# The Naked Scientists Forum

### Author Topic: Weight, Gravity and Acceleration  (Read 1843 times)

#### EdwardHatterly

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##### Weight, Gravity and Acceleration
« on: 21/03/2015 20:16:35 »
If my weight on Earth is a function of F = m x a, where m is my mass, why do I have a weight at all when I am standing on the ground. I understand that the "a" component in the equation for my weight is gravitational acceleration, but my acceleration is zero at that point. If I was in free fall above the earth, I could understand this, but is not my weight while standing on earth equal to my mass times zero acceleration?

Ed

#### chiralSPO

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##### Re: Weight, Gravity and Acceleration
« Reply #1 on: 21/03/2015 20:38:50 »
Ftotal = m*a

You do not accelerate because there is an equal and opposite force from the ground pushing you up.

Fgravity + Fground = m*a = 0 because
Fgravity = –Fground

If you were trying to stand on a flimsy platform that could't supply that equal and opposite force (Fgravity > –Fplatform) then there would be a net force and net acceleration (the platform would deform or break as you moved down)
« Last Edit: 21/03/2015 20:40:38 by chiralSPO »

#### PmbPhy

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##### Re: Weight, Gravity and Acceleration
« Reply #2 on: 21/03/2015 21:18:28 »
If my weight on Earth is a function of F = m x a, where m is my mass, why do I have a weight at all when I am standing on the ground. I understand that the "a" component in the equation for my weight is gravitational acceleration, but my acceleration is zero at that point. If I was in free fall above the earth, I could understand this, but is not my weight while standing on earth equal to my mass times zero acceleration?

Ed
In general relativity one can view this as you being in an accelerating frame of reference and the force that you're experiencing is the force of the ground pushing you, accelerating you through space. That's due to the nature of the equivalence principle which states
Quote
A uniform gravitational field is equivalent to a uniformly accelerating frame of reference.
All gravitational fields match this condition locally, i.e. in a region of space small enough and for a time period short enough for the purposes of the experiment/observation at hand.

#### Colin2B

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##### Re: Weight, Gravity and Acceleration
« Reply #3 on: 21/03/2015 22:42:37 »
...... If I was in free fall above the earth, I could understand this, ....

Your sensation of weight also comes from all parts of your body pressing down and being resisted by each part of the body below them.
If you are in free fall, you would have no sensation of weight, precisely because you would have no force from the ground pushing against your feet. Also, other parts of the body, particularly the organs in the abdominal cavity, are no longer pressed down leading to that peculiar 'pit of the stomach' sensation you get on fairground rides.

#### EdwardHatterly

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##### Re: Weight, Gravity and Acceleration
« Reply #4 on: 22/03/2015 00:33:26 »

Ed

#### PmbPhy

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##### Re: Weight, Gravity and Acceleration
« Reply #5 on: 22/03/2015 01:20:48 »

Ed
You're welcome. And welcome to the forum. Will you be staying for a while or was this just a one time question?

#### The Naked Scientists Forum

##### Re: Weight, Gravity and Acceleration
« Reply #5 on: 22/03/2015 01:20:48 »