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### Course: Physics library>Unit 4

Lesson 3: Newton's law of gravitation

# Gravity for astronauts in orbit

Why do astronauts appear weightless despite being near the Earth? Created by Sal Khan.

## Want to join the conversation?

• ok.so if the space shuttle is moving super fast than how come those astronauts don't feel any thing when they go out of the ship for what ever reason they show it like they are motionless you know like floating away why don't they move downwards to the earth?
• When they step (or float) out of the shuttle, the astronauts are still moving at the same speed as the shuttle. Since there's nothing to slow them down, they continue to move at the same speed and direction as the shuttle. If someone were to jump out of a moving vehicle (don't try this at home!), the person would, just for an instant, be moving at the same exact speed as the vehicle. However, friction from the air around you—and the asphalt below—would quickly slow a person down. Because there's nothing to slow the astronauts down, they continue to move at the same speed as the shuttle.
• Sorry still didn't get it. Sal says, "The astronauts are going so fast, they keep missing the earth".
What is that supposed to mean? How can they "miss" the earth? And if they do miss it at a point, why do they keep orbiting?
Also, is this centripetal force? Then why doesn't the shuttle fly towards the centre of the earth?
I guess I can also ask the same for comets, planets etc.
• Try to imagine it using the centripetal force diagram. For example, tie a stone to a rope and spin it. Here your hand (assuming you are holding one end of the rope and spinning it) is the Earth, the stone is ISS and the Tension of the rope is gravity. I know this might be a bit difficult but hope it helps.
(1 vote)
• My mass is what the stuff im having in my body. if i get into space station i become free from gravity. now there, if i wish to measure my mass or weight, what is the method of doing it.
• Your mass is a measure of how difficult you are to accelerate given a certain amount of force (it can be described as the proportionality constant that relates these two vectors). Like this second definition implies, your mass is constant. Your weight, on the other hand, is the amount of force exerted upon you due to acceleration by a gravitational field. As gravitational acceleration varies depending on the strength of the field (you weight less on the moon or at the Earth's equator), this value is not constant. In space, assuming no gravitational field--or one that could be approximated as zero--you would be weightless. You would still have mass however that can be measured in space using what is called an "inertial balance." These devices use the oscillations of a spring with a known spring constant to determine the mass of an attached object and work in microgravity scenarios.
• Why don't the astronauts fall while spacewalking? I'm guessing it's beacause they are attached to the space station which is moving so they move too. But if they were not attached to it, would they just fall on the surface of the Earth?
• They do fall. In fact, both the astronauts and the space station and all other things in orbit are constantly falling in exactly the same way as a rock falls to the ground when you drop it. We say that things in orbit are in a free fall. The reason why they don't eventually hit the ground is that while they are falling downwards, they are also moving very quickly forwards.

The Earth is round. This means that the ground is curving away from you in every direction. The most visible effect this has is the horizon, which is the distance where the ground has curved so much away from you that you can't see it anymore. The fact that the Earth is curved also means that if you were to move forwards in a perfectly straight line, the ground would drop away from your feet. The faster you move, the faster the ground would drop. At a certain speed, the ground would be dropping away as fast as you would fall towards it. This means that if you move at this speed, even though you are constantly falling towards the ground, you never hit it, since the ground is curving away from you at the same rate. You are falling down, but by the time you have fallen to where the ground was, you have moved so far that the ground is still just as far below you. This is what we call orbiting.

Since you are in a free fall when you are in orbit (and you are also generally above the atmosphere, since that would slow you down enough to hit the ground pretty quickly) there is no force that is pushing on you like the ground pushes on you on the surface. This makes it seem like there is no gravity, but in fact the force of gravity is almost as strong at the space station as it is on the ground. It is just that instead of being canceled out by the force from your shoes or your chair, it is being canceled out by the centrifugal force of your orbit around the Earth. Since both the astronauts and the space station are moving with the same (really fast) velocity, they are at rest relative to each other, and the astronauts can float around the station on their space walks. Constantly falling, but never hitting the ground.
• If they have so high velocity so that they go around the earth that means that we cannot reach the moon.Because when ever we are going little far from earth (let we go little far than setalites) our shuttle would be in free fall and just moving round and round the earth.
• You are right - the space station cannot reach the moon.
When you want to reach the moon, you have to go faster, and on a path that is not a circular, orbital path.
• i cant understand Why do astronauts appear weightless despite being near the Earth?
• Imagine there is no atmosphere in the Earth. You're superman and are throwing rocks horizontally. When you throw the rock really really fast, it is possible for it to fall at the same rate the Earth is curving, which means it will stay rotating around the Earth forever.
This is the same principle that allows satellites and astronauts to stay in orbit. They are falling all the time, but missing the Earth every time. It looks like there is no gravity for them because everything around them is also falling with them.
• Is there any specifically measured distance between the earth and space that any spacecraft or body do not feel gravity?
• the gravitational field of the Earth extends infinitely throughout space but its intensity is so less that it doesn't effects bodies outside a certain limit. just like Andrew said, the strength of the gravitational field declines with the square of the distance which is known as the inverse square law;
1/x^2
• If the astronauts are near the earth and are experiencing gravity to keep them in orbit, why would they float, they should just be sitting on their seats and going around the earth, why is the gravity affecting the space shuttle- keeping it in orbit but not affecting the astronauts inside it and keeping them down in their seats?
• They are indeed going around the earth. As a part of the shuttle-astronaut system, they are acted on by gravity and they go around the earth.
What you are perhaps confusing about, is the force necessary to keep them in their seats. This must be a force that pulls them towards the seat while the seat does not concomitantly move downwards (otherwise they would not be able to sit in the seat). This is sort of a force acting on the astronaut relative (in a fuzzy way) to the seat, which gravity cannot provide since it acts on both the shuttle and the astronaut.
• Does time affect gravity? If it does, is time traveling through space time possible?
• More like the presence of a gravitational field has an influence on time.
• What creates mass and gravitational pull? As in, if we pulled a bunch of space junk together and created a planet-sized hunk of waste, would that have gravitational pull, and, if not, why wouldn’t it?