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Author Topic: Calculating the mass of an object if all you know is the acceleration  (Read 3399 times)

Offline risingsun

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I was wondering if it is possible to calculate the mass of an object (what is commonly referred to as weight) if you know how long it takes to move from point A to point B (on earth).

For example, if I take a barbell and put weights on it and lift it from the floor to above my head, is there a way I can use a calculation to confirm how much weight I just lifted.

I know that mass = force/ acceleration but what if I don't know the force? Since I am lifting upward can gravity be used somehow to determine the force since gravity is a constant (well sort of)? Hopefully this makes sense  :)


 

Offline Geezer

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I was wondering if it is possible to calculate the mass of an object (what is commonly referred to as weight) if you know how long it takes to move from point A to point B (on earth).

For example, if I take a barbell and put weights on it and lift it from the floor to above my head, is there a way I can use a calculation to confirm how much weight I just lifted.

I know that mass = force/ acceleration but what if I don't know the force? Since I am lifting upward can gravity be used somehow to determine the force since gravity is a constant (well sort of)? Hopefully this makes sense  :)

Ah! Sadly, if you don't know the force, you can't measure the mass, but your muscles do give you some sense of the force they are exerting, but it's hard to calibrate what that force is.
 

Offline tommya300

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Also since you are not refering to a free falling object,
 how do you define acceleration?
How fast are you lifting the mass to what distance?
« Last Edit: 10/09/2010 07:32:54 by tommya300 »
 

Offline Soul Surfer

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Yes it is but only under certain conditions.  If you just drop it and watch it accelerate you cannot calculate its mass because neglecting air resistance everything just accelerates in the same way.  however if you had a known mass and a pulley system that was reasonably low loss connected the two together and then watched them accelerate pulling the lighter one upwards and the heavier one downwards (it does not matter which is the heavier)  you can calculate the mass of the unknown object.
 

Offline Geezer

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Yes it is but only under certain conditions.  If you just drop it and watch it accelerate you cannot calculate its mass because neglecting air resistance everything just accelerates in the same way.  however if you had a known mass and a pulley system that was reasonably low loss connected the two together and then watched them accelerate pulling the lighter one upwards and the heavier one downwards (it does not matter which is the heavier)  you can calculate the mass of the unknown object.

Why wouldn't you just use a beam balance with an adjustable fulcrum?
 

Offline Soul Surfer

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That does not achieve the objective of the question.  The questioner was asking about a dynamic method of measuring the mass of an object and not a static one like a balance.
 

Offline Geezer

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That does not achieve the objective of the question.  The questioner was asking about a dynamic method of measuring the mass of an object and not a static one like a balance.

True, but once you introduce a "known mass" for comparison, you might as well use the simplest method.

I think the only way to do it is to calibrate the muscles. To some extent, our muscles give us a pretty good idea of the force they are producing although, except at limit of their capability, they are not too well calibrated.
 

Offline risingsun

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thanks everyone...you've given me a lot to think about...
 

Offline risingsun

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That does not achieve the objective of the question.  The questioner was asking about a dynamic method of measuring the mass of an object and not a static one like a balance.

Yes that is exactly what I am asking. Soulsurfer you mentioned that "neglecting air resistance everything accelerates in the same way". That makes sense to me for falling objects. But what about rising objects?  I am not a scientist so forgive me if these questions do not makes sense.
 

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