# Help! My Brain is Melting!

1. Jun 6, 2009

### Gwenne

Hi physics geniuses!

I've been awake all night (grr!) trying to figure out some stuff we talked about in our calculus-based physics class today about Relativity. I hope you guys can make this all clear to me, because maybe then I can get some sleep!

OK, so our professor told us this week that as a particle gets closer to c, its inertial mass increases and this is why if you keep pushing on something with the same force, it doesn't accelerate as much and so nothing can ever go at the speed of light. I can't read my own notes, but our textbook says: "Einstein deduced from his special theory of relativity that the tangible, measurable mass of a particle is given by the equation m = (gamma)*m0, where m0 is the mass of the particle at rest relative to the observer, called the rest mass, and m is the mass of the particle in motion, called the inertial or relativistic mass. This mass can be measured, for instance, by applying a known centripetal force and measuring the radius of the curvature of the path, or in other words using F=mv^2/r. " Then it talks about E=mc^2 and p=mv, with the m supposed to be the new m.

OK, I sort of think I get that, but I can't get it to work out right. F=mv^2/r is just F=ma from last semester, which I totally understand, I think. F=ma is supposed to come from F=dp/dt, and p=mv and in Relativity m is supposed to be gamma*m0, but when I try to do it that way, I can't get d/dt(gamma*m0*v) to work out to F=ma, or F=gamma*m0*a, or anything like that, and I've been trying all night to understand this, because if m is "inertial mass" then isn;'t it supposed to be the m in F=ma? What am I doing wrong? I'm sure that the problem is that gamma is complicated and I'm not very good at calculus and I can't do math which is why I am getting a C in this course. But I really want to understand this. Help!

2. Jun 6, 2009

### Naty1

f = d/dt(mv) is the derivative of a product...both m and v are variables.