lecture_24 - Lecture #24: Microgravity What is...

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-1 Lecture #24: Microgravity What is weightlessness, really? You’ve probably heard the terms weightlessness, zero-g or zero-gravity, and freefall . They all refer to the same sensation, which is what astronauts experience when they’re either spacewalking or on board a spacecraft that’s in a natural orbit. For obscure reasons, the preferred term among the NASA crowd for this type of environment is microgravity . Weightlessness is one of the most frequently misunderstood concepts in science, and so we should spend some time getting a handle on it. Most people think that astronauts are weightless in space because there is no gravity there, or at least because Earth’s gravity is very weak there. But this is not the case . In fact, the strength of Earth’s gravitational pull on, say, the astronauts on the International Space Station in LEO, is only slightly less than the pull we all feel here on Earth’s surface. So right away, we need to recognize that weightlessness is something that can be experienced in the presence of gravity . In fact, gravity is a universal force, and there is no place in the universe that is entirely free of gravity. So the only way to experience weightlessness is in the presence of gravity. So when do you feel weightless? You feel weightless whenever gravity is the only force acting on you – i.e., when you’re in a natural orbit. Huh? Well, here goes. First let’s make sure we understand what it means to experience weight (as opposed to weightlessness). Here on Earth, there is of course a force of gravity on you which pulls you down toward the center of the planet. But gravity alone is not enough to generate the sensation of weight. When you step on a bathroom scale, the scale does not measure the strength of the Earth’s gravitational pull on you. Instead it measures the compression of the springs inside the scale, which is due to the upward contact force between the bottoms of your feet and the scale. The contact force is a force you feel whenever some part of you is touching a surface. While you’re standing on the scale in Earth normal gravity (what we call 1-g), the upward contact force precisely balances out the downward gravitational force, and so their magnitudes are equal. In this case, then, the scale is effectively reading your true weight. (It is important that the contact force be present – if it weren’t, the force of gravity would accelerate you downward toward the center of the Earth.) But we’ve all had the experience of feeling either heavier or lighter than usual. This can happen on a high-speed elevator. If you were to take a bathroom scale with you on the elevator, it would measure the contact force on you – and that can change depending on the acceleration of the elevator. Suppose your true weight is 150 lbs. When you’re at
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rest, the gravitational and contact forces on you will balance, and the scale will read 150 lbs. When the elevator starts to accelerate upward, the contact force will increase. Suppose the elevator accelerates upward at 0.1 g.
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lecture_24 - Lecture #24: Microgravity What is...

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