# Warm-Up Exercise 37

## Due 12:15 pm, Tue 20 Nov 2012

Physics 123, Fall 2012

Reading assignment: 39.1-39.3

The*Galilean velocity transformation equation*, Eqn 39.2 (8th
edition), *is simple--much simpler than its name implies. It's also
sometimes called the "classical velocity addition" formula. It's "obviously"
true. Use it to answer this question: A pickup truck is traveling forward at 5 m/s. A
man standing in the back of the truck throws a ball forward at 10 m/s (relative
to himself). How fast will someone on the ground measure the ball to be
traveling? What is the faulty assumption on which the equation
relies?
*

*
*
15 m/s. The faulty assumption is that all observers will agree on what time it is at any given instant: t' = t for all observers.

Explain in your own words (in language that your non-physics-major
roommates could understand) how the *Galilean velocity
transformation equation* and the 2nd postulate of special
relativity are at odds with each another.

Einstein said (2nd postulate) that all observers will measure
a given light wave to travel at 3x10^8 m/s, regardless of the
observers'
own
velocities.
This
contradicts
the
normal
velocity
addition
formula,
because
if
the
truck
is
traveling
at
(for
example)
0.5c,
and
the
man
turns
on
a
flashlight
with
photons
traveling
at
c,
then
someone
on
the
ground
would
measure
the
photons'
speed
to
be
1.5c.
That's
too
fast!
And
impossible,
according
to
Einstein.

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