2.003 Engineering Dynamics: Problem Set 6 with Concept Questions Answers

2.003 Engineering Dynamics Problem Set 10 with answer to the concept questions Problem 1 Figure 1. Cart with a slender rod A slender rod of length l (2m) and mass m2(0.5kg)is attached by a frictionless pivot at ‘A’ to a block of mass m1(1kg). The block moves horizontally on rollers. The position of the block is described by the x coordinate. The block is connected to a fixed wall by a spring of constant k(10 N/m)and unstretched length lo(0.5m),and a damper with damping coefficient b (5N-s/m). The position of the block in the inertial frame is xIˆ. Use the results from Pset 6, #1 to find the two equations of motion of this system and linearize them. Use the linearized equations to find the undamped natural frequencies and mode shapes of the system. Concept question: Do you expect to be able to eliminate the terms involving gravity in the equations of motion by choosing coordinates with respect to static equilibrium positions. Answer: Gravity will appear in at least one of the EOMs in this problem because it provides the restoring torque on the pendulum. The moment arm is a function of  and therefore the term involving gravity will involve a coordinate which describes the motion. Problem 2 An Optics experiment is set up on a vibration isolation table which may be modeled as a single degree of freedom system for vertical oscillations. The system has very little damping. The floor has considerable vertical vibration at 30 Hz. What must the natural frequency of the table be in order to reduce the motion of the table and optics experiment by 12 dB, when compared to the motion of the floor? 1 x Stated mathematically this would read 12dB  20log10 ( ) . For this part of the problem y assume the damping is 0%. When working on the table, you notice that after an accidental bump the resulting motion takes a long time to die out. To make it die down sooner you add a damper between the table and the floor, such that the total damping is 10% of critical damping. Compute the value of |x/y| with this amount of damping to that from part one. Concept question: Will the addition of damping increase or reduce the vibration of the table in response to floor motion at 30 Hz? Answer: The addition of damping increases the response of the table to the floor motion when  the frequency ratio is greater than 1.414. I.e.  2 n Problem 3 Prof. Vandiver once measured the vibration response of a coast guard light station, which stood in 20 meters of water. It was a steel, space frame structure with a large house sitting on the top. A simple single degree-of-freedom equivalent model of the structure is a massless beam with an equivalent concentrated mass on the end. The equivalent mass is the sum of the mass of the house and 1/4 of the mass of the space frame structure. The total concentrated mass in this case was M = 2.73*105 Kg. The measured natural frequency of the platform in the first bending mode was 1.0 Hz, and the measured damping was 1.0 % of critical. By watching the output of an accelerometer on an oscilloscope the professor was able to shift the weight of his body from one foot to the other in synchrony with the motion of the structure at 1.0 Hz. He was able to drive the structure to amplitudes in excess of that observed in a storm with 50 knot winds plus 20 foot ocean waves. The shifting of his weight from side to side may be modeled as an horizontal harmonic force. Find the magnitude of the steady state 2 horizontal force that was necessary to drive the structure to a steady state acceleration amplitude of 0.003 g’s. Concept question: What was the steady state response amplitude expressed in meters: a). 0.003m, b). 0.003*9.81 m, c). 0.003*9.81/(6.28)2 m. Answer: The relationship between acceleration and displacement magnitude for steady state harmonic motion is given by x  x  2 , where x is in meters and acceleration is in m/s 2 . x  x  2  0.003( g )9.81 m / s2  0.02943m / s 2 g m / s2 2 /  2  (radians / s) 2  0.00075meters g This is on the order of a millimeter of motion. x  x /  2  0.003( g )9.81 Problem 4 An irregularly shaped object is hinged at one point. It is supported by several springs. Its measured natural frequency is 5.0 Hz. When a static force of 10 N isapplied 0.4m from the pivot, the object rotates through an angle of 0.005 radians. What is the mass moment of inertia of the object about the pivot? Assume the body is horizontal when at rest. Concept question: For small motions about the horizontal do you expect the natural frequency to be a function of gravity. Answer: For small oscillations about the horizontal the torque produced by the weight of the object acting on a moment arm rG/A=0.4m is a constant independent of the angle of oscillation. The natural frequency for small oscillations is not a function of ‘g’. 3 Problem 5 A 1.0 kg mass, m sits on top of a 10.0 kg mass, M. The large mass is connected to a spring, (K=250N/m) and a damper, R, and is free to oscillate horizontally on rollers. The damping ratio of the system is 5%. The coefficient of static friction between the large and small block is 0.35. The large block is driven in steady state vibration by an harmonic force, Fo(t). a. What is the maximum allowable acceleration of the large block, such that the small block does not slide. Express this acceleration in g’s? b. If the harmonic force, Fo(t), is applied to the larger mass at the natural frequency of the system, what is the largest force magnitude, Fo, that may be applied such that the small mass does not slide on top of the large mass. Concept question: When the acceleration of the system is one-half that required to make the mass slide, what is the magnitude of the friction force. a. µMg b. µMg / 2 c. neither Answer: The magnitude is answer b., one half of the maximum possible friction force. The main point of this question is to shine some light on the frequent misconception that friction force is always µMg. In fact friction force is only as large as it needs to be to get the job done. µMg is an upper bound. Problem 6 A mortar shell is shot almost vertically from a rail car. The rail car is spring supported as shown in the figure. It also has a damper as shown. The mass of the rail car and the mortar is M = 10,000 kg. The mass of the shell is 25 kg. The velocity of the shell is 150m/s as it leaves the mortar. The spring stiffness of the suspension system of the rail car is k =15,791 N/m. The damper constant, R, is 251.33 Ns/m. a. Write down the equation of this single degree of freedom system for vertical motion. Determine the natural frequency and damping 4 ratio (obtain numerical values). b. The mortar is loaded and fired when the system is initially at rest (no motion). After firing the shell the car will vibrate. Find an expression for x(t), the time history of the vibration of the car, after the shell has been fired. Be specific about the initial conditions that you use. Sketch the result. c. The vibration of the car will decay with time after firing. Predict the ratio of two vibration amplitude peaks separated by five periods of vibration. Concept question: What initial conditions will be required? A. initial displacement only, B. Initial velocity only, C.Both initial velocity and displacement. Answer: C. both initial velocity and displacement will be needed in the answer. Hint: Use a coordinate system corresponding to the at–rest position of the car after the shell has been fired and the vibration has stopped. 5