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 5c), for
example, its mass in flight (as measured by your friends on Earth)
would be 1 000 000 kilograms—a thousand times its
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).
The graph shows how the mass of an
object increases as the speed of the object approaches
the speed of light.
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