My weight is zero!

Classical Mechanics Level 1

Two friends are discussing: 

Friend 1: Weight of astronauts in a satellite orbiting the Earth is zero.

Friend 2: I disagree, they just feel weightless but their weight is still non-zero.

Who is correct?

Friend 2 Friend 1

Munem Shahriar by Munem Shahriar , 18, Bangladesh

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3 solutions

Mike Davison
Sep 27, 2017

It's true that people in orbit aren't actually weightless, they're constantly accelerating toward the earth which negates any weight they would feel due to the gravitational attraction of the earth.

But I'd argue that people in deep intergalactic space would be essentially weightless seeing as any gravitational forces acting upon them would be utterly negligible...

Looking forward to comments, this could make for a very interesting discussion!

5 Helpful 0 Interesting 0 Brilliant 0 Confused

I agree.

An orbit is a free-fall state, which brings with it a sensation of weightlessness without actually being weightless.

But being subject to net zero forces does mean that a body has no weight.

However, the problem has been updated to include an orbit, so there's not as much need to discuss the potential implications of simply being in outer space.

Jonathan Quarrie - 3 years, 8 months ago

I disagree with assumption that forms the basis of this question.

The assumption is that the weight is defined as "the force of gravity acting on a body of mass". There is also the the definition of the "feeling" of the weight: "the normal force that supports you and prevents you from falling". This is 18th century physics.

Since Einstein developed a theory of general relativity we know that there is no force of gravity, only curvature of space time. The only meaningful definition of weight is the second one. There is no distinction between the force of gravity that a person "feels" and the "real" force of gravity. In the curved space time the natural state of matter is free falling, and on Earth (or Moon e.t.c.) you need a force to keep you static relative to your environment.

To be more specific, there are no experiments that probe your local environment that can distinguish between fee falling close to Earth and floating in deep space. This follows from the exact equivalence of the inertial mass (the mass in F=ma) and the gravitational mass (the mass in F=mg). This, in turn, motivated Einstein's theory.

Las Miha - 3 years, 8 months ago
Steven Chase
Sep 27, 2017

This so-called "weightlessness" is actually a condition in which the only force acting on you is your weight (with the typical reaction force having been removed).

2 Helpful 0 Interesting 0 Brilliant 0 Confused
Syed Hamza Khalid
Sep 27, 2017

Our weight doesn't depend on the factors of gravity. It is just what we feel.

2 Helpful 0 Interesting 0 Brilliant 0 Confused

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