At the same moment, one rock is thrown upward at 4.5 m/s and another thrown downward at 3.4 m/s. What is the relative velocity of the first rock from the perspective of the second rock? Assume that up is positive.
so we can assume the first rock is stationary, by adding the top rocks velocity, to the bottom one, therefore, we have -10.7 ms-1, from the perspective of the first rock, hope this helps
