# The Naked Scientists Forum

### Author Topic: What is weight?  (Read 2382 times)

#### Jamie-Lee

• Guest
##### What is weight?
« on: 11/06/2009 13:30:02 »
Jamie-Lee asked the Naked Scientists:

Hello Everyone @ The Naked Scientist

My name is Jamie, I'm at Toronto, Ontario, Canada.

I've been pondering this question for a while but a fairly recent podcast stuck the question back in my head.

Weight is defined as how much a person weighs?  With that here's my question.

If I weigh 162 lbs on earth, but only 61 pounds on mars, what is "weight". Is it just term we use for non-scientist persons to understand why we break tables when we jump on them?

And if in natural space, we are weightless, does that mean in reality we have no weight and it just depends on the gravity that is pulling? Does the regular scale register the gravities pull then?

Hope it all makes sense!

- Jamie

What do you think?

#### LeeE

• Neilep Level Member
• Posts: 3382
##### What is weight?
« Reply #1 on: 11/06/2009 15:33:51 »
Weight, strictly speaking, is the force required to support your body in a gravitational field.  It's tightly related to your mass, but your mass is something that never changes, regardless of gravity or acceleration.

On Earth, if you have a weight of 50kg, you'll also have a mass of 50kg, because the definition of mass was made upon the Earth's surface, but if you went up to the Moon you'd only weigh 1/6 of that because, although your mass would remain 50kg, the Moon's gravity would only pull you down 1/6 as strongly as on Earth.

Now let's say that before you went to the Moon you took part in a little experiment;  you sat on a wheelchair (with very low friction bearings) that was pushed up against a spring attached to a wall, so that when the spring was released it pushed you away from the wall, and you measured how fast you were going after the spring was fully extended.  If you then try the same experiment on the Moon you'll find that although you now only weigh 8.33 kg, instead of 50kg, you will still only reach the same speed as on Earth.

With the spring experiment, what we've done is to effectively set up an artificial gravity at right angles to real gravity.  When we then move to the Moon we can see by your reduced weight how gravity has changed and how, because you reach the same speed with the spring, that the spring hasn't changed on the Moon.

What you say at the end is correct then; the regular scale just registers the pull of gravity upon your mass.

#### lyner

• Guest
##### What is weight?
« Reply #2 on: 12/06/2009 00:58:03 »
LeeE.
Weight is a Force and should be talked about in Newtons.
Apart from the problem at the greengrocers it avoids the paradoxical statement you made about weight and mass both being in kg.

#### LeeE

• Neilep Level Member
• Posts: 3382
##### What is weight?
« Reply #3 on: 12/06/2009 18:46:42 »
I was just trying to address weight and mass - once that was clear I thought we could move on to forces and Newtons.  I didn't want to complicate it too much to start with, but perhaps I should have left the answer to better teachers.

#### lyner

• Guest
##### What is weight?
« Reply #4 on: 12/06/2009 21:03:38 »
I have the advantage that it was thrashed into me. Weight is a FORCE. Mass is the amount of "substance".
And don't you forget it you horrible little boy. (As I was told.)

#### LeeE

• Neilep Level Member
• Posts: 3382
##### What is weight?
« Reply #5 on: 13/06/2009 17:07:20 »
I didn't forget it, honest sir, it's just that anyone going into their grocer to buy their spuds for dinner will only be able to buy them by the lb or kg and not by the N.

Hmm... if it wasn't so difficult and expensive to get there, it could be worthwhile doing our weekly grocery shopping on the Moon.

#### lyner

• Guest
##### What is weight?
« Reply #6 on: 13/06/2009 19:05:17 »
But did YOU do Pounds Force and Pounds Weight at School?

#### Chemistry4me

• Neilep Level Member
• Posts: 7709
##### What is weight?
« Reply #7 on: 14/06/2009 00:56:51 »
I didn't, that would have been WAY too complicated! It still is actually!

#### The Naked Scientists Forum

##### What is weight?
« Reply #7 on: 14/06/2009 00:56:51 »

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