A motorboat travels 9 miles downstream (with the current) in 30 minutes. The return trip upstream (against the current) takes 90 minutes. Which system of equations can be used to find x, the speed of the boat in miles per hour, and y, the speed of the current in miles per hour? recall the formula d = rt. 9 = 0. 5(x – y) 9 = 1. 5(x + y) 9 = 1. 5(x – y) 9 = 0. 5(x + y) 0. 5 = 9(x – y) 1. 5 = 9(x + y) 1. 5 = 9(x – y) 0. 5 = 9(x + y).

Ateacher attaches a ball to a meter long string and whirls it around in a horizontal circle at constant speed. the teacher explains that the string is what makes the ball go in a circle. why is the string necessary? a) the string is needed to apply a torque to the ball. b) the string is needed to push on the ball to get it up to speed. c) circle motion requires an inertia which is provided by the string. d) circle motion requires a force to give it a centripetal acceleratio