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Re: Mass and weight does it change?

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On 4/22/2016 2:03 AM, Om Prakash wrote:
> I have long passed my schooling and forgotten most about of the my concepts of physics.
> I would like to clarify some of my doubts which i encountered while speaking with my friends
> couple of days back.
>
> As I understand mass of a body remains constant but the weight does changes, as it is based
> on the "acceleration due to gravity" and my body mass. But the question is more like, does
> the mass of a body changes if the body is at rest and if it is motion?

The short answer is "no". More below.

>
> I understand there is relativistic mass, but isn't that supposed to be from an observers
> prespective? which means the mass of the body if the observer is removed, shall remain constant,
> unless some mass is used(in cases of rockets, where the mass of rockets are higher but once
> they take off the fuel mass burns hence reducing the overall mass.)

You're confusing a number of things here, and so setting this aside as 
an irrelevant hairball. You're about to get to the meat of it.

>
> As per Newtonian physics F = ma, using this how will I calculate the mass of the body if I am
> standing at one place or at rest, the only available value to me is the 'a' which is acceleration
> due to gravity and of course there is no force applied on to me, which implies m = F/a hence, 0.
> Is this correct?

Well, no. First of all, if you're at rest, the acceleration is 0, so in 
attempting to use m = F/a you are dividing by zero. The result of 
dividing by zero is not zero, it's indeterminate. So you aren't getting 
a different number with this equation, it's more like you're not getting 
any number at all in this case.

So really your question is, how am I to determine mass of an object at 
rest? The simplest answer is to presume that you can determine it while 
it's accelerating and then make the presumption that the mass doesn't 
suddenly change when it's not accelerating. But that begs the question a 
little. A little more sophisticated answer is to note what F=ma really 
means. The F on the left-hand side is the NET force acting on a body. 
When you are standing motionless on the floor, it's not like there are 
no forces acting on you. You can FEEL the force of the floor pushing on 
your feet, for example. What is true is that the NET force is zero. So 
this means there must be more than one force acting on you, and they 
happen to cancel, so that your acceleration is zero. Well, what are the 
forces acting on you? We've mentioned one: the force of the floor 
pushing upwards on your feet. There's also the pull of the earth (via 
gravitational attraction) down on you. So the way to write Newton's 2nd 
law is this way:
F_net = ma
F_up - F_down = ma
F_floor - F_gravity = ma

Now you happen to know that your acceleration is zero, so this turns into:
F_floor - F_gravity = m(0) = 0.

The rest is just algebra:
F_floor = F_gravity
F_floor = mg
so
m = F_floor/g.

So you can measure your mass by measuring F_floor and dividing that by 
g. In case you think this is tricky, just stick a bathroom scale between 
you and the floor, and the bathroom scale will give you a direct reading 
of the force the floor pushes up on you. (WHY??)

>
> Please excuse, if these questions sound pretty amateurish in nature, I retrying to refresh my
> understanding of the concepts of physics. Please do guide
>


-- 
Odd Bodkin --- maker of fine toys, tools, tables

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Thread

Mass and weight does it change? Om Prakash <oomprakash@gmail.com> - 2016-04-22 00:03 -0700
  Re: Mass and weight does it change? "Y.Porat" <y.y.porat@gmail.com> - 2016-04-22 02:02 -0700
  Re: Mass and weight does it change? Odd Bodkin <bodkinodd@gmail.com> - 2016-04-22 08:02 -0500
  Re: Mass and weight does it change? Sam Wormley <swormley1@gmail.com> - 2016-04-22 08:11 -0500
    Re: Mass and weight does it change? HVAC <mr.hvac@gmail.com> - 2016-04-22 06:26 -0700
      Re: Mass and weight does it change? benj <nobodyxx@gmail> - 2016-04-22 11:58 -0400
        Re: Mass and weight does it change? Odd Bodkin <bodkinodd@gmail.com> - 2016-04-22 11:25 -0500
          Re: Mass and weight does it change? benj <nobodyxx@gmail> - 2016-04-22 13:05 -0400
            Re: Mass and weight does it change? Odd Bodkin <bodkinodd@gmail.com> - 2016-04-22 12:39 -0500
              Re: Mass and weight does it change? benj <nobodyxx@gmail> - 2016-04-22 14:41 -0400
                Re: Mass and weight does it change? Odd Bodkin <bodkinodd@gmail.com> - 2016-04-22 13:46 -0500
        Re: Mass and weight does it change? HVAC <mr.hvac@gmail.com> - 2016-04-22 09:43 -0700
          Re: Mass and weight does it change? benj <nobodyxx@gmail> - 2016-04-22 12:59 -0400
    Re: Mass and weight does it change? Mahipal <mahipal7638@gmail.com> - 2016-04-22 09:18 -0700
  Re: Mass and weight does it change? Yousuf Khan <bbbl67@spammenot.yahoo.com> - 2016-04-22 10:10 -0400
  Re: Mass and weight does it change? "reber g=emc^2" <herbertglazier0@gmail.com> - 2016-04-22 10:11 -0700
    Re: Mass and weight does it change? "hanson" <hanson@quick.net> - 2016-04-22 11:02 -0700
    Re: Mass and weight does it change? benj <nobodyxx@gmail> - 2016-04-22 14:56 -0400

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