This week, I settled down to observe the primary episode of The 100. If you have not seen the present, I will simply level out that it takes place within the close to future (although it ran, on the CW, within the close to previous). For causes that I will not get into, there’s a spacecraft with a bunch of youngsters that’s touring from an area station right down to the floor of the Earth. Through the reentry course of, one child desires to indicate that he’s the grasp of house journey and that he is superior. So what does he do? He will get out of his seat and floats round as an indication of his mastery of weightlessness. One other teenager factors out that he is being fairly dumb—and that he will get harm very quickly.
OK, that’s sufficient of the outline of the scene in order that we will discuss physics. The purpose is that there’s one dude “floating” round within the spacecraft throughout reentry.
Earlier than I over-analyze this brief scene, let me add a caveat about my philosophy on science and tales. I’ve talked about this earlier than, so I will simply give a abstract: The primary job for a author of a present is to inform a narrative. If the author distorts science with a purpose to make the plot transfer alongside—so be it. Nevertheless, if the science might be right with out destroying the plot, then clearly I would choose it.
On to the over-analysis!
What Causes Gravity?
Clearly this scene has to do with gravity, so we should always discuss gravity—proper? In brief, gravity is a elementary interplay between objects with mass. Sure, any two objects which have mass can have a gravitational drive pulling them collectively. The magnitude of this gravitational drive will depend on the space between the objects. The additional aside the objects get, the weaker the gravitational drive. The magnitude of this drive additionally will depend on the plenty of the 2 objects. Higher mass means a higher drive. As an equation, this could be written as:
On this equation, the plenty are described by the variables m1 and m2 and the space between the objects is the variable r. However a very powerful factor is the fixed G—that is the common gravitational fixed and it has a worth of 6.67 x 10-11 Nm2/kg22. Which may appear to be it is necessary, so let me give an instance that everybody can relate to. Suppose you’re standing someplace and your pal is true there with you and also you two are having a dialog. Because you each have mass, there’s a gravitational drive pulling the 2 of you collectively. Utilizing tough approximations for distance and mass, I get a pretty drive of three x 10-7 Newtons. Simply to place that into perspective, this worth is pretty near the drive you’d really feel for those who put a grain of salt in your head (sure, I’ve an approximate worth for the mass of 1 grain of salt).
So, the gravitational drive is tremendous tiny. The one manner we ever discover this drive is that if one of many interacting objects has an excellent enormous mass—one thing just like the mass of the Earth (5.97 x 1024 kg). In the event you substitute your pal with the Earth and put the space between you and your friend-Earth because the radius of the Earth, you then get a gravitational drive of one thing like 680 Newtons—and that could be a drive you possibly can really feel (and also you do).
Is There Gravity in House?
Now for the actual query. Why do astronauts float round in house except there isn’t any gravity? It certain looks like there isn’t any gravity in house—it is even known as “zero gravity.” OK, I’ve answered this earlier than, nevertheless it’s necessary sufficient to revisit the query.
The brief reply is “sure”—there may be gravity in house. Look again on the gravitational equation above. What modifications in that equation as you progress from the floor of the Earth into house? The one distinction is the space between you and the middle of the Earth (the r). In order the space will increase, the gravitational drive decreases—however by how a lot does the gravitational drive change? How a few fast estimation?
Let’s use an Earth radius of 6.371 x 106 meters. With this worth, an individual with a mass of 70 kg would have a gravitational drive of 686.7 Newtons. Now shifting as much as the orbital top of the Worldwide House Station, you’d be an additional 400 km farther from the middle. Recalculating with this higher distance, I get a weight of 608 Newtons. That is about 88 % the worth on the floor of the Earth (you possibly can examine all my calculations right here). However you possibly can see there may be clearly gravity in house.
Oh, right here is a few further proof. Why does the moon orbit the Earth? The reply: gravity. Why does the Earth orbit the Solar? Yup, it is gravity. In each of those circumstances, there’s a vital distance between the 2 interacting objects—however gravity nonetheless “works,” even in house.
However why do astronauts float round in house? Nicely, they float round when in orbit—if there was an excellent tall tower reaching into house, they would not float round. The “weightless” setting is attributable to the orbital movement of the folks inside a spacecraft or house station. Right here is the actual deal. If the one drive appearing on a human is the gravitational drive, that human feels weightless. Standing on a tall tower would end in two forces (gravity knocking down and the tower pushing up). In orbit, there may be solely the gravitational drive—resulting in that feeling of weightlessness.
Truly, you do not even must be in orbit to really feel weightless. You might be weightless by having the gravitational drive as the one factor appearing on you. Here’s a scenario so that you can think about. Suppose you’re standing in a stationary elevator on the high of a constructing. Since you’re at relaxation, the entire drive should be zero—which means the downward gravitational drive knocking down is balanced by the upward pushing drive from the ground. Now take away the drive from the ground. Sure, that is troublesome however it may be achieved. Simply have the elevator speed up down with the identical acceleration as a free falling object. Now you may be falling inside an elevator. The one drive is gravity and you may be weightless.
Some folks suppose this falling elevator is enjoyable. That is why many amusement parks have a journey like The Tower of Terror. Mainly, you get in a automotive that drops off a tower. Through the fall, you are feeling weightless—however you do not crash on the backside. As a substitute, the automotive is on a monitor that one way or the other slows down extra progressively than if it smashed into the bottom. They’ve one among a lot of these rides on the NASA heart in Huntsville. went on this with my youngsters—it was truly scarier than I had imagined.
How about one other instance? If you’re in an airplane and the aircraft flies with a downward acceleration, everybody inside might be weightless. Even a canine. Test it out.
In the long run, there appears to be enormous misunderstanding about gravity. I consider the reasoning follows like so: Astronauts are weightless in house. There isn’t any air in house. Due to this fact, if there isn’t any air, there isn’t any gravity. This no-air/no-gravity thought pops up on a regular basis in motion pictures (incorrectly so).
This is how you will see it: Some dude is floating round in house (that is OK) after which he enters the airlock of a spacecraft, nonetheless floating. The airlock door shuts and air is pumped into the chamber and increase—he falls to the bottom as a result of now there’s gravity.
Here’s what it ought to seem like—from the epic film 2001: A House Odyssey. SPOILER ALERT: Hal is loopy and will not open the pod-bay doorways. Not even for Dave.
Wow. That scene is just about excellent. They even don’t have any sound till the air is available in.
What Occurs Throughout Reentry?
Now again to the occasions in The 100. The scene does not happen in orbit, it happens throughout reentry. That is the half the place the spacecraft enters again into the environment and encounters an air resistance drive (as a result of there may be air). Let me begin with a easy drive diagram exhibiting the spacecraft sooner or later throughout this movement.
Clearly, this not weightless. Sure, there’s a gravitational drive appearing on all the pieces—however there may be additionally that air drag drive that may make the spacecraft decelerate because it strikes down. If the human goes to remain contained in the spacecraft, there should even be an additional drive on that human (from the ground). So, not weightless—in actual fact, the human would really feel extra than regular gravity due to the acceleration. You already know this, although, as a result of the very same factor occurs to you in an elevator. Because the elevator is shifting down and coming to a cease, it is usually slowing down. Throughout this time, you’d really feel slightly bit heavier due to the drive from the ground pushing on you. You are not actually heavier, you simply really feel that manner due to the acceleration.
Once more, there may be one other film instance the place somebody will get this reentry physics proper. It is from Apollo 13. Test it out.
Discover the water falling from the ceiling. On this case, the capsule is shifting downward at an angle. Nevertheless, the air resistance drive is pushing in the other way of movement inflicting the spacecraft to decelerate. However what slows down the water? The water does cling to the floor slightly bit—however the acceleration is an excessive amount of to maintain it there and it “falls” in direction of the astronaut. Notice that “falling” right here does not imply straight in direction of the floor of the Earth however reasonably simply in the other way because the acceleration.
Trying again on the scene from The 100, this is how they might repair the scene—and it is fairly easy. Have the daring floating man transfer round earlier than they get to reentry. Then the opposite guys fall as quickly because the spacecraft begins to work together with the environment. That would not even change the plot—and it could be extra scientifically correct.