# The Naked Scientists Forum

### Author Topic: How fast am I going, relative to the Earth, when standing still?  (Read 3847 times)

#### Anywho

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##### How fast am I going, relative to the Earth, when standing still?
« on: 23/10/2012 05:16:26 »
The title sounds wrong I know, but when I am standing still I am exerting a force on the Earth which can be measured with a set of scales.

How fast would I have to be travelling in space to get the same reading on a set of scales?

#### Ophiolite

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##### Re: How fast am I going, relative to the Earth, when standing still?
« Reply #1 on: 23/10/2012 08:41:16 »
It's not a question of speed, but of acceleration.

#### syhprum

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##### Re: How fast am I going, relative to the Earth, when standing still?
« Reply #2 on: 23/10/2012 09:56:34 »
At relatively low speeds a power source beneath the scales would have to be accelerating you at 9.82 meters per second per second for you to get the same reading on the scales as you would get on the surface of the Earth.

#### Anywho

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##### Re: How fast am I going, relative to the Earth, when standing still?
« Reply #3 on: 23/10/2012 14:09:16 »
I should have mentioned that I only want to know how fast I would have to be going to get the scales to read the same weight (force?) temporarily.

So if I weigh 100kgs on Earth, how fast would I have to be travelling to get the scales to read 100kgs in space?

I realise that in the absence of gravity the scales will only read 100kgs temporarily.

#### syhprum

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##### Re: How fast am I going, relative to the Earth, when standing still?
« Reply #4 on: 23/10/2012 15:33:15 »
As Ophiolite  and myself have pointed out speed is irelevant at any speed the scales would read zero unless you are accelerating
« Last Edit: 24/10/2012 09:21:44 by syhprum »

#### Anywho

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##### Re: How fast am I going, relative to the Earth, when standing still?
« Reply #5 on: 23/10/2012 23:41:30 »
As Ophiolite  and myself have pointed out speed is relevant at any speed the scales would read zero unless you are accelerating

Okay, I don't think I've made myself clear, the scales are stationary and immovable, you hurl yourself at them to get a reading, how fast would you have to be going to get the scales to hit the 100kgs you weigh in Earth?

Or alternatively, if there is a pane of glass that would only just break if you were to stand on it under Earths gravity, how fast would you have to collide with the same pane of glass in space in order to break it?

#### syhprum

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##### Re: How fast am I going, relative to the Earth, when standing still?
« Reply #6 on: 24/10/2012 09:19:16 »
It would depend on how much the scale surface moved when you landed on it if you were moving at one meter per second and the scale surface deflected 1/9.82 meters i.e 10.183 cm your 100kgs weight would be recorded.

#### Ophiolite

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##### Re: How fast am I going, relative to the Earth, when standing still?
« Reply #7 on: 24/10/2012 17:41:16 »
In other words what is your temporary acceleration when you hit the scale? It will be negative, i.e. a deceleration. As syphrum points out, depending upon the 'give' of the scale will depend the speed with which you need to impact the scale.

(We're not being difficult, just needfully pedantic.)

#### Anywho

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##### Re: How fast am I going, relative to the Earth, when standing still?
« Reply #8 on: 25/10/2012 01:14:04 »
(We're not being difficult, just needfully pedantic.)

I'm not being difficult either :), and I understand the points being made, but my understanding of how this works is still counter-intuitive for me so I'm trying to get it all clear.

What if the scales were of a hypothetical kind that were solid and unflexible, and the person was rigid also.

On Earth the rigid person steps onto the immovable scales and they settle on 100kgs (after initially reading more due to dynamic loading)

In space what speed would be needed for the rigid 100kg mass to temporarily read 100kgs on the unflexing scales?

I think the answer is "any speed will register more than 100kgs" which seems very counter-intuitive to me.

Would that mean that a 100kg(mass) person on the moon could not gently step onto a set of scales without it reading 100+kgs before it finally settles on 16kgs?
« Last Edit: 25/10/2012 01:15:48 by Anywho »

#### syhprum

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##### Re: How fast am I going, relative to the Earth, when standing still?
« Reply #9 on: 25/10/2012 07:13:58 »
A moving rigid body stopped in zero time by a completely rigid scale will produce an infinite force, of course completely rigid bodies do not actually exist.
When you step on your scales on the moon provided you step gently your velocity relative to the scale will be zero and the only force on the scale will be due to the gravitational field of the moon

#### The Naked Scientists Forum

##### Re: How fast am I going, relative to the Earth, when standing still?
« Reply #9 on: 25/10/2012 07:13:58 »