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# Weighing a large person

Ranch Hand
Posts: 84
Assume that you have a person that is over 300 lbs. You have two scales that have a max weight of 250 lbs. Is it possible to determine the persons weight by having them stand with one foot on each scale, and if so, what is the formula for determining the weight.

Rancher
Posts: 1197
22

Originally posted by George Harris:
Assume that you have a person that is over 300 lbs. You have two scales that have a max weight of 250 lbs. Is it possible to determine the persons weight by having them stand with one foot on each scale, and if so, what is the formula for determining the weight.

Marshal
Posts: 67420
173

author
Posts: 15385
6
truck scales always work well

Rancher
Posts: 13459
Engineering Free Body - assuming a static system (the person isn't swaying or jumping up and down) then the sum of the forces in the system are zero, so the scales exert enough force to counter the weight of the person. There fore the weight is the sum of the values on the scales assuming one of the scales isn't 'maxed out'.

George Harris
Ranch Hand
Posts: 84
Does it matter if the scales are placed further apart so that the persons legs are spread wide thus making them something like a suspension bridge were there is both forces in the X and Y directions?

Ranch Hand
Posts: 1241

Originally posted by George Harris:
Does it matter if the scales are placed further apart so that the persons legs are spread wide thus making them something like a suspension bridge were there is both forces in the X and Y directions?

Yeah, because if the large person isn't humiliated enough to have to stand on two scales, making them stand in a funny way would do the trick.

BTW I'm assuming that "300 lbs" is a lot, I've got no idea. What's that in kilos? Or more usefully, how many small elephants is that?

[ June 23, 2006: Message edited by: Dave Lenton ]
[ June 23, 2006: Message edited by: Bear Bibeault ]

David O'Meara
Rancher
Posts: 13459
300 lbs is the same as 300 1lb bags of flour which is um...

Look, a liter of water weighs one kilo and is also 10cmx10cmx10cm. At the risk of turning this into a metric debate, what is a pound?

David O'Meara
Rancher
Posts: 13459

Originally posted by George Harris:
Does it matter if the scales are placed further apart so that the persons legs are spread wide thus making them something like a suspension bridge were there is both forces in the X and Y directions?

Nope, in a static system the horizontal forces must cancel, as must the vertical. The weight of the individual is cancelled by the force exerted back by the floor and measured by the scale, and the horizontal force on one scale is cancelled by the force on the other. This assumes that applying a shear force (ie horizontal) to the scale doesn't affect the reading. That would depend in how the scale was made.

lowercase baba
Posts: 12871
62
technically, since a pound is a unit of force, and a Kg is a unit of mass, there is no direct conversion. you can't convert feet to liters, afterall.

a pound is equal to 4.44 newtons, and newton is the force needed to accelerate 1kg at 1m/sec/sec...

but at sea level, 1 lb is about 0.45 kg, so 300lbs is roughly 135 kg.

author and iconoclast
Posts: 24203
43
300 lbs == 21 stone.

Ranch Hand
Posts: 1936

Originally posted by Ernest Friedman-Hill:
300 lbs == 21 stone.

And a stone is what, 100 pebbles?

Wanderer
Posts: 18671
[fred]: technically, since a pound is a unit of force, and a Kg is a unit of mass, there is no direct conversion.

Depends who you ask. I trust NIST. Wikipedia expound on this. When forced to use English units for some reason, it's easy enough to just say lbm or lbf depending which one you mean, and use F = ma/gc. Of you can talk about slugs, or poundals. But most people will more readily understand pounds force and pounds mass, I think.

[EFH]: 300 lbs == 21 stone.

21 stone 6, I believe.

[Ashok]: And a stone is what, 100 pebbles?

A stone is 14 pounds, 224 ounces, 3584 drams, or 98000 grains. Take your pick.
[ June 23, 2006: Message edited by: Jim Yingst ]

Bear Bibeault
Marshal
Posts: 67420
173
I know a lot of football-build type of guys who weigh in at well over 300 lbs and don't have an ounce of fat on them... so let's keep to the adjective "large".

Me? I'm fat.

author
Posts: 9000
19
A pint's a pound the world around...

So that makes these 300 pounders about 37.5 gallons...

Marshal
Posts: 25682
69

Originally posted by Bert Bates:
A pint's a pound the world around...

Or as Wikipedia says about the pint:

It is in use mainly in the U.S., the UK and Ireland, although the value is not the same and the U.S. has two types of pint...

[ June 28, 2006: Message edited by: Paul Clapham ]

Ranch Hand
Posts: 478

Originally posted by George Harris:
Assume that you have a person that is over 300 lbs. You have two scales that have a max weight of 250 lbs. Is it possible to determine the persons weight by having them stand with one foot on each scale, and if so, what is the formula for determining the weight.

aah just chop em up and weigh using the two scales

David O'Meara
Rancher
Posts: 13459
There may be laws about that, but I would have to check.

Bert Bates
author
Posts: 9000
19
oh, oh, oh, i know!
stack one scale on top of the other - then put the 300 lb. guy on top...

the top scale will max out at 250, and the bottom one will pick up the additional 50!

sa-weet!

Trailboss
Posts: 23216
Two scales will work.

I weigh 300 pounds. But I just make sure that I get scales that go up to 350.

I used to drive wheat truck. And we used old scales to calculate how much wheat we brought in. The scale was designed for much smaller (older) trucks. So we would drive on the front axel, weight that, and then drive on the back axel and weigh that. We would do this with a full load and with an empty truck. I think this worked only because the ground was level and the truck was stiff - in other words, I don't think it would work with a person and one scale.

Sheriff
Posts: 13411
State police use multiple scales when weighing trucks at impromptu weigh stations.