Mass of Kilogram?

Question

News stories say the world's official kilogram is loosing mass -- or is it the other test mass that are gaining weight? What do you think should be the definition of the kilogram, and how would you make one in your multi-million dollar lab?

I dont think there is any practical way to use planck mass for measuring things at the 1 kilogram level.

Answer 1

Ultimately it doesn't matter. God gave us units with which to do physics.

c--the speed of light
h-bar--the quantum of angular momentum
G--the gravitational constant

These work just as well as kilograms, meters, and seconds, and using them makes the equations much simpler because you can drop c, h-bar, and G wherever they appear.

So to answer your question, the standard of mass is the planck mass--sqrt (h-bar c / G)--about 4.340 µg. If only we had a better handle on G, we could peg the kg on that.

But anyway, units like kg are for engineers, who don't really care if the kilogram changed relative to its fellow kilograms by the weight of a fingerprint.

Edit--it's not so much that the planck mass is small that makes it unsuitable to peg the kg on it, but the fact that G can't be measured with as much precision as we would want.

Answer 2

If each student must exert an upward force of 400 N, and there are eighteen students, and the car is not accelerating in any direction, it must be exerting a downward force of 18*400N = 7200 N. Assuming that this takes place on earth, every kilogram exerts 9.8 N of downward force. So, 7200N / 9.8 N = 743.7 kg. And that's how much the car weighs.

Answer 3

The MASS remains constant. What varies is the WEIGHT which is due to the gravitational attraction of the MASS and the Earth.
The gravitation is affected by the tides due to the moon and the sun.

Answer 4

I wouldn't pay much attention to that stuff.

And physicists waste a lot of time on stuff that doesn't really matter