B PHYSICS 218 – EXAM 2 Tuesday, October 26, 2010 NAME: SECTION: 513 514 515 516 Note: 513 Recitation & lab Wed 8:00 - 10:50 am 514 Recitation & lab Wed 11:30 am - 2:20 pm 515 Recitation & lab Wed 3:00 - 5:50 pm 516 Recitation & lab Wed 6:10 - 9:00 pm There are a total of 7 problems on this test: • Problems 1, 2, and 3 are worth 6 or 7 points each. For these three problems, points will be deducted for the wrong units or wrong number of significant digits. Other than that, no partial credit will be awarded for incorrect answers. • Problems 4, 5, 6, and 7 are worth 20 points each. For these four problems, partial credit will be awarded where appropriate. For all 7 problems: • You must show your work and/or explain your reasoning to receive any credit for a problem; merely stating the answer is NOT sufficient. • Write your final answer(s) in the blanks provided. • You may use the backs of the pages for scratch calculations if you wish, but only the work in the spaces provided on the front of the pages will be graded. • For numerical values, assume that all specified digits are significant, including trailing zeros. Also remember, an answer CAN NOT be completely correct if it has the wrong units or the wrong number of significant digits. G O O D L U C K ! ! ! ! !B For problems 1, 2, and 3, do your work in the space provided, and write your final answer in the blank. Points will be deducted for the wrong units or wrong number of significant digits. Other than that, no partial credit will be awarded for incorrect answers. 1. (6 points) In the figure below, block A weighs 46.0 N and block B weighs 24.0 N. Once block B is set into downward motion, it descends at constant speed. Calculate the coefficient of kinetic friction between block A and the tabletop. Coefficient of friction ___________________________ 2. (7 points) A 2.50 kg block is pushed against a spring with negligible mass and force constant k = 450 N/m, compressing it 0.220 m. When the block is released, it moves along a frictionless, horizontal surface and then up a frictionless incline with slope 37.00. How far does the block travel along the incline before starting to slide back down again? Distance _______________________________ 3. (7 points) A person pulls horizontally on block B in the figure below. Both blocks A and B are seen to move together toward the right. The coefficient of static friction between block B and the table is 0.45. The coefficient of kinetic friction between block B and the table is 0.35. To the right of the figure, draw a free-body diagram for block B that would satisfy an Engineering professor.B For problems 4, 5, 6, and 7, do your work in the space provided, and write your final answer in the blank. For these problems, partial credit will be awarded where appropriate, based on the work that you show. 4. (20 points) To study damage to aircraft that collide with large birds, you design a test gun that will accelerate 1.6-kg chicken-sized objects so that their displacement along the gun barrel is given by: x = (9.9 x 103 m/s2)t2 – (8.0 x 104 m/s3)t3. The object leaves the end of the barrel at t = 0.026 s. (a) What will be the speed of the objects as they leave the end of the barrel? (b) What net force must be exerted on an object at (i) t=0 and (ii) t = 0.026 s? Speed ________________________________ Force (i) _____________________________ Force (ii) _____________________________B 5. (20 points) A box is sliding with a speed of 4.30 m/s on a horizontal surface when, at point P, it encounters a rough section. On the rough section, the coefficient of friction is not constant, but starts at 0.100 at P and increases linearly with distance past P, reaching a value of 0.600 at 12.5 m past point P. (a) How far does the box slide before stopping? (b) What is the coefficient of friction at the stopping point? (c) How far would the box have slid if the friction coefficient didn’t increase but instead had the constant value of 0.100 everywhere after point P? Distance (a) __________________________________________ Coefficient of friction __________________________________ Distance (c) __________________________________________B 6. (20 points) An object is moving in the x-y plane under the influence of a force F. (a) Assume F = Cy2i, where C is a constant with units of N/m2. Is the force conservative or nonconservative? (Encircle your answer below.) In the appropriate space below, either (i) find the potential energy function U and encircle your result, or (ii) prove that no such function exists. (b) Now assume F = Cy2j, where C is a constant with units of N/m2. Is the force conservative or nonconservative? (Encircle your answer below.) In the appropriate space below, either (i) find the potential energy function U and encircle your result, or (ii) prove that no such function exists. Part (a): Conservative Nonconservative Proof: ____________________________________________________________________________ Part (b): Conservative Nonconservative Proof:B 7. (20 points) You are driving in a circle of radius R on a flat surface that is inclined at an angle θ from the horizontal, as shown in the figure below. At the instant shown in the figure, you are driving directly out of the page (i.e., in the horizontal direction). The coefficient of static friction between the tires and the inclined surface is μs. Find the maximum speed vmax that you can be moving in the circular path to avoid slipping at the moment shown in the figure if: (a) at the moment in the figure, you are at the highest point as you go around your circular path; (b) at the moment in the figure, you are at the lowest point as you go around your circular path. (In both cases, express your answer in terms of g and the constants specified above.) Speed (a) _______________________________________ Speed (b)
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