课程名称︰普通物理学甲上 课程性质︰系必修 课程教师︰张宝棣 开课学院：理学院 开课系所︰数学系 考试日期（年月日）︰103.10.23 考试时限（分钟）：120(有再延长20分钟) 是否需发放奖励金：是 （如未明确表示，则不予发放） 试题 : 1.Please answer the following questions. 　　‧Why do we use vector in physics? If we add a constant term to the 　　　potential energy of an object, why doesn't this change the object's 　　　motion? (5%) 　　‧What is inertial frame? What is the advantage to use the center of 　　　mass frame as a reference frame? (5%) 　　‧Velocity and acceleration are frame dependent but force and mass in 　　　classical physics are not. Why is there no conflict for observers 　　　in different frames to describe a motion of an object? Are the laws 　　　of energy and momentum conservations hold in any frame? 　　　Explain your answer? (5%) 2.The water in a river flows uniformly at a constant speed of 2.50 m/s between two parallel banks 80.0 m apart. You are to deliver a package directly across the river, but you can swim only at 1.50 m/s. (a) If you choose to minimize the time you spend in the water, in what direction should you head? (b) How far downstream will you be carried? (c) What if? If you choose to minimize the distance downstream that the river carries you, in what direction should you head? (d) How far downstream will you be carried? (16%) 3.To prevent a box from sliding down an inclined plane, student A pushes on the box in the direction parallel to the inclined, just hard enough to hold the box stationary. In an identical situation student B pushes on the box horizontally. Regard as known the mass m of the box, the coefficient of static fricion μs between box and incline, and the inclination angle θ. (a) Determine the force A has to exert. (b) Determine the force B has to exert. (c) If m = 2.00 Kg, θ = 25.0°, and μs = 0.160, who has easier job? (d) What if μs = 0.380? Whose job is easier? (12%) 4.Two curved inclines of two identical wedge-shaped masses M smoothly merge with a horizontal frictionless surface as indicated in Fig. 1. The curved surfaces of the inclines are also frictionless. Another mass m is released at rest from a height h on one of the inclines. (a) When m reaches the horizontal plane, is the mechanical energy of m and M conserved? Is the total momentum of m and M conserved when m reaches the horizontal surface? Why?(5%) (b)After m reaches the horizontal plane, it encounters the right-hand wedge and ascends it to some height h'? Please explain.(5%) (c)Please express h'? in terms of m, M, and h using the ideas in (a) and (b).(10%) Ｍ Ｍ │\▄← m /｜↑ │﹨ ∕｜｜ │ \ / ｜｜ │ Λ Λ ｜｜ │ ﹨ ∕ ｜｜ │ ﹨ 两个物体「相连」 ∕ ｜｜h │ ˋ 但是没有「相黏」 ˊ ｜｜ │ ╲ ╱ ｜｜ ｜ ╲ │ ╱ ｜｜ ｜ ︿ ↓ ︿ ｜｜ ｜ ‵*-,,-*′ ｜↓ ￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣￣ FIG. 1: An object slides in two identical blocks. (图改自99-1考古题) 5.A ball at the end of a pendulum of length L is released at rest from an initial position in which the pendulum string is horizontal (Fig. 2) The floor is just beneath the low point of the swing. The string is cut after the ball has passed the low point with the string at an angle α = θ- 90 to the vertical. Find the horizontal distance that the ball travels from the low point before it bounces on the floor. (7%) ＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿＿ 　　　　　　　｜←－Ｌ－→ 　　　　　　　○－－－－－●ｍ 　　　　　　／θ 　　　　　／ 　　　　／(cut) 　　　／ String is cut at 　　／ angle θ 　● 　　　　　　Fig. 2 6.A curve of radius 30 m is banked so that a 950-kg car traveling at 40.0 km/h can round it even if the road is so icy that the coefficient of static friction is approximately zero. You are commissioned to tell the local police the range of speeds at which a car can travel around this curve without skidding. Neglect the effects of air drag and rolling friction. If the coefficient of static friction between the road and the tires is 0.300, what is the range of speeds you tell them? (8%) 7.A 5-kg block is held against a spring of force constant 20 N/cm, compressing in 3 cm. The block is released and the spring extends, pushing the block along a rough horizontal surface. The coefficient of friction between the surface and the block is 0.2. (a) Find the work done on the block by the spring as it extends from its compressed position to its equilibrium position. (b) Find the energy dissipated by friction while the block moves 3 cm to the equilibrium position of the spring. (c) What is the speed of the block when the spring is at its equilibrium position? (d) If the block is not attached to the spring, how far will it slide along the rough surface before coming to the rest? (20%) --

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