PHYS 172/1720 EXAM 2 m (200 Points) Spring 2022 Problems 20 - 21: l u" + d i A box of mass M contains a length of string of _, __ negligible mass. Friction makes it difficult to pull the F F string though a tight hole in the side of the box. Initially, the box is at rest on a very slippery surface Ff" with a short length of string outside the box. You start to pull on the string with a force of constant magnitude .F as shown in the figure below. A short time later you have pulled an additional length d of string out of the box and the box has moved as distance w. As you pull the string through the tight hole in the box ofthe previous problem you expect the string and box to get warmer because of the friction interaction between them. Problem 20: [6 points] What is the change of the box-string system's translational kinetic energy in the situation? A] Fw B) Fd C) F (w + a!) D] F (d w) E] None of the above