jm1234567890
Premium Member
Quote from google
Put another way, the reason you don't feel gravity is because you actually have no organs for detecting it. Seriously! We think we do, but in fact we can only detect acceleration. (For that matter, even scientific instruments designed to detect gravity actually don't -- they measure acceleration, and gravity is deduced from that.)
I had weightlessness explained to me once in a science class documentary, and although it was many years ago, the explanation really stuck with me.
If I stand normally on a scale placed firmly on the floor, the scale will read 150 lbs. (Yes, I'm a woman, and I've just given my true body weight in public! )
Now let's say I put my arm out against the wall. I lever off of that and push myself down into the scale. The weight climbs, perhaps to 170 lbs. Do I really weigh 170 lbs? Going the other way, lets say I put my arms on a table standing by the scale. I push on the scale, lifting a lot of my body weight up. The scale now reads 75 lbs. Do I really only weigh 75 lbs? My mass is unchanged.
So scales don't measure mass. They measure force. Since the amount of gravitational attraction exerted by the Earth remains basically constant (there are very slight differences in your weight in different places, but your average scale isn't sensitive enough to pick them out), we can work out mass based on how much force our bodies apply to the scale because of gravity pulling us downward. But we can screw it up if we add or decrease the amount of force our bodies are applying to the scale.
Since weight, unlike mass, is dependent on the amount of force we are imparting to the scale, we can screw it up by changing the amount of force. We've tried moving my body towards or away from the scale; what if the scale is moving too?
One important point: when gravity pulls you firmly to the ground, what it is really doing is imparting an acceleration to you. The only reason you don't actually speed up is because the floor is in the way. Take the floor away, and you will fall, accelerating all the way until you hit the actual ground.
I'm standing in an elevator at the ground floor of a truly enormous building. Imagine it is many miles tall, just for the sake of argument. On the floor of the elevator is a scale. I step on to the scale. The elevator begins to ascend rapidly. Now, the scale is moving upwards. But I am being pulled down by gravity. So the scale is moving up while my body is trying to move down in response to gravity. This means that the scale registers more force than my weight can produce by itself. It is now reading 180 lbs, but I still have a mass of only 150 lbs. I am experiencing what is called a "G force".
Let's say the elevator is rising *really* fast, and accelerating all the way. I'm now feeling 3 Gs, similar to what the astronauts in the Space Shuttle feel while they are rising into space. That means that I am feeling three times as much acceleration as I'm used to from gravity pulling me down. So I weigh three times as much. I now weigh 450 lbs. (No wonder it's so tough for the astronauts to move around during launch!)
Imagine now that my elevator is at the top of the impossibly tall building. For the sake of argument, let's also assume that the elevator rails are perfectly frictionless (in other words, it's a huge safety hazard!!!). I'm standing on the scale and the elevator starts to descent. It is now acclerating downwards at about half the acceleration of gravity. Since we're imagining it's frictionless, that means it's working to slow our descent. We're not just falling. My scale now says I weigh 75 lbs.
Now imagine somebody cuts the rope holding the elevator. There is nothing now to keep it from falling as fast as gravity wants it to. It is accelerating rapidly, and I'm accelerating inside of it. But I'm accelerating at exactly the same rate that it is, in exactly the same direction. Therefore, my feet are no longer putting any pressure on the scale at all. My weight now reads as 0 lbs. I still have a mass of 150 lbs, but my weight is now 0.
Of course, sooner or later the elevator is going to hit the bottom of the shaft. It'll be going very fast by then. It's true what they say: the fall won't kill you, but the stop will be simply smashing! Good thing this is only imaginary.
This is the same thing the famous Vomet Comet aircraft does, only unlike the elevator, it can safely pull out of the freefall. When you are in orbit (even an orbit that takes you as far away from Earth as the Moon), you're actually just falling the whole time, so you will be like me in the falling elevator -- you will be weightless.
It seems odd that you can be travelling *away* from the Earth and still be weightless. The trick to understanding that is to understand that in that case, you aren't accelerating. Like ricimer said, you are simply coasting. And if you're not accelerating, you can't perceive the true effect of gravity (which is actually acting very strongly on you, pulling what would be a straight line trajectory into a huge ellipse). You don't feel a wind rush, you don't feel the stuff inside your ears sloshing around, you don't feel pressure on the soles of your feet, you have no way to tell that gravity is there. So you're weightless, but not massless.
Actually, strictly speaking even in low orbit you aren't *totally* weightless. Just very close to it. And every time the spacecraft fires any of its thrusters, you'll feel the acceleration as if gravity was coming from the direction of that particular thruster, sometimes as much as a tenth of a G. The Apollo astronauts would have felt something that felt exactly like gravity, only vastly weaker, when their Saturn V third stage (the S-IVB) fired to put them on course for the Moon. And they would have felt it again when their Service Module fired to break them out of lunar orbit, pushed into their couches as if it were indeed gravity.
Put another way, the reason you don't feel gravity is because you actually have no organs for detecting it. Seriously! We think we do, but in fact we can only detect acceleration. (For that matter, even scientific instruments designed to detect gravity actually don't -- they measure acceleration, and gravity is deduced from that.)
I had weightlessness explained to me once in a science class documentary, and although it was many years ago, the explanation really stuck with me.
If I stand normally on a scale placed firmly on the floor, the scale will read 150 lbs. (Yes, I'm a woman, and I've just given my true body weight in public! )
Now let's say I put my arm out against the wall. I lever off of that and push myself down into the scale. The weight climbs, perhaps to 170 lbs. Do I really weigh 170 lbs? Going the other way, lets say I put my arms on a table standing by the scale. I push on the scale, lifting a lot of my body weight up. The scale now reads 75 lbs. Do I really only weigh 75 lbs? My mass is unchanged.
So scales don't measure mass. They measure force. Since the amount of gravitational attraction exerted by the Earth remains basically constant (there are very slight differences in your weight in different places, but your average scale isn't sensitive enough to pick them out), we can work out mass based on how much force our bodies apply to the scale because of gravity pulling us downward. But we can screw it up if we add or decrease the amount of force our bodies are applying to the scale.
Since weight, unlike mass, is dependent on the amount of force we are imparting to the scale, we can screw it up by changing the amount of force. We've tried moving my body towards or away from the scale; what if the scale is moving too?
One important point: when gravity pulls you firmly to the ground, what it is really doing is imparting an acceleration to you. The only reason you don't actually speed up is because the floor is in the way. Take the floor away, and you will fall, accelerating all the way until you hit the actual ground.
I'm standing in an elevator at the ground floor of a truly enormous building. Imagine it is many miles tall, just for the sake of argument. On the floor of the elevator is a scale. I step on to the scale. The elevator begins to ascend rapidly. Now, the scale is moving upwards. But I am being pulled down by gravity. So the scale is moving up while my body is trying to move down in response to gravity. This means that the scale registers more force than my weight can produce by itself. It is now reading 180 lbs, but I still have a mass of only 150 lbs. I am experiencing what is called a "G force".
Let's say the elevator is rising *really* fast, and accelerating all the way. I'm now feeling 3 Gs, similar to what the astronauts in the Space Shuttle feel while they are rising into space. That means that I am feeling three times as much acceleration as I'm used to from gravity pulling me down. So I weigh three times as much. I now weigh 450 lbs. (No wonder it's so tough for the astronauts to move around during launch!)
Imagine now that my elevator is at the top of the impossibly tall building. For the sake of argument, let's also assume that the elevator rails are perfectly frictionless (in other words, it's a huge safety hazard!!!). I'm standing on the scale and the elevator starts to descent. It is now acclerating downwards at about half the acceleration of gravity. Since we're imagining it's frictionless, that means it's working to slow our descent. We're not just falling. My scale now says I weigh 75 lbs.
Now imagine somebody cuts the rope holding the elevator. There is nothing now to keep it from falling as fast as gravity wants it to. It is accelerating rapidly, and I'm accelerating inside of it. But I'm accelerating at exactly the same rate that it is, in exactly the same direction. Therefore, my feet are no longer putting any pressure on the scale at all. My weight now reads as 0 lbs. I still have a mass of 150 lbs, but my weight is now 0.
Of course, sooner or later the elevator is going to hit the bottom of the shaft. It'll be going very fast by then. It's true what they say: the fall won't kill you, but the stop will be simply smashing! Good thing this is only imaginary.
This is the same thing the famous Vomet Comet aircraft does, only unlike the elevator, it can safely pull out of the freefall. When you are in orbit (even an orbit that takes you as far away from Earth as the Moon), you're actually just falling the whole time, so you will be like me in the falling elevator -- you will be weightless.
It seems odd that you can be travelling *away* from the Earth and still be weightless. The trick to understanding that is to understand that in that case, you aren't accelerating. Like ricimer said, you are simply coasting. And if you're not accelerating, you can't perceive the true effect of gravity (which is actually acting very strongly on you, pulling what would be a straight line trajectory into a huge ellipse). You don't feel a wind rush, you don't feel the stuff inside your ears sloshing around, you don't feel pressure on the soles of your feet, you have no way to tell that gravity is there. So you're weightless, but not massless.
Actually, strictly speaking even in low orbit you aren't *totally* weightless. Just very close to it. And every time the spacecraft fires any of its thrusters, you'll feel the acceleration as if gravity was coming from the direction of that particular thruster, sometimes as much as a tenth of a G. The Apollo astronauts would have felt something that felt exactly like gravity, only vastly weaker, when their Saturn V third stage (the S-IVB) fired to put them on course for the Moon. And they would have felt it again when their Service Module fired to break them out of lunar orbit, pushed into their couches as if it were indeed gravity.
