If we feel weightless because there is no contact force acting on our bodies. Then why are there no contact forces for people in the international space station but there is for people on airplanes?
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2$\begingroup$ I have felt weightless in an airplane. Doesn't happen offten in airline flights because the passengers really don't like it. $\endgroup$– Solomon SlowCommented Dec 3, 2020 at 19:00
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$\begingroup$ Lol, but that would be if the plane is free falling, which it is not does that mean the space station is free falling? And if it is...why isn’t it falling? Is it cuz of the centripetal force and stuff ? $\endgroup$– HELCommented Dec 3, 2020 at 19:04
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3$\begingroup$ An orbit is free fall. The space station is attracted toward the center of the Earth by gravity, but it is moving horizontally at tens of thousands of kilometers per hour. Douglas Adams wrote in his "Hitchhiker's guide" novels, that the secret to flying was to throw yourself at the ground and miss. That literally is how spacecraft stay in orbit. $\endgroup$– Solomon SlowCommented Dec 3, 2020 at 19:06
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$\begingroup$ It's cuz a stuff . $\endgroup$– my2ctsCommented Dec 3, 2020 at 20:44
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5 Answers
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why are there no contact forces for people in the international space station
"Contact force" is just what it sounds like: The force between two (approximately) rigid bodies that are in contact with each other. Astronauts in the ISS feel "contact force" any time they touch something.
The reason why they can "float" without touching anything is that both the astronaut and the space station itself are in practically identical orbits. They both are "falling" around the Earth together.
but there is for people on airplanes?
Ideally, when you're flying on a commercial airline, the plane will not "fall." Ideally, the plane will fly a "straight and level" course. In order for your path to be "level" in a plane that is traveling at MUCH less than orbital speed, something has to be holding you up---to stop you from falling. That would be your seat and the floor beneath your feet. Those, in turn are held up by the airframe, and the airframe (including the wings) is held up by continuously beating down the air through which the plane flies.
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$\begingroup$ Ok so firstly what I understood, is if their feet are touching the ground they will feel a reaction force but feel lighter because the reaction force is much smaller than on earth? $\endgroup$– HELCommented Dec 3, 2020 at 19:29
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$\begingroup$ And secondly they are floating because they are falling on earth without actually falling, the force of gravity only changes their direction? But if that’s so they must have a constanr mass velocity and radius right? Or else it wouldn’t work? $\endgroup$– HELCommented Dec 3, 2020 at 19:30
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$\begingroup$ No, something doesn't need to be holding you up for you to stay at a constant height above the earth's surface. If you go fast enough, your trajectory under pure gravity would be circular and thus you won't need anything holding you up. Freefall doesn't mean you fall to the earth. $\endgroup$– user87745Commented Dec 3, 2020 at 19:36
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1$\begingroup$ Just a comment to the word level: a satellite is also typically in a level path in its orbit. So whether a level path requires "something to hold you up" depends on altitute and speed. $\endgroup$– SteevenCommented Dec 3, 2020 at 19:44
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1$\begingroup$ @Hamza An astronaut in the ISS "standing" on the "floor" doesn't feel any normal force acting on their feet because they & the floor are both falling with the same velocity (speed & direction). BTW, the ISS has an altitude around 410 km, which is small compared to Earth's radius, and g at that height is about 0.89 of its value at sea level. $\endgroup$– PM 2RingCommented Dec 3, 2020 at 19:47
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For people on ISS , there is no external agency which can bring two surfaces in contact all the time because all the surfaces are falling due to gravity by the same rate but if they (by their own) touch any surface , they do feel the contact forces.
The main reason for your question is that the ISS is experiencing only gravitational force as seen from earth while the aeroplane is experiencing the gravitational force and a force which makes the aeroplane fly too. Thus the gravitational pull on the aeroplane is balanced by the uplifting force. So comparing the two cases as similar is meaningless.
And for people in aeroplane , their weight and the uplifting force both favours two surfaces in coming close to each other and hence they do experience contact force.
Hope it helps 🙂.
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Apparent weightlessness is when you only feel the force of gravity acting on you. The plane would have to be in freefall at an acceleration of approximately $9.8\;\rm m/s^2$ for you to feel weightless.
Astronauts feel apparent weightlessness if they are orbiting the earth. If they and the spacecraft are travelling at the 'right' orbital speed, then the force acting on them will only be the gravitational force.
If they touch something, they will feel a contact force.
answered Dec 3, 2020 at 19:08
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$\begingroup$ It uses fuel to get into orbit and achieve the necessary orbital speed. But once that speed is achieved, it will stay in orbit. It will use a bit of fuel to counteract some small amounts of drag to fix its course. $\endgroup$ Commented Dec 3, 2020 at 19:56
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The point is that if you are freely falling, i.e., if your motion is purely under the influence of gravity, then you feel weightless. However, it is not just because the pseudo force cancels out the force of gravity. Even when you are standing on the surface of the earth, the normal force from the ground cancels out the force of gravity on you--but you obviously don't feel weightless. The point is that
If you are falling purely under the force of gravity then the pseudo force acting on you will exactly cancel out the force of gravity -- for each part of your body.
Thus, parts of your body will not be pushed or pulled against the rest of your body, i.e., it will not induce any stress/pressure in your body. This is what is weightlessness. You feel the weight while standing up on the ground because while the normal force from the earth still cancels out the force of gravity acting on you, it doesn't do so uniformly across your body. Thus, your feet are under stress/pressure.
So, we need to aim to be moving purely under the force of gravity and without the influence of any other force if we want to feel weightless. Is the plane moving purely under the force of gravity? No. Why? Again, unlike what other answers suggest, it is not because the plane needs to be at a constant height above the ground and thus it can't afford to fall to the ground under the force of gravity. No, you don't need to fall to the ground under the force of gravity to feel weightless. You simply need to move purely under the force of gravity to feel weightless. Are there any other options than falling to the ground if you were to move purely under the force of gravity? Of course! One can move in a circular orbit around the earth and they will always remain at a constant height above the ground while moving purely under the force of gravity. However, in order for your motion to be in this circular orbit purely under the force of gravity, you need to have a certain specific tangential speed. Otherwise, if your speed is lower, your motion under pure gravity would be elliptical (or linear) and you will likely (or certainly) hit the ground. So, what is this tangential speed that you need to have for you to move in a circular orbit under the influence of pure gravity? Well, it varies with height but near the surface of the earth, it is $7.8\text{ km/s}$. A plane flies at around $0.25 \text{ km/s}$. That is the reason it won't move in a circular orbit purely under the force of gravity and would have to be influenced by additional forces to keep it at a fixed height. On the other hand, satellites do exactly what is required to follow a circular orbit under the influence of pure gravity. They have the required high tangential speed and that is why they can move under the influence of pure gravity and still not run into the earth.
Obviously, this is a somewhat idealized answer in order to elucidate the concept. In practice, the atmosphere also plays a big role in making sure that the motion of a plane is not purely under the influence of gravity.
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- You are in free fall on an orbiting space station.
- You are not on free fall in an airplane.
Basically, when the space station is on orbit then you and everything inside it is also in orbit. That is a free-fall condition where you are constantly falling (but missing) with no other forces than gravity acting on you (nothing is holding you up). It is the same situation as if a skydiver jumped from a plane with a chair. He and the chair would be falling equally fast beside one another and the chair does not lift anything.
In an airplane you are being held up by the plane, which feels lift forces and updrift forces in the atmosphere that hold up the plane. Here you are not falling.
