Q
qibbish
Guest
Hello - I'm an undergrad studying physics and today, while tutoring a student on gravitational issues, I stumbled out of my depth. <br /><br />The student I was assisting asked if the earth would become a blackhole if compressed to the size of a ping pong ball and at first I entertained the possibiliy. After a moment, however, I corrected that view under the reasoning density = mass/vol, and gravitational attraction is based on mass and (inverse square) of radial distance. <br />In other words, no matter how much you alter the volume of an object its gravitational strength *per unit distance* stays the same, right?<br /><br />To say this again, the ping pong ball version of earth and the uncompressed version will exert the same gravitational effect at the same distance from the center - so if I stood 120,000 miles above the center of either they would exert the same pull, right? I say it like this because, due to the reduction in vol of the ping pong ball, the surface gravity would be more than our earth due to that surface being closer to the center. <br /><br />Anyhow, if this is right - and I assume it is not - then how can a star ever become a black-hole? If compression doesn't effect mass, and mass is all gravity is based upon, then where does the extra mass come from that tips the scale and warps space to such a degree that light can't escape? If that mass was present before the star went nova then shouldn't the pre-nova star have had the same gravitational field (or a greater one, as the nova ejects matter into space) ? <br /><br /><br /><br />