. This is exactly what we see. As the balloon decreases in size, the ship moves.
If we look at it from a forces perspective, it gets even spicier! Here's a thought experiment, if you blow up a balloon and tie it, then place it on a table, why does it just stay there? Why doesn't it move? It's a simple question with a simple but probably not intuitive answer. The reason it doesn't move is that the air inside is pressing on the balloon equally, in all directions. This begs the question, what if the air was pressing on the balloon unequally? What would happen then?
That's exactly the behavior we saw in the experiment, little one! As the nozzle is opened (the balloon is untied) the rubber contracts and this causes the balloon to push air out of the nozzle. Additionally, we know (from our fav, Newton) that every action has an equal and opposite reaction. This means that while the air is being pushed out of the nozzle, the air pushes back on the balloon. This results in a net force forwards, causing the ship to be propelled forward. While we used a balloon to demonstrate propulsion, the same principles are applied to real-life engines!