Spring Balance: Mass or Weight? Clearing Out Confusion

[Peter G.]

By definition, the difference between Mass and Weight is easy to comprehend. However, I am really confused, confused to the level I feel like ripping all the hair out of my head, with spring balances:

After researching a lot, I think that as I step on a spring balance the spring will extend according to my weight, the force I'm exerting on it (Hooke's Law F = kx). For example, I "weigh" 70 kg, so as I step on the balance, the spring will extend by a certain amount which is correspondent to 700N.

But it does not display 700N. It displays 70 KG. Does that mean that the balance divides my mass by 10 (the pull of gravity) and displays my MASS.

Please, can someone help me clear out what a balance actually displays?

 

[rock.freak667]

If it displays kg, then it is measuring your mass. Meaning that it is reducing the actual value by the factor of 'g' (acceleration due to gravity).

 

[Peter G.]

Ok thanks a lot

So if I wanted to check my weight in different planets I would be better off with a Newtons balance!

 

[D H]

If it displays kg, then it is measuring your mass. Meaning that it is reducing the actual value by the factor of 'g' (acceleration due to gravity).

Aa spring scale does not measure your mass. Take a perfect spring scale to Nome Alaska and then to the Mauna Kea Observatory. Put an object of a known mass, say 100 lbm on the scale, and the scale will register 100.16 pounds in Alaska but only 99.66 pounds in Hawaii.

 

[rock.freak667]

Aa spring scale does not measure your mass. Take a perfect spring scale to Nome Alaska and then to the Mauna Kea Observatory. Put an object of a known mass, say 100 lbm on the scale, and the scale will register 100.16 pounds in Alaska but only 99.66 pounds in Hawaii.

I meant the actual display scale itself will show the mass and not the value of 'mg'.

 

[stevenb]

Please, can someone help me clear out what a balance actually displays?

I think you would have to look at the detailed design of the scale to know exactly what is displayed. Is the design a simple mechanical design with no electronics, or a microprossessor controlled unit with electronic display?

If it's a simple mechnanical system, then the measurement is a force measurement which means the measurement is weight with a scale factor used to convert to mass, assuming an average value of g.

If it's a modern scale with microprocessor and electronics, then you don't really know. It might be very similar to the mechanical system with force measurement and simple unit conversions. However, there is nothing to prevent a manufacturer from installing an inexpensive accelerometer into the scale to measure g and use the measured value in the unit conversion. In this case, the system is trying to measure true mass and is properly displaying the units of kg. The benefit of this latter approach is that errors due to small angle offsets in your floor can be calibrated out of the measurement.

There has been a long-standing issue that scales for measuring human body weight/mass are not necessarily consistent with units and actual measurement. In the old days, a doctor's counter balance scale in the US typically would give a reading only in pounds (although the kg scale is also provided in recent years). Of course, a counter balance measures mass, so the system was (and still is, if you look only at the pound reading) inconsistent. And, as you see, a simple spring system may measure weight, but display in kg which is also inconsistent.

 

[Lame One]

If you are referring to a conventional spring scale with the "actual display scale", then you cannot simply convert it to Newtons. A spring scale's weight 'value' is entirely dependent on gravity. If you went to the moon, for instance, a spring scale would show a smaller value due to the fact that less Newtons are being pressed down on it. A method which generally is used as an example of how mass WOULD be obtained correctly on any "planet", as you said, is the triple beam balance. This uses objects of a known mass, then determines how many of them are needed to weigh an equal amount as whatever happens to be on the other side. Because both objects are effected by the change in gravity, any change in gravity can be safely ignored. One example of a triple beam balance that you might recognize is the one the doctor uses to measure your weight, with the multiple metal sliders. Hope this helps!