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UMass Amherst KIN 430 - problem-set3-2018sp

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KIN 430 - Problem Set 1KIN 430 – Biomechanics Spring Semester 2018 Problem Set #3 (20 points) – Due on Wednesday, April 11th, 2018, in class To receive full credit, you must hand the completed problem set directly to one of the course instructors (do not slip it under a door or leave it in the front office) Answer the following questions by placing your answers in the spaces provided. Do all of your calculations on separate sheets of paper, clearly indicating which calculations go with which problem. Staple the separate sheets of paper to this problem set before you turn it in. You must show your calculations to receive full credit, even if you have the correct answers. Don’t forget to include proper units! 1. (4 pts.) A person with a body mass of 68 kg is pushing off against the ground to perform a broad jump while wearing a 7.5 kg weighted vest to generate a training overload. There is a wind that has a component blowing in the person’s face of -121 N and a component blowing downward of -72 N. At one point in the push-off the person generated a vertical ground reaction force of 2637 N and an anterior-posterior ground reaction force of 389 N. What was the resultant acceleration of the person’s center of mass (magnitude and direction)? Center of mass acceleration _________________ 2. (4 pts.) During the warm-ups for the 2018 World Vertical Jumping Games, nearly everyone on the planet did a maximal-effort jump together. Seven billion people all jumped at the same time, producing an acceleration of the Earth of 2.4  10-12 m/s2. If the mass of the Earth is 5.9  1024 kg, what was the average force generated per person? Average force per person ________________ 3. (4 pts.) While landing from a jump, a person generated the vertical ground reaction force shown in the diagram below (solid dark line). Also shown is the magnitude of the person’s body weight (500 N, dashed dark line). Using the graph, determine the total positive impulse, the total negative impulse, and the net impulse acting on the body over the time interval from 0.0 to 0.4 s. Total positive impulse _______________ Total negative impulse _______________ Net impulse _______________ time (s) force (N) 0.0 0.2 0.4 1000 0 0.1 0.3 0.5 1500 500KIN 430 - Problem Set 24. (4 pts.) A woman with a mass of 67 kg is squatting on a force platform while holding two 5 kg dumbbells (one in each hand). You record the vertical force acting on her body as she performs the following tasks: A) she remains still while holding the squatting position for 1 s, B) she steadily stands up over the course of 1 s, C) she stands still in the upright position for 1 s, then D) she lets go of the dumbbells, such that they do not land on the force platform, and remains standing still for 1 more second. In the spaces provided below, draw: A) A free body diagram showing the vertical forces acting on the woman. B) A graph showing the vertical force that would be recorded by the force plate as the woman performs this sequence of tasks (be sure to indicate some numerical values on the vertical and horizontal axes). done 5. (4 pts.) Ideally, you want the friction force between cross-country skis and the snow to be low so the skis will glide freely; however, there needs to be some friction in order to be able to go up hills. A person is skiing up a 10º slope, as shown in the figure below, and generates a 1500 N force oriented directly downward. What is the minimum value of the static coefficient of friction (S) between the base of the ski and the snow that will prevent the ski from sliding backwards when the person generates this force? Minimum magnitude of S ____________ A - free body diagram B - force plate graph force (N) time (s) 10 1500


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