5128_Ch03_pp098-184

More documents

Recommendations

Info

Section 3.5 Derivatives of Trigonometric Functions 143 Radian Measure in Calculus In case you have been wondering why calculus uses radian measure when the rest of the world seems to measure angles in degrees, you are now ready to understand the answer. The derivative of sin x is cos x only if x is measured in radians! If you look at the analytic confirmation, you will note that the derivative comes down to cos x times lim sin h . h→0 h We saw that lim sin h 1 h→0 h in Figure 3.36, but only because the graph in Figure 3.36 is in radian mode. If you look at the limit of the same function in degree mode you will get a very different limit (and hence a different derivative for sine). See Exercise 50. EXAMPLE 1 Revisiting the Differentiation Rules Find the derivatives of (a) y x 2 sin x and (b) u cos x1 sin x. SOLUTION (a) d y x dx 2 d d • sin x sin x • x d x d x 2 Product Rule x 2 cos x 2x sin x d d (b) d 1 sin x • cos x cos x • 1 sin x u d x d x Quotient Rule dx 1 sin x 2 1 sin xsin x cos x 0 cos x 1 sin x 2 sin x sin 2 x cos 2 x 1 sin x 2 1 sin x 1 sin x 2 sin 2 x cos 2 x 1 1 Now try Exercises 5 and 9. 1 sin x –5 Simple Harmonic Motion The motion of a weight bobbing up and down on the end of a spring is an example of simple harmonic motion. Example 2 describes a case in which there are no opposing forces like friction or buoyancy to slow down the motion. 0 5 Rest position Position at t 0 EXAMPLE 2 The Motion of a Weight on a Spring A weight hanging from a spring (Figure 3.38) is stretched 5 units beyond its rest position s 0 and released at time t 0 to bob up and down. Its position at any later time t is s 5 cos t. What are its velocity and acceleration at time t? Describe its motion. —2 3 s SOLUTION We have: Figure 3.38 The weighted spring in Example 2. Position: s 5 cos t; Velocity: v d s d 5 cos t 5 sin t; dt d t Acceleration: a d v d 5 sin t 5 cos t. s, v dt d t Notice how much we can learn from these equations: 5 0 –5 v –5 sin t –2 s 5 cos t 2 —2 5 1. As time passes, the weight moves down and up between s 5 and s 5 on the s-axis. The amplitude of the motion is 5. The period of the motion is 2p. 2. The velocity v 5 sin t attains its greatest magnitude, 5, when cos t 0, as the t graphs show in Figure 3.39. Hence the speed of the weight, v 5 sin t, is greatest when cos t 0, that is, when s 0 (the rest position). The speed of the weight is zero when sin t 0. This occurs when s 5 cos t 5, at the endpoints of the interval of motion. 3. The acceleration value is always the exact opposite of the position value. When the Figure 3.39 Graphs of the position and weight is above the rest position, gravity is pulling it back down; when the weight is velocity of the weight in Example 2. below the rest position, the spring is pulling it back up. continued
144 Chapter 3 Derivatives 4. The acceleration, a 5 cos t, is zero only at the rest position where cos t 0 and the force of gravity and the force from the spring offset each other. When the weight is anywhere else, the two forces are unequal and acceleration is nonzero. The acceleration is greatest in magnitude at the points farthest from the rest position, where cos t 1. Now try Exercise 11. Jerk A sudden change in acceleration is called a “jerk.” When a ride in a car or a bus is jerky, it is not that the accelerations involved are necessarily large but that the changes in acceleration are abrupt. Jerk is what spills your soft drink. The derivative responsible for jerk is the third derivative of position. DEFINITION Jerk Jerk is the derivative of acceleration. If a body’s position at time t is s(t), the body’s jerk at time t is j(t) d a d 3s . dt dt 3 Recent tests have shown that motion sickness comes from accelerations whose changes in magnitude or direction take us by surprise. Keeping an eye on the road helps us to see the changes coming. A driver is less likely to become sick than a passenger who is reading in the back seat. EXAMPLE 3 A Couple of Jerks (a) The jerk caused by the constant acceleration of gravity g 32 ftsec 2 is zero: d j g 0. d t This explains why we don’t experience motion sickness while just sitting around. (b) The jerk of the simple harmonic motion in Example 2 is j d a d 5 cos t dt d t 5 sin t. It has its greatest magnitude when sin t 1. This does not occur at the extremes of the displacement, but at the rest position, where the acceleration changes direction and sign. Now try Exercise 19. Derivatives of the Other Basic Trigonometric Functions Because sin x and cos x are differentiable functions of x, the related functions sin x 1 tan x , sec x , c os x co s x cot x c os x 1 , csc x sin x sin x are differentiable at every value of x for which they are defined. Their derivatives (Exercises 25 and 26) are given by the following formulas.
Page 1 and 2: Chapter 3 Derivatives Shown here is
Page 3 and 4: 100 Chapter 3 Derivatives f(a + h)
Page 5 and 6: 102 Chapter 3 Derivatives We comple
Page 7 and 8: 104 Chapter 3 Derivatives Slope f(
Page 9 and 10: 106 Chapter 3 Derivatives 18. dy/dx
Page 11 and 12: 108 Chapter 3 Derivatives Standardi
Page 13 and 14: 110 Chapter 3 Derivatives Later in
Page 15 and 16: 112 Chapter 3 Derivatives An Altern
Page 17 and 18: 114 Chapter 3 Derivatives Quick Rev
Page 19 and 20: 116 Chapter 3 Derivatives 3.3 What
Page 21 and 22: 118 Chapter 3 Derivatives EXAMPLE 1
Page 27 and 28: 124 Chapter 3 Derivatives 8. If f x
Page 29 and 30: 126 Chapter 3 Derivatives 55. Multi
Page 31 and 32: 128 Chapter 3 Derivatives Figure 3.
Page 33 and 34: 130 Chapter 3 Derivatives The rate
Page 35 and 36: 132 Chapter 3 Derivatives -1 6 4 2
Page 37 and 38: 134 Chapter 3 Derivatives 0 y (doll
Page 39 and 40: 136 Chapter 3 Derivatives 8. At the
Page 41 and 42: 138 Chapter 3 Derivatives 26. Movin
Page 43 and 44: 140 Chapter 3 Derivatives 47. Findi
Page 45: 142 Chapter 3 Derivatives X -.03 -.
Page 49 and 50: 146 Chapter 3 Derivatives 4. Domain
Page 53 and 54: 150 Chapter 3 Derivatives “Outsid
Page 57 and 58: 154 Chapter 3 Derivatives 49. Let x
Page 59 and 60: 156 Chapter 3 Derivatives Explorati
Page 61 and 62: 158 Chapter 3 Derivatives y 2 -25
Page 63 and 64: 160 Chapter 3 Derivatives The norma
Page 65 and 66: 162 Chapter 3 Derivatives 2. y 1
Page 67 and 68: 164 Chapter 3 Derivatives 56. The l
Page 69 and 70: 166 Chapter 3 Derivatives EXPLORATI
Page 71 and 72: 168 Chapter 3 Derivatives -1 y y
Page 73 and 74: 170 Chapter 3 Derivatives Quick Rev
Page 77 and 78: 174 Chapter 3 Derivatives EXPLORATI
Page 79 and 80: 176 Chapter 3 Derivatives We could
Page 81 and 82: 178 Chapter 3 Derivatives [-5, 10]
Page 83 and 84: 180 Chapter 3 Derivatives Explorati
Page 85 and 86: 182 Chapter 3 Derivatives 29. y cs
Page 87: 184 Chapter 3 Derivatives 75. Searc

5128_Ch03_pp098-184

Create successful ePaper yourself

Delete template?

Save as template?