Question

At the same moment, one rock is thrown upward at 4.5 m/s and another thrown downward at 6.2 m/s. What is the relative velocity of the second rock from the perspective of the first rock? Assume that up is positive.
A) 10.7 m/s
B) -1.7 m/s
C) -10.7 m/s
D) 1.7 m/s

Answer 1

The correct answer is 
C) -10.7 m/s 

In fact, the first rock is moving upward with velocity +4.5 m/s, while the second rock is moving downward with velocity -6.2 m/s, with respect to a fixed reference frame. In the reference frame of the first rock, instead, the second rock is moving with velocity equal to its velocity in the fixed frame minus the velocity of the reference frame of the first rock:
$v=-6.2 m/s -(+4.5 m/s) = -10.7 m/s$