##
Special Relativity in 14 Easy (Hyper)steps

**5. Two ways to measure
the length of a moving object**
Imagine
that we'd like to measure the length of *Sulaco* as it coasts with
speed v past *Nostromo*.

Here
are two ways that come to mind:

Method 1:

Using a single
clock, time how long it takes (call this interval
Dt) for *Sulaco* to coast past
the location of the clock. At
the end of the time interval, disable the clock so that its reading is
frozen at Dt. *Sulaco's* length is
vDt.

Method 2:

Using a large number
of **synchronized**
clocks (labeled 1 - 13 in the illustration), determine which clocks are
closest to *Sulaco's* nose
and tail
at the instant at which
each clock reads zero. The length of *Sulaco* is the same as
the separation
between these two clocks. Inscribe the result in *Nostromo's*
ship's log. In the drawing,
*Sulaco's* length is the same as the separation between clocks 11
and 3.

Methods 1 and 2 will always
yield
the same answer.