# HW_5 - P112 Spring 2007 Homework#5 Due Friday Feb 23rd 2007...

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P112 Spring 2007 Homework #5 Due: Friday Feb 23rd, 2007 – At lecture Reading: 4.3-4.6, 5.1, 5.2 Not all concepts will be covered in class – you are responsible for all material in the reading and in the homework problems. Warm-up Exercises: (Answer only will be marked) 1. Three boxes, with masses M 3 < M 1 and M 1 < M 2 , are connected by two massless ropes, Rope A and Rope B, as shown in the picture. A person's hand pulls the third box with a constant force F , accelerating the system to the right. The system slides on a frictionless horizontal surface. a) Draw a free body diagram for each box and for each of the two ropes. Please identify each force by interaction, the body providing it, and the body on which it is exerted. b) Identify all Third Law interaction pairs shown in your free-body diagrams . Mark your pairs with x, xx, etc. as we did in the tutorials. If there are none, write NONE. c) How does the tension T A in the rope between M 1 and M 2 compare to the tension T B in the rope between M 2 and M 3 ? 2. An athlete whose mass is 90.0 kg is performing weight-lifting exercises. Starting from the rest position he lifts, with constant acceleration, a barbell that weighs 490 N. He lifts the barbell a distance of 0.60 m in 1.6 s. a) Draw a clearly labeled free-body diagram for the barbell and for the athlete. b) Use the diagrams in part a and Newton’s laws to find the total force that his feet exert on the ground as he lifts the barbell.

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