Question

Mass is the amount of matter an object has; weight is the amount of matter in the object times the gravitational force on the object. True or False

Answer 1

true

Answer 2

IDK
this might not be entirely true anymore

Answer 3

@nincompoop
What do oyu mean by that?

This is an ALMOST true statement - weight is the FORCE caused by a body due to it's MASS x acceleration of gravity

The statement says 'times the gravitational force' - but it is truly 'time the gravitational acceleration'

Mass has dimension of [Mass]
Weight has dimensions Massx acceleration = [MLT^-2]

Answer 4

The answer is not true - but I think it has been badly worded.

Just using the trivial dimensional analysis above:

the question says it is
"weight is the amount of matter in the object times the gravitational force "

But mass x force has dimensions [M^2LT^-2] and hence the statement is false.

Answer 5

There is another way we define weight, (and quite franlky that definition makes more sense) and that is the force with which the floor or whatever u are standing on .. pushes on u

Answer 6

@mashy
I disagree entirely - a body in free fall experience a force acting down, which is equal to its weight (=mg) there is NO floor - but the force still exists

Answer 7

but you FEEL nothing.. u don't feel ur weight.. if u had a weighing scale.. it shows zero.. seeing is believing..

if u are in a plane in free fall.. everything inside appears floating as if there is no force acting.. feeling is believing :P.. all experiments inside that plane will confirm that..

so .. it makes more sense to DEFINE WEIGHT that way.. m not arguing that there is no force acting on u.. ofcourse there is gravity.. m just saying.. its really not all that great to call that force as WEIGHT..

Answer 8

We have disagreed on this before (over your insistence on the use of the term centrifugal force).

The weight is = mg - it is a FORCE and is given by the universal gravitational equation
\[F=\frac{ GMm }{ r ^{2} }\]

On the surface of earth this simplifies to mg where g is acceleration of gravity on earth.

The free body diagram is the only precise way to interpret what is happening to a body. The force IS weight and exists whether you are falling in a vacuum or in a plane.

Answer 9

MASS DEFINED AS THE AMOUNT OF MATTER THAT AN OBJECT HAS might not be the entire story about mass.
ADGAF about weight, because that can be easily defined under F=ma

Answer 10

http://physics.aps.org/articles/v6/111

Answer 11

http://sciencesprings.wordpress.com/2014/07/25/from-fermilab-the-higgs-gives-mass-to-matter-too/

Answer 12

@MrNood

why are you not trying to understand what i am saying? :P.. its a definition.. and u can't ask someone to agree on what you wanna define as weight.. and infact if you look up, you ll see that many people (including me) like to define weight as the way i did.. cause its MORE INTUITIVE!..
you heard of being weightlessnessin free fall.. why do they say that? CAUSE YOU DON"T FEEEEELLLLL YOUR WEIGHTTTTTTTTTT.. ?? that makes no sense.. unless u define the weight as i did..

ITS JUST A MATTER of definition.. mg = weight is just a poor definition.. :P.. its not god given..

Answer 13

during free-fall you are countering gravity and that is why you "feel" weightless
you counter gravity by "falling"

Answer 14

http://youtu.be/HneFM-BvZj4

Answer 15

"countering gravity" ? thats the worst way to put it :P..
M not saying there is no gravity :P..
and exactly you "FEEL" weightless.. and thats why we call that weightlessness..
and define weight that way..

Answer 16

just watch

Answer 17

general theory of relativity ;)

Answer 18

ok

Answer 19

thats a great video in deed :D

Answer 20

omg 10:00 is really funny xD

Answer 21

besides.. i don't believe thats what einstien said.. that its the earth that comes and hits.. cause.. acceleration is very well defined.. and its absolute.. u don't need a frame of reference to calculate acceleration. (unlike velocity) :O

Answer 22

you do need to account he frame of reference. that is ingrained to physics students from the beginning.

you will find out the extent of its application when you take nuclear chemistry also

Answer 23

since acceleration = dv/dt
and velocity = ds/dt

it is evident that you MUST have a frame of reference from which to measure displacement.

Answer 24

@mashy
Intuition is not a good basis for analysis.

It is not just a matter of definition. A body with mass experiences a force under gravity known as its weight = mg

IF that force is resited by a floor then you can equate the two, but the force is there regardless. I AM trying to understand what you are saying - I just don't.

Answer 25

I return to your statements about 'centrifugal force' being 'more intuitive cos that's wht we feel' - but it is well established that there is no such force. IF you draw free body diagrams all the forces will become well defined. WEIGHT = mg and centriPETAL forces will feature on those diagrams.

Answer 26

@MrNood i don't think u are trying to understand..

why don't you look at this site
http://en.wikipedia.org/wiki/Weight
check out the second paragraph. .

also check out the lecture from a renown physics teacher Professor Walter Lewin..
https://www.youtube.com/watch?v=zfTE4J87aQQ

These are some references i am citing.. i am saying weight is a matter of definition..

you give me one good reason why we CAN'T or should NOT define weight as the normal reaction of the floor? and call mg = gravitational force..

i gave you tons of reasons why it makes more sense to define it that way.. and call mg as just the gravitational force.. :P



second of all..
acceleration is NOT RELATIVE.. acceleration is an absolute quantity.. (non relativistic of course) what i mean is

if i say a dude is going at 5m/s.. its ill defined.. cause i need to mention the velocity WITH RESPECT to some reference..

but if i say a dude is going at 5m/s^2.. it is very well defined.. regardless of which reference frame u take.. you ll end up calculating that that dude is indeed moving at 5m/s^2

Answer 27

Also i told countless times.. there is no centrifugal force.. i agree..

but when u are in a non inertial reference frame.. JUST IMAGINE.. u are in a circulating reference frame..
you feel you are being thrown out.. how will you make sense of it?

also for some physics calculation.. for example.. if i ask u to find the time period of a pendulum held by a person going in a circle.. it would be foolish to do the calculation from the inertial reference frame.. (where u see the pendulum not only oscillating but also going around in a circle)
but it would be damn easy to enter the non inertial reference frame.. and imaging having a centrifugal force.. (I KNOW ITS NOT REAL.. but its better :P).. and then do the math!

Answer 28

So the guy in your 'weightless' plane who is falling with an acceleration og 9.82m/s^2 WRT earth - is not accelerating WRT to the plane - where is the absolute in this?

Answer 29

yes.. but .. the plane is a non inertial reference frame.. so if you are inside this plane.. there is a way in which you can figure out that.. u are indeed falling.. (and no in outer space drifting across)..

Answer 30

infact.. u should support this :P

if you are inside a vehicle.. rotating.. and u see an object kept in front of you NOT moving..
you may say for once, that .. that object is not accelerating

but since u know you are rotating, and tehre is a centriPETAL force, that object MUST be accelerating towards the centre..

acceleration is absolute, cause FORCE is absolute

velocity is relative because, displacement and positions are relative..

Answer 31

so the guy in the plane - is he accelerating towards earth - or not accelerating towards the plane's fuselage?

Answer 32

yes he is accelerating towrads.. the earth.. so yes there is gravitational force on him.. YES I AGREE where you trying to go :P..
but if i ask him.. hey dude.. whats ur weight?.. do you feel ur normal weight.. he will say
no.. i feel totally weightless bro!!..

Answer 33

it is possible for him to do some complicated experiment WITH LIGHT.. and figure out .. that YES INDEED he is accelerating towards the earth..

thats why.. acceleration is absolute..

Answer 34

BUt he is NOT accelerating toward the plane - so it is RELATIVE to your frame of reference, as is your velocity

Answer 35

But you can't do this argument from the planes reference frame.. cause that reference frame is non inertial.. laws of physics don't work here..

Answer 36

its like this

if i ask you.. BOttom line..
IS THERE A FORCE ON THAT FELLOW?

is it dependent on the frame of reference?? no right?.. you ll say.. YES.. there is a force.. = mg..


force is ABSOLUTE..

and so is accelration

Answer 37

I think we have totally destroyed the purpose of this question :P

Answer 38

no keep going i like this argument :D

Answer 39

@ganeshie8 check this :D
its very educated

Answer 40

Yeah - but the OP hasn't been back anyhow!
I'm outta here for now:

All inertial frames are in a state of constant, rectilinear motion with respect to one another; an accelerometer moving with any of them would detect zero acceleration


If bodies, any how moved among themselves, are urged in the direction of parallel lines by equal accelerative forces, they will continue to move among themselves, after the same manner as if they had been urged by no such forces.

—Isaac Newton: Principia Corollary VI, p. 89, in Andrew Motte translation

This principle generalizes the notion of an inertial frame. For example, an observer confined in a free-falling lift will assert that he himself is a valid inertial frame, even if he is accelerating under gravity, so long as he has no knowledge about anything outside the lift

Answer 41

"so long as he has no knowledge about anything outside the lift"

but if i look outside .. then i CAN figure out experimentally that it is indeed my elevator that is accelerating.. or in other words.. indeed its the elevator that is experiencing this force and is rushing down..

but thats not the case if the elevator is moving with constant speed.. in that case if i look outside.. then whatevr experiment i do, ll say that i am at rest.. and the whole world is moving

Answer 42

i know W = mg

Answer 43

lol @ganeshie8

Answer 44

atleast it should work in steady state when nothing is moving :P

Answer 45

*accelerating

Answer 46

mashy this :
" then whatevr experiment i do, ll say that i am at rest.. and the whole world is moving "

remined me with this

“Stop acting so small. You are the universe in ecstatic motion.”

Answer 47

this is the longest thread on weight
weight watchers, these physics students are coming after you!

Answer 48

:P