# The Naked Scientists Forum

### Author Topic: Theoretically, can you 'weigh' nothing?  (Read 6993 times)

#### Chemistry4me

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##### Theoretically, can you 'weigh' nothing?
« on: 28/02/2009 07:51:47 »
Let me explain...
So I was standing on the bathroom scales  and of course we all know that when you crouch down suddenly you momentarily become lighter and then heavier again right?
My question: is it possible for you to 'weigh' nothing if you crouch down fast enough? Is this even remotely possible?
I don't seem to be able to go down any less than 15kg
What am I doing wrong? How can I improve?

#### lyner

• Guest
##### Theoretically, can you 'weigh' nothing?
« Reply #1 on: 28/02/2009 08:15:58 »
If you could bring your knees up fast enough so they left the floor you would not be pushing down on the scales. Practise!

#### Chemistry4me

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##### Theoretically, can you 'weigh' nothing?
« Reply #2 on: 28/02/2009 08:19:12 »
How do you do that? Bring my knees up?
How do you bring your knees up?

#### BenV

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##### Theoretically, can you 'weigh' nothing?
« Reply #3 on: 28/02/2009 09:32:57 »
I think it's commonly known as jumping - and is probably not advised for the health of your weighing scales!

#### Damo the Optics Monkey

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##### Theoretically, can you 'weigh' nothing?
« Reply #4 on: 28/02/2009 09:36:25 »
by jumping...

this is similar to a physics problem that has come up in m class, except the person is standing on a set of scales in an elevator.

If the elevator accelerates downwards at the same rate as gravity (9.8ms-2),then he reaction forces would result in you becoming 'weightless'.

Remember, that weight and mass are 2 different things.

HEre is an interactive animation about weightlesness.

#### Chemistry4me

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##### Theoretically, can you 'weigh' nothing?
« Reply #5 on: 28/02/2009 09:41:37 »
But when I jump I become heavier!!

#### lyner

• Guest
##### Theoretically, can you 'weigh' nothing?
« Reply #6 on: 28/02/2009 09:59:41 »
How do you do that? Bring my knees up?
How do you bring your knees up?
You can surely stand on one leg and 'bring one knee up to your chest'. Just do the same with both knees at the same time. If they accelerate upwards faster than g, they will leave the ground.
This is not proposed as a serious means of transport. It will last only a few ms.
« Last Edit: 28/02/2009 10:03:21 by sophiecentaur »

#### Chemistry4me

• Neilep Level Member
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##### Theoretically, can you 'weigh' nothing?
« Reply #7 on: 28/02/2009 10:01:18 »
Okay, so most likely, it would be imposible for me to weigh nothing.

#### Damo the Optics Monkey

• Sr. Member
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##### Theoretically, can you 'weigh' nothing?
« Reply #8 on: 28/02/2009 10:06:07 »
ahem...

by jumping...

this is similar to a physics problem that has come up in m class, except the person is standing on a set of scales in an elevator.

If the elevator accelerates downwards at the same rate as gravity (9.8ms-2),then he reaction forces would result in you becoming 'weightless'.

Remember, that weight and mass are 2 different things.

HEre is an interactive animation about weightlesness.

#### Chemistry4me

• Neilep Level Member
• Posts: 7709
##### Theoretically, can you 'weigh' nothing?
« Reply #9 on: 28/02/2009 10:07:17 »
No, I meant, when I jumped, I pushed down on the scales and I became heavier!

#### Damo the Optics Monkey

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##### Theoretically, can you 'weigh' nothing?
« Reply #10 on: 28/02/2009 10:22:28 »
yes, that would be true - the example I gave is going down becoming lighter - so going up would indeed result in you becoming heavier.

The reason is the opposite to the weightless explanation I gave in my post.  Going up suddenly on your scales is analogous to going up in an elevator - where when you go up, you feel havier.

Here is some further information.

#### Chemistry4me

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##### Theoretically, can you 'weigh' nothing?
« Reply #11 on: 28/02/2009 10:25:09 »
Thanks mate

#### Damo the Optics Monkey

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##### Theoretically, can you 'weigh' nothing?
« Reply #12 on: 28/02/2009 10:36:43 »
No problem at all.

#### Karen W.

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##### Theoretically, can you 'weigh' nothing?
« Reply #13 on: 28/02/2009 11:28:44 »
He needs one of those helicopter blade caps strapped to his head to give him extra lift...LOL

#### Chemistry4me

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##### Theoretically, can you 'weigh' nothing?
« Reply #14 on: 28/02/2009 11:48:00 »
Or I can start my scale at -75 kgs!

#### Karen W.

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##### Theoretically, can you 'weigh' nothing?
« Reply #15 on: 28/02/2009 12:27:26 »
LOL...That works!

#### lyner

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##### Theoretically, can you 'weigh' nothing?
« Reply #16 on: 28/02/2009 13:17:29 »
I think the word 'weight' should really be restricted to very straightforward situations, like standing or hanging up.
There is nothing different between the force which your feet are overcoming when you stand up and the force which keeps a satellite in its orbit. But the practical differences between the two situations make it far more dodgy to talk of weight whilst in orbit. That's because we think of weight as a force we can 'feel' and you don't 'feel' what's pulling you into orbit.
When in doubt use MASS and the appropriate FORCE, be it due to gravity or a rocket engine.

#### Damo the Optics Monkey

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##### Theoretically, can you 'weigh' nothing?
« Reply #17 on: 28/02/2009 13:23:35 »
or an elevator..

Yup and the connection between mass and force is Newton's 2nd Law:

Force = mass x acceleration

(As force is a vecor, the direction is in the direction of acceleration)

and:

Weight = mass x acceleration due to gravity

#### lyner

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##### Theoretically, can you 'weigh' nothing?
« Reply #18 on: 28/02/2009 14:39:43 »
Did you ever ask the greengrocer for 50N of potatoes?

#### Damo the Optics Monkey

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##### Theoretically, can you 'weigh' nothing?
« Reply #19 on: 28/02/2009 15:13:58 »
Did you ever ask the greengrocer for 50N of potatoes?

Actually, I have - and 20N of tomatoes... the greengrocer thought it was funny as hell

#### LeeE

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• Posts: 3382
##### Theoretically, can you 'weigh' nothing?
« Reply #20 on: 28/02/2009 17:05:10 »
As the others have said, weight is a very limited concept.  Less energetic ways of weighing less, when measured using bathroom scales, are to prop the scales up against a wall and lie on the floor with your feet only lightly touching them, or simply hold the scales upside down above you, with them lightly touching your head.

#### lyner

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##### Theoretically, can you 'weigh' nothing?
« Reply #21 on: 28/02/2009 17:21:31 »
Sounds a bit like doing 'pressdowns' instead of pressups in a workout. Much easier.

#### Chemistry4me

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##### Theoretically, can you 'weigh' nothing?
« Reply #22 on: 01/03/2009 04:56:27 »
Less energetic ways of weighing less, when measured using bathroom scales, are to prop the scales up against a wall and lie on the floor with your feet only lightly touching them, or simply hold the scales upside down above you, with them lightly touching your head.
How does that work??

#### LeeE

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##### Theoretically, can you 'weigh' nothing?
« Reply #23 on: 01/03/2009 12:33:11 »
It doesn't, which just goes to show how careful you must be when using 'weight'.

#### swansont

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##### Theoretically, can you 'weigh' nothing?
« Reply #24 on: 02/03/2009 01:03:32 »
No, I meant, when I jumped, I pushed down on the scales and I became heavier!

What you want to do is fall, rather than jump.  Bring your knees up (or bend them) but without raising your center-of-mass

#### The Naked Scientists Forum

##### Theoretically, can you 'weigh' nothing?
« Reply #24 on: 02/03/2009 01:03:32 »