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How does the mass of the spaceship change? While your ship was on Earth waiting to take off, it had a rest mass of 1000 kilograms. But to observers on Earth, the mass of your ship changed while you were in flight. Did you notice the increase in mass shown on the control panel? Relative to an observer not traveling with it, an object always has more mass in motion than it has at rest. For jet planes or even the space shuttle, this increase is so small that we ignore it. But at speeds close to the speed of light, an increase in mass is significant. If your spaceship were traveling at 0.999 999 5 times the speed of light (0.999 999 5 c ), for example, its mass in flight (as measured by your friends on Earth) would be 1 000 000 kilograms—a thousand times its rest mass. You can think of mass as the resistance an object has to any change in its motion. So the greater the mass of an object, the greater its resistance to increases in speed. In fact, as the speed of an object approaches the speed of light, its mass approaches infinity. To accelerate an object up to the speed of light, therefore, would require an infinite amount of energy.That’s why anything that has an initial rest mass can’t move at or faster than the speed of light ( c ).
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