A parachutist relies on air resistance (mainly on her parachute) to decrease her downward velocity. She and her parachute have a mass of m and air resistance exerts a total upward force of f on her and her parachute.

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