95.141 Physics I (Navitas) EXAM #2 Spring 2015 Name, Last Name First Name Student Identification Number: Write your name at the top of each page in the space provided. Answer all questions, beginning each new question in the space provided. Show all work. Show all formulas used for each problem prior to substitution of numbers. Label diagrams and include appropriate units for your answers. You may use an alphanumeric calculator (one which exhibits physical formulas) during the exam as long as you do not program any formulas into memory. By using an alphanumeric calculator you agree to allow us to check its memory during the exam. Simple scientific calculators are always OK! SCORE 1. (15) 2. (20) 3. (20) 4. (20) Total Score (out of 75 pts) Total Score (scaled up to 100 pts)
Problem 1: (15 points) - 3 pts each -- No partial credit on this problem only. Circle the letter corresponding to the most correct answer. 2 1-1 Alice and Tom dive from an overhang into the lake below. Tom simply drops straight down from the edge, but Alice takes a running start and jumps with an initial horizontal velocity of 25 m/s. Neither person experiences any significant air resistance. Just as they reach the lake below A) the speed of Alice is larger than that of Tom. B) the splashdown speed of Alice is larger than that of Tom. C) they will both have the same speed. D) the speed of Tom will always be 9.8 m/s larger than that of Alice. E) the speed of Alice will always be 25 m/s larger than that of Tom. 1-2 An electrical motor spins at a constant 2857.0 rev/min. If the armature radius is 2.685 cm, what is the acceleration of the outer edge of the armature? A) 84.40 m/s2 B) 844.4 m/s2 C) 241,100 m/s2 D) 2403 m/s2 1-3 An object is moving forward with a constant velocity. Which statement about this object MUST be true? A) No forces are acting on the object. B) The net force on the object is in the forward direction. C) The net force on the object is zero. D) The acceleration of the object is in the forward direction.
3 1-4 A person is in an elevator that is traveling upward but is gradually slowing down at a rate of 9 m/s2? f is the force of the floor on the person and g is the force of gravity on the person and their magnitudes are indicated by the length of the vectors. Which one of the following free-body diagrams best represents the free-body diagram take only the correct relative force magnitudes A) B) C) D) E) 1-5 A fish weighing 16 N is weighed using two spring scales, each of negligible weight, as shown in the figure. What will be the readings of the scales? A) The bottom scale will read 16 N, and the top scale will read zero. B) Each scale will read 16 N. C) The top scale will read 16 N, and the bottom scale will read zero. D) The scales will have different readings, but the sum of the two readings will be 16 N. E) Each scale will read 8 N.
For problems 2, 3 and 4 4 NO credit will be given for just correct answer without appropriate formulae/logic. Circle your numerical answers. Problem 2a: (10 points) A traffic-light of mass, M is supported by two ropes as shown in the figure. A) Draw a free body diagram indicating all the forces and their components; label the coordinate system. B) If M = 8 kg and θ = 37 o, determine the tensions in the ropes. Problem 2b: (10 points) A construction worker pulls a box (mass M) on a rough horizontal floor with rope (of tension T), in a direction of θ above the horizontal. The coefficient of kinetic friction is μ k. A) Draw a free body diagram for the box. B) How hard does the floor push up on the box? C) Determine the acceleration of the box.
Problem 3: (5 points each part total of 20 points) 5 An object with a mass of 175 kg is launched from the ground with a speed of 120 m/s at an angle of 60 degrees above the surface. The object hits a vertical wall which is at a distance of 250 m from the position that the object is launched. A) Draw a suitable diagram, label the coordinate system and show the trajectory of the ball. Label the variables. B) How long the object is in the air before hitting the wall? C) Determine the maximum height reached by the object. D) Determine both the magnitude and direction of the velocity just before the object hits the wall. Write this in the unit vector notation.
Problem 4: (5 points each part total of 20 points) 6 The block of mass M 1, on an rough inclined plane is connected to a second block of mass M 2 by a cord passing over a pulley as shown. M 2 is greater than M 1 A) Draw and label clearly the free-body diagram for each block. B) Using Newton s Second Law, write down the equations that will allow the determination of the motion of the blocks. C) Solve the equations in order to determine an expression for the acceleration of the blocks in terms of M 1, M 2, θ, μ k and g show all steps clearly. D) If M 1 = 1.2 kg, M 2 = 1.8 kg, θ = 34 o, μ k = 0.15, determine for the acceleration of the blocks and the tension in the string.