This week, I settled down to look at 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 all the way down to the floor of the Earth. Throughout the reentry course of, one child desires to point out that he’s the grasp of area journey and that he is superior. So what does he do? He will get out of his seat and floats round as an illustration 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 are able to 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 before, 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. Nonetheless, if the science might be appropriate with out destroying the plot, then clearly I would desire it.
On to the over-analysis!
What Causes Gravity?
Clearly this scene has to do with gravity, so we should always discuss gravity—proper? Briefly, gravity is a basic interplay between objects with mass. Sure, any two objects which have mass could have a gravitational power pulling them collectively. The magnitude of this gravitational power is dependent upon the space between the objects. The additional aside the objects get, the weaker the gravitational power. The magnitude of this power additionally is dependent upon the plenty of the 2 objects. Higher mass means a larger power. As an equation, this is able to 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 crucial factor is the fixed G—that is the common gravitational fixed and it has a price of 6.67 x 10-11 Nm2/kg22. That may seem to be it is vital, so let me give an instance that everybody can relate to. Suppose you’re standing someplace and your buddy is correct there with you and also you two are having a dialog. Because you each have mass, there’s a gravitational power pulling the 2 of you collectively. Utilizing tough approximations for distance and mass, I get a lovely power of three x 10-7 Newtons. Simply to place that into perspective, this worth is pretty near the power you’ll really feel in the event you put a grain of salt in your head (yes, I have an approximate value for the mass of one grain of salt).
So, the gravitational power is tremendous tiny. The one manner we ever discover this power 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). For those who substitute your buddy with the Earth and put the space between you and your friend-Earth because the radius of the Earth, then you definately get a gravitational power of one thing like 680 Newtons—and that may be a power you may really feel (and also you do).
Is There Gravity in Area?
Now for the true query. Why do astronauts float round in area until there is no such thing as a gravity? It positive looks as if there is no such thing as a gravity in area—it is even known as “zero gravity.” OK, I’ve answered this before, nevertheless it’s vital sufficient to revisit the query.
The brief reply is “sure”—there’s gravity in area. Look again on the gravitational equation above. What adjustments in that equation as you progress from the floor of the Earth into area? The one distinction is the space between you and the middle of the Earth (the r). In order the space will increase, the gravitational power decreases—however by how a lot does the gravitational power change? How a couple of 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 power of 686.7 Newtons. Now shifting as much as the orbital peak of the Worldwide Area Station, you’ll be an additional 400 km farther from the middle. Recalculating with this larger distance, I get a weight of 608 Newtons. That is about 88 p.c the worth on the floor of the Earth (you can check all my calculations here). However you may see there’s clearly gravity in area.
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 instances, there’s a important distance between the 2 interacting objects—however gravity nonetheless “works,” even in area.
However why do astronauts float round in area? Nicely, they float round when in orbit—if there was an excellent tall tower reaching into area, they would not float round. The “weightless” setting is attributable to the orbital movement of the individuals inside a spacecraft or area station. Right here is the true deal. If the one power appearing on a human is the gravitational power, that human feels weightless. Standing on a tall tower would end in two forces (gravity flattening and the tower pushing up). In orbit, there’s solely the gravitational power—resulting in that feeling of weightlessness.
Truly, you do not even must be in orbit to really feel weightless. You could be weightless by having the gravitational power as the one factor appearing on you. Here’s a scenario so that you can take into account. Suppose you’re standing in a stationary elevator on the high of a constructing. Since you’re at relaxation, the overall power have to be zero—meaning the downward gravitational power flattening is balanced by the upward pushing power from the ground. Now take away the power from the ground. Sure, that is troublesome however it may be completed. Simply have the elevator speed up down with the identical acceleration as a free falling object. Now you can be falling inside an elevator. The one power is gravity and you can be weightless.
Some individuals suppose this falling elevator is enjoyable. That is why many amusement parks have a trip like The Tower of Terror. Mainly, you get in a automobile that drops off a tower. Throughout the fall, you’re feeling weightless—however you do not crash on the backside. As a substitute, the automobile is on a monitor that in some way slows down extra progressively than if it smashed into the bottom. They’ve certainly one of most of these rides on the NASA heart in Huntsville. went on this with my children—it was really scarier than I had imagined.
How about one other instance? In case you are in an airplane and the aircraft flies with a downward acceleration, everybody inside will likely be weightless. Even a canine. Test it out.
In the long run, there appears to be enormous misunderstanding about gravity. I imagine the reasoning follows like so: Astronauts are weightless in area. There is no such thing as a air in area. Due to this fact, if there is no such thing as a air, there is no such thing as a gravity. This no-air/no-gravity concept pops up on a regular basis in motion pictures (incorrectly so).
This is how you will see it: Some dude is floating round in area (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 Area 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 would 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 power (as a result of there’s air). Let me begin with a easy power diagram exhibiting the spacecraft sooner or later throughout this movement.
Clearly, this not weightless. Sure, there’s a gravitational power appearing on every little thing—however there’s additionally that air drag power 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 power on that human (from the ground). So, not weightless—the truth is, 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, additionally it is slowing down. Throughout this time, you’ll really feel just a little bit heavier due to the power from the ground pushing on you. You are not actually heavier, you simply really feel that manner due to the acceleration.
Once more, there’s 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. Nonetheless, the air resistance power is pushing in the wrong way of movement inflicting the spacecraft to decelerate. However what slows down the water? The water does cling to the floor just a little bit—however the acceleration is an excessive amount of to maintain it there and it “falls” in direction of the astronaut. Observe that “falling” right here doesn’t suggest straight in direction of the floor of the Earth however slightly simply in the wrong way because the acceleration.
Wanting again on the scene from The 100, here is how they may 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.