S E C T I O N 3 9 . 6 • The Lorentz Velocity Transformation Equations
1265
Note that u"
y
and u"
z
do not contain the parameter v in the numerator because the
relative velocity is along the x axis.
When v is much smaller than c (the nonrelativistic case), the denominator of
Equation 39.16 approaches unity, and so u"
x
% u
x
#
v, which is the Galilean velocity
transformation equation. In another extreme, when u
x
!
c, Equation 39.16 becomes
From this result, we see that a speed measured as c by an observer in S is also measured
as c by an observer in S"—independent of the relative motion of S and S". Note that
this conclusion is consistent with Einstein’s second postulate—that the speed of light
must be c relative to all inertial reference frames. Furthermore, we find that the speed
of an object can never be measured as larger than c. That is, the speed of light is the
ultimate speed. We return to this point later.
To obtain u
x
in terms of u"
x
, we replace v by # v in Equation 39.16 and interchange
the roles of u
x
and u"
x
:
(39.18)
u
x
!
u
"
x
&
v
1 &
u
"
x
v
c
2
u
"
x
!
c # v
1 #
c
v
c
2
!
c
"
1 #
v
c
#
1 #
v
c
!
c
Two spacecraft A and B are moving in opposite directions,
as shown in Figure 39.15. An observer on the Earth
measures the speed of craft A to be 0.750c and the speed of
craft B to be 0.850c. Find the velocity of craft B as observed
by the crew on craft A.
Solution To conceptualize this problem, we carefully
identify the observers and the event. The two observers
are on the Earth and on spacecraft A. The event is the
motion of spacecraft B. Because the problem asks to find
an observed velocity, we categorize this problem as one
requiring the Lorentz velocity transformation. To analyze
the problem, we note that the Earth observer makes two
measurements, one of each spacecraft. We identify this
observer as being at rest in the S frame. Because the
velocity of spacecraft B is what we wish to measure, we
identify the speed u
x
as # 0.850c. The velocity of
spacecraft A is also the velocity of the observer at rest in
the S" frame, which is attached to the spacecraft, relative
to the observer at rest in S. Thus, v ! 0.750c. Now we
can obtain the velocity u"
x
of craft B relative to craft A
by using Equation 39.16:
▲
PITFALL PREVENTION
39.5 What Can the
Observers Agree On?
We have seen several measure-
ments that the two observers O
and O" do not agree on: (1) the
time interval between events that
take place in the same position in
one of the frames, (2) the distance
between two points that remain
fixed in one of their frames,
(3) the velocity components of a
moving particle, and (4) whether
two events occurring at different
locations in both frames are simul-
taneous or not. Note that the two
observers can agree on (1) their
relative speed of motion v with
respect to each other, (2) the
speed c of any ray of light, and
(3) the simultaneity of two events
which take place at the same posi-
tion and time in some frame.
Quick Quiz 39.8
You are driving on a freeway at a relativistic speed. Straight
ahead of you, a technician standing on the ground turns on a searchlight and a beam
of light moves exactly vertically upward, as seen by the technician. As you observe the
beam of light, you measure the magnitude of the vertical component of its velocity as
(a) equal to c (b) greater than c (c) less than c.
Quick Quiz 39.9
Consider the situation in Quick Quiz 39.8 again. If the
technician aims the searchlight directly at you instead of upward, you measure the
magnitude of the horizontal component of its velocity as (a) equal to c (b) greater than
c (c) less than c.
S
′ (attached to A)
y
′
0.750c
–0.850c
B
A
x
′
O
′
S (attached
to the Earth)
y
x
O
Figure 39.15 (Example 39.7) Two spacecraft A and B move in
opposite directions. The speed of B relative to A is less than c and
is obtained from the relativistic velocity transformation equation.
Example 39.7 Relative Velocity of Two Spacecraft