When your spaceship is waiting to be launched, it has a rest mass of m ₀ . When it’s in flight, it has a mass m, also known as its relativistic mass or moving mass. The relationship of these two masses is shown in this equation:

where  and v is the speed of the spaceship.

Note that is always greater than 1, so m is always greater than m ₀ .

Try this!

Enter two values into the calculator—a fraction of the speed of light (from 0 to 0.999 999 999) and the rest mass of an object (your own mass, perhaps)—and the Moving Mass will tell you how massive the object would be at that speed.

	Speed (1 = speed of light = 299 792 km/sec) Rest mass kilograms     Moving Mass kilograms

If you don't know how many kilograms your mass is, you can divide your weight in pounds by 2.2 to find out. Mass and weight are different quantities, but they're directly proportional, so we can convert from one to the other. At the surface of Earth, an object with the mass of 1 kilogram weighs 2.2 pounds.

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