Share this thread: 
#1
Feb1708, 02:29 AM

P: 13

When I step on a scale I see that it says....70kg. We typically call this value weight? However, kg is used for "mass" and not "weight". So when we say 70kg do we mean that my mass is 70kg OR do we mean that my weight is 70 N?
So if it does mean mass, then my ACTUAL weight is 686N!!! lol And if it means weight, then my mass is roughly 7kg. See where I'm coming from? And just to let you guys know, b4 u waste ur time, I def. know the difference between mass and weight. I'm just uncertain about what we are actually measuring. I truly appreciate anyones help! thanks 


#2
Feb1708, 02:39 AM

P: 1

you are measuring your mass!!



#3
Feb1708, 07:20 AM

Mentor
P: 41,304

What a scale actually measures depends on the type of scale. A typical "bathroom" scale measures forceyour "weight". Bring it to the moon and you'll weigh less according to the scale, but your mass is unchanged. But a balance scale compares masses; it will read the same on the moon. 


#4
Feb1708, 07:55 AM

Sci Advisor
HW Helper
PF Gold
P: 12,016

Scale?, mass or weight
Well, what the scale actually actually measures is how much a spring within the apparatus has been compressed, or, if a torsional spring has been used, the displacement angle.
And then that is correlated to the effective weight the person standing upon it must have exerted upon the scale.. 


#5
Feb1708, 08:24 AM

Mentor
P: 15,065

A spring scale (e.g., the typical bathroom scale) comes close to measuring your weight (within about 0.3% on the Earth). What a spring scale measures is the normal force needed to keep a person from sinking into the floor. This normal force is your apparent weight, not your "actual" weight. The difference: weight is tautologically the force due to gravity while apparent weight is the sum of all forces acting on a body except gravity.



#6
Feb1708, 12:32 PM

P: 13

So if I'm reading this right are you guys saying that my mass IS 7kg? 


#7
Feb1708, 12:38 PM

Mentor
P: 41,304




#8
Feb1708, 12:42 PM

P: 1,127

You have to differentiate between the formal definition in physics and the everyday definition in commerce. The weight of a head of lettuce will be in kg as will the weight of a baby at the pediatrician. In America, both will be in pounds. That's just the system that exists.



#9
Feb1708, 12:47 PM

P: 13

I'm not trying to be stupid but i'm trying to understand this 100% thanks for your patience 


#10
Feb1708, 12:49 PM

P: 13




#11
Feb1708, 12:52 PM

Mentor
P: 41,304




#12
Feb1708, 01:15 PM

P: 1,127

There is, I think, no understanding available in such matters. Like the difference between aircraft heading and engineering standard angles, it's just a matter of usage. 


#13
Feb1708, 01:54 PM

P: 13

I now understand that it is measuring your force. I see that it's dependant on the gravataional pull. The scale is reading your mass affected by gravity. So therefore, gravity is by default factored in? So that's my problem. I don't see why you would multiply it by 9.8 when gravity was already factored in to give you the reading of 70kg. That's why I was saying that it would make more sense if we would say 70N. And I do understand the difference. Mass is absolute....doesn't change. Weight is dependant on gravity...so it can change. 


#14
Feb1708, 02:13 PM

Mentor
P: 41,304




#15
Feb1708, 02:18 PM

P: 18

Bathroom scales assume you are under the influence of Earth's gravity at 9.8m/s/s, and calculate your assumed mass off of that assumption. A quick way to fool your scale is to get on an elevator that accelerates upwards and downwards, making it look like you weigh more then less of what you actually do.
Should your scale read 70kg, it believes you have a mass of 70kg, as you are probably exerting a force of 686N on the scale. If the world were less complicated, your scale would read out in Newtons, or we would say you mass yourself on a scale, instead of weighing yourself. Do keep in mind, though, that pounds are a unit of force, so you can weigh yourself in pounds or newtons, and mass yourself in kilograms (or slugs). 


#16
Feb1708, 02:20 PM

Mentor
P: 15,065

Derived units can be used in lieu of the standard ones to enable the use Newton's second law in the form applied by our metric systemusing cohorts (i.e., F=ma). With lbm and lbf, one has to use the more generic form F=kma. 


#17
Feb1708, 05:40 PM

P: 13

How did we get the 1kg measurement? Obviously it had to come from weighing it on a scale to get the mass...NOT the weight. B/C if the scale gave us our weight then there would be no need to multiply it by 9.8 I emailed the admin at "http://www.mathsisfun.com" b/c he had an article on weight vs. mass which can be found at http://www.mathsisfun.com/measure/weightmass.html He said and I quote "The scale estimates your mass based on the force your body exerts on it. If the scale measures kg, you can work out how much force your body is exerting on it by multiplying by 9.8 (to convert kg into Newtons)." According to him the scale measures mass and not weight. This is more consistent than saying that the scale measures weight. B/c if you said that the scale measures weight than you would be multiplying by 9.8 two times (one from the scale figuring out your weight and the other from multiplying it by 9.8). So I think by weight, you REALLy mean mass. 


#18
Feb1708, 06:03 PM

Mentor
P: 15,065

Dr. Al is most certainly not saying 1kg = 9.8N. Your bathroom scale measures force, not mass. Pick the scale up and give it a good squeeze with your hands. It will register some value.
If you want to be real picky, it isn't even measuring force. As Arildno said earlier, it is actually measuring the displacement of a spring. The spring has a known spring constant, so the displacement of the spring varies linearly with the force that caused the displacement. The normal force that keeps a person standing on the scale from sinking into the scale is proportional to the person's mass; the constant of proportionality is the Earth's gravitational acceleration. Your scale automatically converts the spring displacement (units=length) to mass via the known spring constant and the known gravitational acceleration. 


Register to reply 
Related Discussions  
Mass vs. weight with a falling rock and a spring scale  Introductory Physics Homework  8  
Weight on a Scale Down an Incline vs. True Weight  Introductory Physics Homework  1  
How do I calculate scale weight for testing a scale glider?  Mechanical Engineering  2  
Lose weight (mass) or train?  effect of losing weight (mass) on power  Introductory Physics Homework  1  
What is the force that the scale reads (the weight ) as the mass slides down the slo  Introductory Physics Homework  15 