# Difference between Wieghtless and Freefall?

## Main Question or Discussion Point

It is say when an astronaut is in a spaceshuttle orbiting the Earth, he feels "weightless".

My physics teacher says that is not true, he said that it is mere the sensation of the astronaut not physical reality.

But from my knowledge, the weighless felt by the astronaut is because of he is constantly frefalling towards earth. And according to relativity, a freefalling frame of reference is equivalent to an inertial frame of reference. So, doesn't that means that the sensation felt by the astronaut is the same as physical reality?

Related Other Physics Topics News on Phys.org
Well you say the astronaut "feels" weightless, then your physics teacher says it's his "sensation." Are they not the same thing?

I think all your teacher was saying is that the astronaut is not actually weightless, in that there is a substantial gravitational force acting between the astronaut and the earth, he just merely feels weightless because of his acceleration. At least that's what I read into his comment.

jcsd
Gold Member
Yep, that's what he means. Weight is the force dragging you towards the centre of the Earth (= mg), this force is still acting on the astronaut even though he's in orbit and therefore freefall.

Integral
Staff Emeritus
Gold Member
But the astronaught is weightless! Weight measured is in lbs or Newtons and is only felt on the surface of the earth where there is a reaction force (the earth pushing up on your feet). The astronaught is not massless but he is weightless.

jcsd
Gold Member
The force's still acting on you, therefore you have 'weight'.

neutroncount
Originally posted by Integral
But the astronaught is weightless! Weight measured is in lbs or Newtons and is only felt on the surface of the earth where there is a reaction force (the earth pushing up on your feet). The astronaught is not massless but he is weightless.
In deep space he'd be "weightless". It's different in orbit. That's just freefall. Like falling out of a plane.

russ_watters
Mentor
Tomato...

Staff Emeritus
Gold Member
Dearly Missed
What we feel as weight is not the downward force -gm (where m is our mass) but rather the upward force +gm which we have to exert to resist the downward one. This tenses our muscles and we feel it when we do squats or push ups.

In free fall the force -gm is still there, but it is unresisted (that is what is meant by 'free'), so the astronaut is falling. Being in orbit means to fall. So "the force is with him", but what he is accustomed to call his weight is not.

jcsd
Gold Member
What we feel as weight is not the downward force -gm (where m is our mass) but rather the upward force +gm which we have to exert to resist the downward one. This tenses our muscles and we feel it when we do squats or push ups.

In free fall the force -gm is still there, but it is unresisted (that is what is meant by 'free'), so the astronaut is falling. Being in orbit means to fall. So "the force is with him", but what he is accustomed to call his weight is not.
The standard defintion of weight is the force of graviational attraction not the reaction force.

Integral
Staff Emeritus
Gold Member
What we measure, and call weight is the reaction force. What we feel and call weight is the reaction force. So for all practical purposes the reaction force IS the weight.

russ_watters
Mentor
Originally posted by jcsd
The standard defintion of weight is the force of graviational attraction not the reaction force.
Hey, so what are you measuring if you flip over your bathroom scale?

Katsup....

Ketchup.

This is a really pointless thread.

All matter has gravity. We attract the earth just as the earth attracts us. The astronauts are attracted to the spacecraft and vice-versa. This force is not as strong and may not even be measureable, but is there none-the-less wherever they are.

Originally posted by Artman
All matter has gravity. We attract the earth just as the earth attracts us. The astronauts are attracted to the spacecraft and vice-versa. This force is not as strong and may not even be measureable, but is there none-the-less wherever they are.
Fo real? :)

Originally posted by AndersHermansson
Fo real? :)
I know, but they all seem to have neglected that in the previous responses, so I thought I would throw it in.

What the astronaut no longer feels is the reaction (sensation) of (his/her) "inertial mass" to the gravitational field of the Earth as the Earth's field effect has been subsumed by the ongoing free fall event.

That help?

In my previous post I said that the astronaut freefalling alone with the spaceshuttle, in this scenario, and I'm fairly convinced that he is not be free of forces because of the mutual gravitational atraction between him and the space shuttle.

But would the astronaut (or anyone) be truely free of all external forces if only he or she is present freefalling with respect to Earth and with no other matter in the system?

I read a book that says if you fall through an imaginary tunnel right through the center of a earth, during that time, you would be in an inertial reference frame.

If what I recalled above is true, then surely the anyone who is frefalling with no other matter freefalling in the system, it would be correct to say he or she is "physically" weightless.

LURCH
Originally posted by Hyperreality
In my previous post I said that the astronaut freefalling alone with the spaceshuttle, in this scenario, and I'm fairly convinced that he is not be free of forces because of the mutual gravitational atraction between him and the space shuttle.

But would the astronaut (or anyone) be truely free of all external forces if only he or she is present freefalling with respect to Earth and with no other matter in the system?

I read a book that says if you fall through an imaginary tunnel right through the center of a earth, during that time, you would be in an inertial reference frame.

If what I recalled above is true, then surely the anyone who is frefalling with no other matter freefalling in the system, it would be correct to say he or she is "physically" weightless.
But this reproduces the situation of a person inside a shuttle. Inside the tunnel, there is something exerting gravitational influence on the person, but from all directions around him. In the shuttle, the vehical pulls on the astronaut, but from all around him (although I suppose the pull is greater from below and near the tail).

But I'm fairly certain that none of this is what the physics etacher in the original post was reffering to. He seems to mean that an individual in freefall is not weightless because if they were, they wouldn;t be "falling". With this I must respectfully dissagree. If an object is following it's own worldline without acceleration forces, it is weightless.