Falling object, weight or no weight?!

[RyanJ]

Hi everyone,

 

Firstly let me say thank you to all that replied to my last thread here - it was very informative and very interesting to read and still people are posting - WOW!

 

Now to my question:

 

If I were to fall out of a plane with a weighing scales and I were to attept to stand on the scales, I would register 0 (Right?) as I am falling at a constant rate relative to the scales.

 

My question is would I actually have 0 weight? it seems that the moment you stop resisting the pull of gravity you become weightless.

 

I can't answer this question myself so I'm looking ay you (The physics experts) to answer my question

 

 

Cheers,

 

Ryan Jones

[swansont]

You are weightless in freefall. Accoding to GR, a gravitational field is an accelerated frame, so accelerating at that value actually puts you at rest.

[5614]

You are weightless in freefall

I can see how if you place some scales beneath you they will not register any weight due to the fact that you and the scales are moving at the same speed and you're not exerting any force on it (lets ignore fractions of a Newton due to different air resistances).

 

But if you take the definition of weight to be mass x gravity then surely you'd still get a value, as you always have mass and gravity exists in freefall (obviously!) so if you say w=mg then you still get a value for w so do you still have a weight in that respect.

 

But then in freefall you can never exert a force... but if the thing below you was a parachute which has more air resistance and thus moves slower when you come into contact with it you exert a force on it, this force must surely be weight.

 

I've never really thought about weight in freefall, anyone want to expand on the above?

[Douglas]

But then in freefall you can never exert a force... but if the thing below you was a parachute [/b']which has more air resistance and thus moves slower when you come into contact with it you exert a force on it, this force must surely be weight.

But what if the parachute is above you?

[Mobius]

weight is a force... The force we exert on the ground due to our mass and gravity. In freefall we still have mass but we are weightless as the force is causing us to accelerate.

[swansont]

I can see how if you place some scales beneath you they will not register any weight due to the fact that you and the scales are moving at the same speed and you're not exerting any force on it (lets ignore fractions of a Newton due to different air resistances).

 

But if you take the definition of weight to be mass x gravity then surely you'd still get a value' date=' as you always have mass and gravity exists in freefall (obviously!) so if you say w=mg then you still get a value for w so do you still have a weight in that respect.

 

But then in freefall you can never exert a force... but if the thing below you was a parachute which has more air resistance and thus moves slower when you come into contact with it you exert a force on it, this force must surely be weight.

 

I've never really thought about weight in freefall, anyone want to expand on the above?[/quote']

 

 

You will register no weight on a scale, but by one definition you have a weight based on W=mg, as you say. So it depends on what definition you use. So it's a matter of semantics - one needs to define what they mean.

[Tom Mattson]

But if you take the definition of weight to be mass x gravity then surely you'd still get a value' date=' as you always have mass and gravity exists in freefall (obviously!) so if you say w=mg then you still get a value for w so do you still have a weight in that respect.

[/quote']

 

You're right, it does depend on how you define "weight". If you define "weight near Earth" as "the gravitational force exerted on a mass by the Earth" then surely you are not weightless even when in freefall near the Earth. When teaching Physics I, I used to assign an exercise in which students used Newton's law of gravitation to calculate the weight of astronauts orbiting the Earth. This was necessary because the visual image of the astronauts floating around inside their orbiter would lead them to just assume that the result would be zero, despite the fact that the gravitational force in Newton's theory never goes to zero. Go figure.

 

Anyway, the statement "objects in free fall are not weightless" is wholly dependent on whether you are working from Newton's gravity or Einstein's.

[MattC]

You would have weight, in two senses.

 

Weight is a force. You have mass no matter what (unless your entire body is somehow converted to light). That force is due to your mass being tugged on by the Earth's gravity, and vica-verca. The force pulls you toward the Earth. Just because youa re falling does not mean there is no force on you - it just means there is no normal force acting in the opposite direction on you.

 

The normal force is the force the earth's surface exerts against you as gravity pulls you toward it. Any object at rest has all of the forces acting on it balanced. Since you are not moving (relative to the Earth) when you are standing still, and since you have a weight (a downward force), you must have an upward force. Hence the normal force.

 

So, you still have that force of gravity/weight acting on you in freefall. It's there, of course .. it's just not counter balanced.

 

The scales, however, do not measure weight/force, exactly. They measure relative weight. Or, better put, they measure the force that is actually being put against them.

 

Now, since you are falling and the scale is falling, it will not register your weight (not entirely true - I'll explain why at the end). Both you and the scale fall, so there is no pressure on you that is pushing the scales against the Earth.

 

Now, the exception. There IS a force pushing the scales back against you, even in Freefall.

 

Unless the mass of the scales were so great (so much greater than yours) that air resistence did not play any role, the scales would be buffed by the air more than you, and therefor there would be a force pushing the scales upward, relative to you, and thus you would appear to have a weight while standing on those scales. How much weight would depend upon the mass of the scales, the shape of the scales, the density of air (your altitude, basically), and perhaps a few other little things I'm overlooking.

 

I hope that helps!

[RyanJ]

Thanks all.

 

So I am to understand that there can be weight depending on:

 

1) Whos formulatiosn you use.

2) How you actually define weight.

 

 

Thanks for the information, very insightful

 

Cheers,

 

Ryan Jones