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Chapter 15: Oscillations COURSE THEME: NEWTON’S LAWS OF MOTION! • Chs. 5 - 13: Methods to analyze dynamics of objects in Translational & Rotational Motion using Newton’s Laws! Chs. 5 & 6: Newton’s Laws using Forces (translational motion) Chs. 7 & 8: Newton’s Laws using Energy & Work (translational motion) Ch. 9: Newton’s Laws using Momentum (translational motion) Chs. 10 & 11: Newton’s Laws (rotational language; rotating objects). Ch. 14: Newton’s Laws for fluids (fluid language) NOW • Ch. 15: Methods to analyze the dynamics oscillating objects. First, need to discuss (a small amount of) VIBRATIONAL LANGUAGE Then, Newton’s Laws in Vibrational Language! Simple Harmonic Motion • Vibration Oscillation = back & forth motion of an object • Periodic motion: Vibration or Oscillation that regularly repeats itself. Motion of an object that regularly returns to a given position after a fixed time interval. • A special kind of periodic motion occurs in mechanical systems when the force acting on the object is proportional to the position of the object relative to some equilibrium position – If the force is always directed toward the equilibrium position, the motion is called simple harmonic motion • Simplest form of periodic motion: Mass m attached to an ideal spring, spring constant k (Hooke’s “Law”: F = -kx) moving in one dimension (x). Contains most features of more complicated systems Use the mass-spring system as a prototype for periodic oscillating systems Spring-Mass System • Block of mass m attached to ideal spring of constant k. Block moves on a frictionless horizontal surface • When the spring is neither stretched nor compressed, block is at the Equilibrium Position x = 0 • Assume the force between the mass & the spring obeys “Hooke’s Law” Hooke’s Law: Fs = - kx x is the displacement Fs is the restoring force. Is always directed toward the equilibrium position • Therefore, it is always opposite the displacement from equilibrium Block Motion Possible Motion: (a) Block displaced right of x = 0 – Position x is positive (right) – Restoring force Fs to the left (b) Block is at x = 0 – Spring is neither stretched nor compressed – Force Fs is 0 (c) Block is displaced left of x = 0 – Position x is negative (left) – Restoring force Fs to the right Terminology & Notation • Displacement = x(t) • Velocity = v(t) • Acceleration = a(t) Constant!!!!!!! Constant acceleration equations from Ch. 2 DO NOT APPLY!!!!!!!!!!! • • • • Amplitude = A (= maximum displacement) One Cycle = One complete round trip Period = T = Time for one round trip Frequency = f = 1/T = # Cycles per second Measured in Hertz (Hz) A and T are independent!!! • Angular Frequency = ω = 2πf = Measured in radians/s. • Repeat: Mass-spring system contains most features of more complicated systems Use mass-spring system as a prototype for periodic oscillating systems. • Results we get are valid for many systems besides this prototype system. • ANY vibrating system with restoring force proportional to displacement (Fs = -kx) will exhibit simple harmonic motion (SHM) and thus is a simple harmonic oscillator (SHO). (whether or not it is a mass-spring system!) Acceleration • Force described by Hooke’s Law is the net force in Newton’s 2nd Law FHooke FNewton The acceleration is proportional kx ma to the block’s displacement x k • Direction of the acceleration ax x m is opposite the displacement • An object moves with simple harmonic motion whenever its acceleration is proportional to its position and is oppositely directed to the displacement from equilibrium NOTE AGAIN! • The acceleration is not constant! – So, the kinematic equations can’t be applied! From previous slide: - kx = max – If block is released from position x = A, initial acceleration is – kA/m – When block passes through equilibrium position, a = 0 – Block continues to x = - A where acceleration is + kA/m Block Motion • Block continues to oscillate between –A and +A – These are turning points of the motion • The force is conservative, so we can use the appropriate potential energy U = (½)kx2 • Absence of friction means the motion will continue forever! – Real systems are generally subject to friction, so they do not actually oscillate forever Simple Harmonic Motion Mathematical Representation • Block will undergo simple harmonic motion model • Oscillation is along the x axis 2 d x k • Acceleration a 2 x dt m k 2 • Define: w m • Then a = -w2x. To find x(t), a function that satisfies this equation is needed – Need a function x(t) whose second derivative is the same as the original function with a negative sign and multiplied by w2 – The sine and cosine functions meet these requirements Simple Harmonic Motion Graphical Representation d 2x k a 2 x dt m • A solution to this is x(t) = A cos (wt + f) • A, w, f are all constants • A cosine curve can be used to give physical significance to these constants Definitions x(t) = A cos (wt + f) • A the amplitude of the motion – This is the maximum position of the particle in either the positive or negative direction w is called the angular frequency – Units are rad/s A & f: Determined by position & velocity at t = 0 If the particle is at x = A at t = 0, then f = 0 • The phase of the motion is (wt + f) • x (t) is periodic and its value is the same each time wt increases by 2p radians Period & Frequency • Period, T time interval required for mass to go through one full cycle of its motion – Values of x & v for the mass at time t equal the values of x & v at t + T 2p T w • Frequency, f Inverse of period. – Number of oscillations that mass m undergoes per unit time interval ƒ 1 w T 2p • Units: cycles per second = hertz (Hz) Summary Period & Frequency • Frequency & period equations can be rewritten to solve for w. Period & frequency can also be written: 2p w 2p ƒ T m T 2p k 1 ƒ 2p k m • Frequency& period depend only on mass m of block & the spring constant k • They don’t depend on the parameters of motion A, v • The frequency is larger for a stiffer spring (large values of k) & decreases with increasing mass of the block x(t), v(t), a(t) for Simple Harmonic Motion x(t ) A cos (wt f ) dx v w A sin(w t f ) dt 2 d x a 2 w 2 A cos(w t f ) dt • Simple harmonic motion is one-dimensional& so directions can be denoted by + or – sign • Obviously, simple harmonic motion is not uniformly accelerated motion. CANNOT USE kinematic equations from earlier. NOTE ONE MORE TIME! The acceleration is not constant! That is, the 1 dimensional kinematic equations for constant acceleration (from Ch. 2) DO NOT APPLY!!!!!! v f vi at x f xi vit at v f vi 2ax x f xi vt 2 2 1 2 THESE ARE WRONG AND WILL GIVE YOU WRONG ANSWERS!! 2 TO EMPHASIZE THIS! THROW THESE AWAY FOR THIS CHAPTER!! v f vi at x f xi vit at v f vi 2ax x f xi vt 2 2 1 2 2 THESE ARE WRONG AND WILL GIVE YOU WRONG ANSWERS!! Maximum Values of v & a x(t) = Acos(wt + f) v(t) = - wAsin(wt + f) a(t) = - w2Acos(wt + f) • Sine & cosine functions oscillate between 1, so we can easily find the maximum values of velocity and acceleration for an object in SHM v max amax k wA A m k 2 w A A m Graphs: General • The graphs show (general case) (a) Displacement x(t) as a function of time x(t) = Acos(wt + f) (b) Velocity v(t) as a function of time v(t) = - wAsin(wt + f) (c ) Acceleration a(t) as as a function of time a(t) = - w2Acos(wt + f) • The velocity is 90o out of phase with the displacement and the acceleration is 180o out of phase with the displacement Example 1 x(t) = Acos(wt + f) v(t) = - wAsin(wt + f) a(t) = - w2Acos(ωt + f) • Initial conditions at t = 0: • • • x(0) = A; v(0) = 0 This means f = 0 x(t) = Acos(wt) v(t) = - wAsin(wt) Velocity reaches a maximum of wA at x = 0 a(t) = - w2Acos(wt) Acceleration reaches a maximum of w2A at x = A Example 2 x(t) = Acos(wt + f) v(t) = - wAsin(wt + f) a(t) = - w2Acos(ωt + f) • Initial conditions at t = 0 x(0) = 0; v(0) = vi • This means f = p/2 x(t) = Asin(wt) v(t) = wAcos(wt) a(t) = - w2Asin(wt) • The graph is shifted one-quarter cycle to the right compared to the graph of x (0) = A