physics 8.pdf - Chapter 4 Oscillatory Motion 4.1 The...

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Unformatted text preview: Chapter 4 Oscillatory Motion 4.1 The Important Stuff 4.1.1 Simple Harmonic Motion In this chapter we consider systems which have a motion which repeats itself in time, that is, it is periodic. In particular we look at systems which have some coordinate (say, x) which has a sinusoidal dependence on time. A graph of x vs. t for this kind of motion is shown in Fig. 4.1. Suppose a particle has a periodic, sinusoidal motion on the an axis, and its motion takes it between x = +A and cc 2 —A. Then the general expression for x(t) is x(t) = Acos(wt + gt) (4.1) A is called the amplitude of the motion. For reasons which will become clearer later, w is called the angular frequency. We say that a mass which has a motion of the type given in Eq. 4.1 undergoes simple harmonic motion. From 4.1 we see that when the time t increases by an amount 27”, the argument of the cosine increases by 271' and the value of a: will be the same. So the motion repeats itself after a time interval 27”, which we denote as T, the period of the motion. The number of Figure 4.1: Plot of 3: vs. t for simple harmonic motion. (t and a: axes are unspecified!) 69 70 CHAPTER 4. OSCILLATORY MOTION oscillations per time is given by f = %, called the frequency of the motion: T : 21 1 a} w Rearranging we have a formula for w in terms of f or T: _27r T Though a; (angular frequency) and f (frequency) are closely related (with just a factor of 271' between them, we need to be careful to distinguish them; to help in this, we normally express a; in units of %1 and f in units of Cysde, or HZ (Hertz). However, the real dimensions of both are i in the 81 system. From 33(15) we get the velocity of the particle: to = 27rf (4.3) 11(75) 2 (31—: = —wA sin(wt + d) (4.4) and its acceleration: d a(t) = CT: = —w2Acos(wt + a) (4.5) We note that the maximum values of v and a are: vmax = wA amax = @0214 (4.6) The maximum speed occurs in the middle of the oscillation. (The slope of 90 vs. t is greatest in size when a: = 0.) The magnitude of the acceleration is greatest at the ends of the oscillation (when a: = iA). Comparing Eq. 4.5 and Eq. 4.1 we see that d2a: 2 which is the same as a(t) = —w2m(t). Using 4.1 and 4.4 and some trig we can also arrive at a relation between the speed |v(t)| of the mass and its coordinate 50(25): |v(t)| = wA|sin(wt+gb)|=wa/1—cos2(wt+gb) z a 14%)? (4.8) We could also arrive at this relation using energy conservation (as discussed below). Note, if we are given a: we can only give the absolute value of 11 since there are two possibilities for velocity at each 3: (namely a j: pair). 4.1. THE IMPORTANT STUFF 71 Figure 4.2: Mass m is attached to horizontal spring of force constant k; it slides on a frictionless surface! 4.1.2 Mass Attached to a Spring Suppose a mass m is attached to the end of a spring of force constant k (whose other end is fixed) and slides on a frictionless surface. This system is illustrated in Fig. 4.2. Then if we measure the coordinate a: of the mass from the place where it would be if the spring were at its equilibrium length, Newton’s 2nd law gives d2 Fgcz—kxzmalazmfij, and then we have d2x _ k (4 9) dt2 — mm ' Comparing Eqs. 4.9 and 4.7 we can identify of with % so that k z _ 4.10 From the angular frequency w we can find the period T and frequency f of the motion: 2 1 1 T=—7T=Z7n/% f:— E (4.11) w T:%m It should be noted that w (and hence T and f) does not depend on the amplitude A of the motion of the mass. In reality, of course if the motion of the mass is too large then then spring will not obey Hooke’s Law so well, but as long as the oscillations are “small” the period is the same for all amplitudes. In the lab, it’s much easier to work with a mass bobbing up and down on a vertical spring. One can (and should!) ask if we can still use the same formulae for T and f, or if gravity (9) enters in somehow. In fact, the same formulae (Eq. 4.11) do apply in this case. To be more clear about the vertical massispring system, we show such a system in Fig. 4.3. In (a), the spring is oriented vertically and has some unstretched length. (We are ignoring the mass of the spring.) When a mass m is attached to the end, the system will be 72 CHAPTER 4. OSCILLATORY MOTION (a) (b) (C) Figure 4.3: (a) Unstretched vertical spring of force constant k; (assumed massless). (b) Mass attached to spring is at equilibrium when the spring has been extended by a distance mg/k. (c) Mass will undergo small oscillations about the new equilibrium position. at equilibrium when the spring has been extended by some length y; balancing forces on the mass, this extension is given by: E I: When the mass is disturbed from its equilibrium position, it will undergo harmonic oscil— lations which can be described by some coordinate x, where x is measured from the new equilibrium position of the end of the spring. Then the motion is just like that of the horizontal spring. ky=mg : y: Finally, we note that for more precise work with a real springimass system one does need to take into account the mass of the spring. If the spring has a total mass m5, one can show that Eq. 4.10 should be modified to: k w = —m (4.12) m + ?5 That is, we replace the value of the mass m by m plus one—third the spring’s mass. 4.1.3 Energy and the Simple Harmonic Oscillator For the massispring system, the kinetic energy is given by K = $771212 = émw2A2 sin2(wt + gb) (4.13) and the potential energy is U = ékfi = £16142 cos2(wt + gb) . (4.14) Using w2 = g in 4.13 we then find that the total energy is E = K + U = ékA2[sin2(wt + ¢) + cos2(wt + ¢)] 4.1. THE IMPORTANT STUFF 73 and the trig identity sin2 6 + cos2 6 = 1 gives E = am? (4.15) showing that the energy of the simple harmonic oscillator (as typified by a mass on a spring) is constant and is equal to the potential energy of the spring when it is maximally extended (at which time the mass is motionless). It is useful to use the principle of energy conservation to derive some general relations for lidirnensional harmonic motion. (We will not use the particular parameters for the massi spring system, just the quantities contained in Eq. 4.1, which describes the motion of a mass m along the 3: axis. From Eq. 4.13 we have the kinetic energy as a function of time K = %mv2 = %mw2A2 sin2 (wt + qfi) Now the maximum value of the kinetic energy is émw2A2, which occurs when :1: = 0. Since we are free to fix the “zeroipoint” of the potential energy, we can agree that U (3:) = 0 at a: = 0. Then the total energy of the system must be equal to the maximum (i.e. a: = 0 value of the kinetic energy: E = émw2A2 Then using these expressions, the potential energy of the system is U = E — K = émw2A2 — émw2A2 sin2(wt + gb) = émw2A2(1 — sin2 (wt + gb)) : émw2A2 cos2(wt + (M 2 1mafia? 2 Of course, for the massispring system U is given by ékfl, which gives the relation mw2 = k, or w = &, which we’ve already found. If we use the relation vmax = wA then the potential energy can be written as 2 mvmaxx2 (4.16) U($) : % A2 l 2 4.1.4 Relation to Uniform Circular Motion There is a correspondence between simple harmonic motion and uniform circular motion, which is illustrated in Fig. 4.4 (a) and (b) In (a) a mass point moves in a horizontal circular path with uniform circular motion at a radius R (for example, it might be glued to the edge of a spinning disk of radius R). Its angular velocity is w, so its location is given by the timeivarying angle 6, where 9(t)=wt+¢ 74 CHAPTER 4. OSCILLATORY MOTION (a) (b) Figure 4.4: (a) Mass point moves in a horizontal circle of radius R. The angular velocity of its motion is w. A guy with a big nose (seen from above) is observing the motion of the mass at the level of the circle. He sees only the so coordinate of the point’s motion. (b) Motion of the mass as seen by the guy with the big nose. The projection of the motion is the same as simple harmonic motion with angular frequency w and amplitude R. (a) (b) Figure 4.5: (a) Simple pendulum. (b) Physical pendulum. In 4.4 (b) we show the motion of the mass as it would be seen by someone looking toward the +3; direction at the level of the disk. Such an observer sees only the changing w coordinate of the mass’s motion. Since a: = Rcos0, the observed coordinate is a:(t) = Rcos(0(t)) = Rcos(wt + ng) , the same as Eq. 4.1. The motion of the corresponding (projected) harmonic oscillator has an angular frequency of w and an amplitude of R. 4. 1.5 The Pendulum We start with the simple pendulum, which has just a small mass m hanging from a string of length L whose mass we can ignore. (See Fig. 4.5 (a).) The mass is set into motion so 4.1. THE IMPORTANT STUFF 75 that it moves in a vertical plane. One can show that if 6 is the angle which the string makes with the vertical, it obeys the differential equation: 2 g 2 —f Sin 6 One should note that this is not of the form given in Eq. 4.7. Things are much simpler when we restrict 6 to be “small” at all times. If that is the case, then we can use the approximation sin 6 z 6, which is true if we are measuring 6 in radians. Then the differential equation becomes d26 9 Comparison of this equation with Eq. 4.7 lets us identify the angular frequency of the motion: 9 —_ _ 4.18 w (/L ( ) 271' (L w 1 g T=—=2 _ :_:_ — 4.19 w 7T 9 f 271' 271' L ( ) The (perhaps) surprising thing about Eqs. 4.18 and 4.19 is that they have no dependence on the mass suspended from the string or on the amplitude of the swing. . . as long as it is a small angle! 6(t) = 6max cos(wt + gb) (4.20) We must always keep our assumption of “small” 6 in the back of our minds whenever we do a problem with a pendulum. The formulae giving T and f become less accurate as 6max gets too big. An important generalization of the simple pendulum is that of a rigid body which is free to rotate in a plane about some (frictionless!) pivot. Such a system is known as a physical pendulum and is diagrammed in Fig. 4.5 (b). Suppose we look at the line which joins the pivot to the center of mass of the object. If 6 is the angle which this line makes with the vertical, and if we again use the approximation sin 6 s 6, one can show that it obeys the differential equation 2 L9 = _W (4.21) dt2 I where d is the distance between the pivot and the center of mass, M is the mass of the object and I is the moment of inertia of the object about the given axis. (Note: the axis is probably not at the center of mass; if it were, the mass wouldn’t oscillate!) Following the usual procedure we find the period T: 271' I T=—=2 (l— 4.22 w W Mgd < > 76 CHAPTER 4. OSCILLATORY MOTION 4.2 Worked Examples 4.2.1 Simple Harmonic Motion 1. The displacement of a particle at t = 0.25s is given by the expression x = (4.0m) cos(3.07rt + 7r) Where cc is in meters and t is in seconds. Determine (a) the frequency and period of the motion, (b) the amplitude of the motion, (c) the phase constant, and (d) the displacement of the particle at t = 0.25s. [Ser4 13—1] (a) We compare the given function 33(15) with the standard form for simple harmonic motion given in Eq. 4.1. This gives us the angular frequency w: rad and from this we can get the frequency and period: w 3.07rfl = — = S = 1. H f 27r 277 50 Z 1 1 T = — = — = 0.667 f (1.50s71) S (b) We easily read off the amplitude as the factor (a length) which multiplies the cosine function: A = 4.0 m (c) Again, comparison with Eq. 4.1 gives ¢ = W (d) At t = 0.25s the displacement (i.e. the coordinate) of the particle is: 33(025 s) = (4.0 m) cos((3.07r)(0.25) + 7r) 2 (4.0m) cos((1.75)7r) = (4.0m)(0.707) = 2.83m 2. A loudspeaker produces a musical sound by means of the oscillation of a diaphragm. If the amplitude of oscillation is limited to 1.0 X 10’3 mm, what fre- quencies will result in the magnitude of the diaphragm’s acceleration exceeding 9? [HRW5 16-5] We are given the amplitude of the diaphragm’s motion, A = 1.0x 10’3 mm = 1.0x 10’6 m. From Eq. 4.6, the maximum value of the acceleration is amax 2 Am? So then the angular frequency that results in a maximum acceleration of g is max 98% _ w2=a z¥=98><10682 A (1.0 x 10-6 m) 4.2. WORKED EXAMPLES 77 : w=3.1><103s_1. This corresponds to a frequency of 3.1 103 *1 fzi:—( X S >25.0><102Hz 271' 271' At frequencies larger than 500 HZ, the acceleration of the diaphragm will exceed g. 3. The scale of a spring balance that reads from 0 to 15.0kg is 12.0cm long. A package suspended from the balance is found to oscillate vertically with a frequency of 2.00 HZ. (a) What is the spring constant? (b) How much does the package weigh? [HRW5 16—6] (a) The data in the problem tells us us that the spring within the balance increases in length by 12.0 cm when a weight of W = mg 2 (15.0kg)(9.80 3%) = 147N is pulls downward on its end. So the force constant of the spring must be F 14F k:_: ( 7) — = 1225 E x (12 X 10’2 m) m (b) Eq. 4.11 we have the frequency of oscillation of the massispring system in terms of the spring constant and the attached mass. We have the frequency and spring constant and we can solve to get the mass of the package: —27r\/m m_4w2f2 _ (1225 g) _ 4w2(2.00s-1)2 That’s the mass of the package; its weight is Plug in the numbers: m = 7.76kg W = mg = (7.76 kg)(9.80 8%) = 76N 4. In an electric shaver, the blade moves back and forth over a distance of 2.0mm in simple harmonic motion, with frequency 120 HZ. Find (a) the amplitude, (b) the maximum blade speed, and (c) the magnitude of the maximum acceleration. [HRW5 16-9] (a) The problem states that the full distance that the blade travels on each back—and—forth swing is 2.0mm, but the full swing distance is twice the amplitude. So the amplitude of the motion is A = 1.0mm. 78 CHAPTER 4. OSCILLATORY MOTION (b) From Eq. 4.6 we have the maximum speed of an oscillating mass in terms of the amplitude and frequency: vmax 2 (MA = 2wa = 27r(120s’1)(1.0 >< 10*3m) = 0.75% (c) From Eq. 4.6 we also have magnitude of the maximum acceleration of an oscillating mass in terms of the amplitude and frequency: amax = 61214 2 (2mm = 4w2(120s*1)2(1.0 >< 10*3m) = 570g 5. The end of one of the prongs of a tuning fork that executes simple harmonic motion of frequency 1000 HZ has an amplitude of 0.40 mm. Find (a) the maximum acceleration and (b) the maximum speed of the end of the prong. Find (c) the acceleration and (d) the speed of the end of the prong when the end has a displacement of 0.20 mm [HWR5 16—22] (a) Since we have the amplitude A of the prong’s motion, and we can easily find the angular frequency w: w = 27rf = 27r(1000 Hz) 2 6.28 x 103 s71 we can use Eq. 4.6 to find the maximum value of a: am 2 61214 = (6.28 x 103 s’1)2(0.400 X 10*3 m) = 1.6 x 104 8% (b) Likewise, from the same equation we find the maximum speed of the prong’s tip: vmax = wA = (6.28 x 103s—1)(0.400 >< 10—3m) = 2.52 (c) Equation 4.7 relates the acceleration a and coordinate a: at all times. When the dis— placement of the prong’s tip is 0.20 mm (half of its maximum) we find a 2 W26 2 —(6.28 X 103 s—1)2(0.20 x 10‘3 m) = —7.9 x 103 8% (d) We have already given a relation between |v| (speed) and :6 in Eq. 4.8. We use it here to find the speed when x = 0.20 mm: M z .A 14%? (6.28 x 103s—1)(0.40 >< 10—3m) 1 — < 2.2 E S 0.20 mm) 2 0.40 mm | | 4.2. WORKED EXAMPLES 79 Frictionless Figure 4.6: Mass M is attached to a spring and oscillates on a frictionless surface. Another block of mass m is on top! 4.2.2 Mass Attached to a Spring 6. A 7.00 — kg mass is hung from the bottom end of a vertical spring fastened to an overhead beam. The mass is set into vertical oscillations having a period of 2.60s. Find the force constant of the spring. [Ser4 13—11] The formulae in Eq. 4.11 hold even if the massispring system oscillates vertically (just as long as we can neglect the mass of the spring). Then we can solve for the force constant: m 4W2m 4W2m ”V k 3 k 3 T2 and the numbers give us The force constant of the spring is 40.9 g. 7. Two blocks (m = 1.0 kg and M = 10kg) and a spring (k = 200 g) are arranged on a horizontal, frictionless surface as shown in Fig. 4.6. The coefficient of static friction between the two blocks is 0.40. What is the maximum possible amplitude of simple harmonic motion of the spring—block system if no slippage is to occur between the blocks? [HRW5 16-25] We first look at what happens when the two blocks oscillate together. In that case it is legal to regard the mass on the spring as a single mass whose value is M = M +m = 11.0 kg. We know the spring constant, so using Eq. 4.10 the angular frequency of the motion is 80 CHAPTER 4. OSCILLATORY MOTION mg Figure 4.7: The forces acting on mass m in Example 7. (The force of static friction changes direction and magnitude during the motion of mass m.) During the motion, the large mass oscillates with this frequency and so does the small mass since they move together. But note, the spring is attached only to the large mass; what is making the small mass move back and forth? The answer is static friction. We make a diagram of the forces which act on the small mass. This is shown in Fig. 4.7. We have the force of gravity mg pointing down, the normal force N from the big block pointing up and also the force of static friction fS7 which can point either to the right or to the left, depending on the current position of m during the oscillation! The magnitude and direction of the static friction force fS are not constant; the value of fS depends on the acceleration of the co—moving blocks (assuming there is no slipping so that they are indeed co—moving). There is no vertical motion of the small block so clearly N = mg 2 (1.00 kg)(9.80 g) = 9.80N . But having the normal force (between the surfaces of the two blocks) we know the maximum possible magnitude of the static friction force, namely: fsmax = HsFN = Msmg and since that is the only sideways force on mass m, from Newton’s 2nd Law, the maximum possible magnitude of its acceleration 7 assuming no slipping 7 is max noislip : fs : [1“st : a #59 . max m m Now, if the two blocks are moving together and oscillating with amplitude A, then the maximum value of the acceleration is given by Eq. 4.6, namely amax = w2A, which of course will get larger if A gets larger. By equating this maximum acceleration of the motion to the value we just found, we arrive at a condition on the maximum amplitude A such that no slipping will occur: no—sli 2 2 Cl p Z w Amax : Msg 2 w Amax max 4.2. WORKED EXAMPLES 81 which gives: _ [usg _ (0.40)(9.80 8%) Amax W2 (4.26 s71)2 = 0.216m 4.2.3 Energy and the Simple Harmonic Oscillator 8. A particle executes simple harmonic motion with an amplitude of 3.00 cm. At what displacement from the midpoint of its motion does its speed equal one half of its maximum speed? [Ser4 13—23] The maximum speed occurs in the center of the motion, where there is no potential energy. So the total energy is given by 2 _ 1 E — Emvmax At the point(s) where U = %vmax the potential energy is not zero; rather it is given by U = E — émv2 = émviax — émv2 2 _ 1 2 1 ”max — Emvmax — §m( 2 > = 1—1mv2 =§mv2 2 8 max 8 max But we also have from Eq. 4.16 the result 2 mvm X U(:1:) = % A; 3:2 And combining these expressions gives the corresponding value of 3:: 2 %m:n21ax H32 : gmvfnax Solve for x: x2 = 2A2 : x — __\/7§A — __§(3.00 cm) : $2.60 cm The mass has half its maximum speed at x = ::2.60 cm. The problem can also be worked just using Eqs. 4.1 and 4.4. The problem gives no data about any specific value of 25 so we are free to choose (fl = 0 for simplicity. Then 33(15) 2 Acos(wt) and 2105) = —wA sin(wt) = —vmax sin(wt) 82 CHAPTER 4. OSCILLATORY MOTION and for the times t at which the speed of the mass is half the maximum value, we must have the condition sin(wt) = 22% . But when this is true we have c0s2(wt) = 1— sin2(wt) = 1 — i = 3 or 3 cos(wt) = ::£ 2 and that gives 3 3 m==4A%;2::4amnmn%;==i260mn 4.2.4 The Simple Pendulum 9. A simple pendulum has a period of 2.50s. (a) What is its length? (b) What would its period be on the Moon, where gMoon = 1.67 8%? [Ser4 ...
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