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Unformatted text preview: 439 Chapter 10 1. The problem asks us to assume v com and ϖ are constant. For consistency of units, we write v com mi h ft mi 60min h ft min = F H G I K J = 85 5280 7480 b g . Thus, with ∆ x = 60 ft , the time of flight is com (60 ft) /(7480 ft/min) 0.00802 min t x v = ∆ = = . During that time, the angular displacement of a point on the ball’s surface is θ ϖ = = ≈ t 1800 0 00802 14 rev min rev . b gb g . min 2. (a) The second hand of the smoothly running watch turns through 2 π radians during 60 s . Thus, 2 0.105 rad/s. 60 π ϖ = = (b) The minute hand of the smoothly running watch turns through 2 π radians during 3600 s . Thus, ϖ = = × 2 3600 175 10 3 π . rad / s. (c) The hour hand of the smoothly running 12hour watch turns through 2 π radians during 43200 s. Thus, ϖ = = × 2 43200 145 10 4 π . rad / s. 3. Applying Eq. 215 to the vertical axis (with + y downward) we obtain the freefall time: 2 2 1 2(10 m) 1.4 s. 2 9.8 m/s y y v t gt t ∆ = + ⇒ = = Thus, by Eq. 105, the magnitude of the average angular velocity is avg (2.5 rev)(2 rad/rev) 11 rad/s. 1.4 s π ϖ = = CHAPTER 10 440 4. If we make the units explicit, the function is θ = + 4 0 30 10 3 . . . rad / s rad / s rad / s 2 2 3 b g c h c h t t t but generally we will proceed as shown in the problem—letting these units be understood. Also, in our manipulations we will generally not display the coefficients with their proper number of significant figures. (a) Eq. 106 leads to ϖ = + = + d dt t t t t t 4 3 4 6 3 2 3 2 c h . Evaluating this at t = 2 s yields ϖ 2 = 4.0 rad/s. (b) Evaluating the expression in part (a) at t = 4 s gives ϖ 4 = 28 rad/s. (c) Consequently, Eq. 107 gives α ϖ ϖ avg 2 rad / s = = 4 2 4 2 12 . (d) And Eq. 108 gives α ϖ = = + =  + d dt d dt t t t 4 6 3 6 6 2 . c h Evaluating this at t = 2 s produces α 2 = 6.0 rad/s 2 . (e) Evaluating the expression in part (d) at t = 4 s yields α 4 = 18 rad/s 2 . We note that our answer for α avg does turn out to be the arithmetic average of α 2 and α 4 but point out that this will not always be the case. 5. The falling is the type of constantacceleration motion you had in Chapter 2. The time it takes for the buttered toast to hit the floor is 2 2 2(0.76 m) 0.394 s. 9.8 m/s h t g ∆ = = = (a) The smallest angle turned for the toast to land butterside down is min 0.25 rev / 2 rad. θ π ∆ = = This corresponds to an angular speed of min min / 2 rad 4.0 rad/s. 0.394 s t θ π ϖ ∆ = = = ∆ 441 (b) The largest angle (less than 1 revolution) turned for the toast to land butterside down is max 0.75 rev 3 / 2 rad. θ π ∆ = = This corresponds to an angular speed of max max 3 / 2 rad 12.0 rad/s....
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This note was uploaded on 08/09/2010 for the course PHY 202 101A2 taught by Professor Prof.yang during the Summer '10 term at National Taiwan University.
 Summer '10
 Prof.Yang
 Physics, Light

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