15.A ball is tossed from an upper-story window of a building. The ball is given an initial velocity of 8.00 m/s at an angle of 20.0°below thehorizontal. It strikes the ground 3.00s later.(a)How far horizontally from the base of the building does the ball strike the ground?(b) Find the height from which the ball was thrown.(c)How long does it take the ball to reach a point 10.0m below the level of launching? 27.Figure P4.27 represents the total acceleration of a particle moving clockwise in a circle of radius 2.50m at a certain instant of time. At this instant, find(a)the radial acceleration, (b)the speed of the particle, and(c)its tangential acceleration. http://www.wretch.cc/album/show.php?i=j780719&b=4&f=1348217184&p=1 31.A river has a steady speed of 0.500m/s. A student swims upstream a distance of 1.00km and swims back to the starting point. If the student can swim at a speed of 1.20m/s in still water, how long does the trip take? Compare this answer with the time interval required for the trip if the water were still. 39.A pendulum with a cord of length r=1.00m swings in a vertical plane (Fig.P4.39). When the pendulum is in the two horizontal positions θ = 90.0°andθ270°,its speed is 5.00m/s (a)Find the magnitude of the radial acceleration and tangential acceleration for these positions.(b)Draw vector diagrams to determine the direction of the total acceleration for these two positions. (c)Calculate the magnitude and direction of the total acceleration. http://www.wretch.cc/album/show.php?i=j780719&b=4&f=1348217185&p=2 46.A person standing at the top of a hemispherical rock of radius R kicks a ball (initially at rest on the top of the rock)to give it horizontal velocity Vi（向量上面有→）as shown in Figure P4.46.(a)What must be its minimum initial speed if the ball is never to hit the rock after it is kicked?(b)With this initial speed, how far from the base of the rock does the ball hit the ground? http://www.wretch.cc/album/show.php?i=j780719&b=4&f=1348217186&p=3 54.In the What If? section of Example 4.5, it was claimed that the maximum range of a ski jumper occurs for a launch angle θgiven by θ=45°-φ/2 where φ is the angle that the hill makes with the horizontal in Figure 4.14. Prove this claim by deriving this equations. 密碼：mooix --

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