The answer is that she sees the ball as stationary. The sum of the velocities equals
zero, because it is the sum of +5 m/s (the velocity of the object as measured in
reference frame B) and í5 m/s (the velocity of frame B as measured from frame A).Relative velocity equation
vOA = vOB + vBA
vOA = velocity of object measured in
reference frame A
vOB = velocity of object measured in
reference frame B
vBA = velocity of frame B measured in
frame A
Sarah observes the train moving
to the left at í5 m/s. Fred, on the
train, sees the ball moving to the
right at +5 m/s. What is the ball’s
velocity in Sarah’s reference
frame?
vOA = vOB + vBA
vOA = (5 m/s) + (í5 m/s) = 0 m/s
4.16 - Gotchas
A ball will land at the same time if you drop it straight down from the top of a building or if you throw it out horizontally. Yes, the ball will hit the
ground at the same time in both cases. Velocity in the y direction is independent of velocity in the x direction.
An object has positive velocity along thex andy axes.Along the y axis,it accelerates, has a constant velocity for a while, then accelerates
some more. What happens along the x axis? You have no idea. Information about motion along the yaxis tells you nothing about motion along
the x axis, because they can change independently.
A projectile has zero acceleration at its peak. No, a projectile has zero y velocity at its peak. Even though it briefly comes to rest in the vertical
dimension, the projectile is always accelerating at í9.80 m/s^2 due to the force of gravity.(^80) Copyright 2000-2007 Kinetic Books Co. Chapter 04