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Unformatted text preview: 483 Chapter 11 1. The velocity of the car is a constant ( 29 ? 80 km/h (1000 m/km)(1 h/3600 s) i ( 22m s)i, v = + = + r and the radius of the wheel is r = 0.66/2 = 0.33 m. (a) In the car’s reference frame (where the lady perceives herself to be at rest) the road is moving towards the rear at r v v road m s = - = - 22 , and the motion of the tire is purely rotational. In this frame, the center of the tire is “fixed” so v center = 0. (b) Since the tire’s motion is only rotational (not translational) in this frame, Eq. 10-18 gives top ˆ ( 22m/s)i. v = + r (c) The bottom-most point of the tire is (momentarily) in firm contact with the road (not skidding) and has the same velocity as the road: bottom ˆ ( 22m s)i. v = - r This also follows from Eq. 10-18. (d) This frame of reference is not accelerating, so “fixed” points within it have zero acceleration; thus, a center = 0. (e) Not only is the motion purely rotational in this frame, but we also have ϖ = constant, which means the only acceleration for points on the rim is radial (centripetal). Therefore, the magnitude of the acceleration is 2 2 2 3 top (22 m/s) 1.5 10 m s . 0.33 m v a r = = = × (f) The magnitude of the acceleration is the same as in part (d): a bottom = 1.5 × 10 3 m/s 2 . (g) Now we examine the situation in the road’s frame of reference (where the road is “fixed” and it is the car that appears to be moving). The center of the tire undergoes purely translational motion while points at the rim undergo a combination of translational and rotational motions. The velocity of the center of the tire is ˆ ( 22m s)i. v = + r (h) In part (b), we found r v v top,car = + and we use Eq. 4-39: top, ground top, car car, ground ? ? i i 2 i v v v v v v = + = + = r r r CHAPTER 11 484 which yields 2 v = +44 m/s. This is consistent with Fig. 11-3(c). (i) We can proceed as in part (h) or simply recall that the bottom-most point is in firm contact with the (zero-velocity) road. Either way – the answer is zero. (j) The translational motion of the center is constant; it does not accelerate. (k) Since we are transforming between constant-velocity frames of reference, the accelerations are unaffected. The answer is as it was in part (e): 1.5 × 10 3 m/s 2 . (1) As explained in part (k), a = 1.5 × 10 3 m/s 2 . 2. The initial speed of the car is ( 29 80 km/h (1000 m/km)(1 h/3600 s) 22.2 m/s v = = . The tire radius is R = 0.750/2 = 0.375 m. (a) The initial speed of the car is the initial speed of the center of mass of the tire, so Eq. 11-2 leads to com0 22.2 m/s 59.3 rad/s. 0.375 m v R ϖ = = = (b) With θ = (30.0)(2 π ) = 188 rad and ϖ = 0, Eq. 10-14 leads to ( 29 2 2 2 2 (59.3 rad/s) 2 9.31 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