What is the formula for how much mass increase there is based on how fast an item is moving?

Example: If an object of "X"lbs mass at rest is moving at "Y"mph, what is it's mass at that speed?

Thanks!

Edit 10/10/05:
See
http://www.jackson8.com/science/relativisticmass.html

I'm making a little headway. I've scrounged up a couple of formulas that apply, but I just don't remember how to use them.

http://www.jackson8.com/science/images/speedmass.jpg
And
http://www.jackson8.com/science/images/speedmass2.jpg

I know c is the speed of light, v is velocity, and m is mass, but

1. what is M?
And
2. what is γ?

How can I use this formula with my example?

Thanks!

... or is this the more applicable formula?:
http://www.jackson8.com/science/images/rm.gif
... or is this the same as the 2nd one, above?

...or is this the more applicable formula?:
http://www.jackson8.com/science/images/rm.gif
...or is this the same as the 2nd one, above?how about looking at a kenetic energy formular

m - the mass of an ojbect as perceived for a mover by a stationary frame.
mo - the mass of an object as perceived by a mover for the mover.
v - the velocity (as a fraction of c) of the mover relative to the stationary observer.
c - the speed of light in a vacuum which is a constant value of 300,000,000 meters/second or 186,000 miles/second. This value is constant as viewed by all observers.

I understand that "mo" should be shown as a percentage of "c", but how do I show "c"? It's not '1" is it?

See
http://www.jackson8.com/science/relativisticmass.html
For a summary of where I'm at with this if you think you might be able to help.
Thanks!

This question is old, but surprisingly, it doesn't appear that anyone ever answered it. Your example problem asked the weight of a 100 pound person traveling at 0.6 times the speed of light.

Answers: The units don't really matter as long as they match, e.g. the speeds must have the same dimensions.

You were correct that two of the equations you furnished (containing M's and m's) were equivalent.

I'm not sure what the equation containing "lambda" is intended to represent, but it appears merely to omit the m's and M's, so it essentially is an inflation factor for calculating increased mass at speed.

For the example above, the ratio of v-squared over c-squared is .36, the difference under the radical is 0.64, the square root of that is 0.8, so the 100-pounder would weigh 125 pounds at 0.6c.

firstly, it's gamma not lambda. Yes it's essentially a ratio.

Rick, you're having a problem because you don't quite understand relativity. The man who is moving doesn't see his mass increase because he isn't moving. He sees the rest of the universe increase in mass, this is essential.

so in this question, your m0 is 100, your v is 0.6 and your c is 1. So makdan is correct that you get 100/0.8 = 125 lbs.

New example, 47000 pound TRAM moving at 25 mph what is the pounds per square inch force it applies on an armored window? The surface that hits the window is 6" wide by 1/2" thick