Alfke.PASA@Xerox.ARPA (07/24/85)
From: Peter Alfke <Alfke.pasa@Xerox.ARPA> This is in reply to Jim Gardner's message... >Operationally, how does one measure mass? One subjects an object to >a known force and sees how it accelerates, then one applies F=ma. >If the object is more or less stationary in your own frame of >reference, you can for example put the object on a weigh scale. The >force of gravity acts on the object, accelerating it downward a >small distance before the known force of the springs in the scale >decelerate the object. The distance that the object has moved >(entirely a function of the acceleration given by the two forces) is >used to determine the object's mass. Not quite: in a spring scale one subjects the object to a known *accelleration* (9.8 m/s^2), which causes a downwards force on the object. The spring provides an upwards force which is linear with the distance it's compressed/expanded, and one sees how far the spring must move before the forces balance. (A small point, but let's keep things straight.) >[The formula for mass] does not apply to particles going at the speed >of light, since the formula would involve division by zero. >Nevertheless, there are many many "things" that move at the speed >of light (light being a prime example). We can say that the mass >formula is not correct for "things" whose speed is >= the speed of >light; or we can say that mass is not a meaningful concept for such >things. Photons (and anything else which travels at lightspeed) are massless. You can multiply their mass by any gamma factor (the thing that goes infinite at c) and it stays zero: thus, the formula still applies. Interestingly enough, photons do have momentum, which varies with wavelength, not speed. If we could get rid of ALL of a starship's mass, we could get it to go at lightspeed very easily ... hmmm. Sounds like a great gimmick for a space-opera. --Peter Alfke