¥In 1 second I see the
ball move 5 m. Its velocity
is thus dx/dt = 5/1 = 5 m/s.
¥In 1 second I see
observer 2 move 10 m. The velocity
of observer 2 is thus dx/dt = 10 m/s.
¥In 1 second I also see
the ball move (10 + 5) = 15 m. Its
velocity is thus dx/dt = 15
m/s.
¥The velocity I observe
is higher than the velocity he observes!