91年 - 91 學士後西醫：物理#85626

1.A 300N weight, 5m long rigid uniform ladder is leaning against a smooth vertical wall. A 750N weight man stands on a rung 4m up along the ladder from its base as shown in fig.1. What is the minimum coefficient of the statical friction between the ladder and the floor required to keep the system in equilibrium? The angle is 53.1° on a horizontal beam. (cos53.1° = 0.6, sin53.1° = 0.8) (A)0.22 (B)0.42 (C)0.46 (D)0.54 (E)1.90

2.An imaginary closed surface S of radius R is centered on the origin. A positive charge is originally at the origin, and the flux through hte surface is . The positive charge is slowly moved from the origin to a point R/2 away from the origin. Now the flux is . (A) (B)(C) (D) (E)

3.An incompressible fluid flows through a horizontal pipe. At one point in the pipe the pressure in the fluid is p₁ and the fluid speed is v₁. Further down the pipe pressure is p₂ and the flued speed is 2v1. What can be concluded about p₁ and p₂? (A) (B) (C) (D) (E)only that

5.A clear sheet of polaroid is placed on top of a simmilar sheets so that their polarizing axes make an angle of 30° with each other. The ratio of the intensity of emerging light to incident unpolarized light its . (  cos30^o,sin30^o    ) (A)1/4 (B)1/3 (C)1/2 (D)3/4 (E)3/8

6.What is the intensity ratio of sound for the heavy truck at 1.5m(90dB) to normal conversation at 1m(60dB)? (A)1.5 (B)10³ (C)0.67 (D)10³⁰ (E)

8.Some sunglasses are coated on one side with a thin film of MgF2 to reduce reflection from the glass surface. The index of refraction of MgF2 is 1.38; that of the glass is 1.50. What is the least coating thickness that eliminates (via interference) the reflections in the range of the visible spectrum (≒ 550nm)? Assume that the light is approximately perpendicular to the glass surface. (A)40nm (B)60nm (C)80nm (D)100nm (E)120nm

11.An ideal monatomic gas undergoes a quasistatic expansion to one-third of its initial pressure. Find the ratio of the final volume to the initial volume if the process is adiabatic. (⁰ = Cp / Cv 5/3 for ideal monatomic gas, log3 0.5) (A) (B)10^0.3 (C)10^0.5 (D)10^0.6 (E)10^0.7

12.A block with mass m 250g is fastened to a spring with spring constant k 85N/m. The system is excited to oscillate. Assume that there is a velocity-dependent damping force F_d -bv in the system with b 70g/s. How long does it take for the amplitude of the damped oscillation to drop to half its initial value? (ln2 0.693) (A)3s (B)4s (C)5s (D)6s (E)7s

13.A thin rod whose length L is 10cm and whose mass m is 100g, suspended at its midpoint from a long wire. Its period T_a of angular SHM (Simple Harmonic Motion) is measured to be 2.50 s. An irregularly shaped object is then hung from the same wire and its period T_b is found to be 5.0s. What is the rotational inertia of the irregular object about its suspension axis? (The rotational inertia for a rod is  mL²  ) (A) (B) (C) (D) (E)

14.In high-speed motion through air, the resisting force is approximately  F = cv² . For a human body falling through air in a spread-eagle position, c is about 0.25kg/m. An 80kg sky diver jumps out of an airplane and reaches terminal speed before opening the parachute. What is the terminal speed? (A)49m/s (B)56m/s (C)72m/s (D)84m/s (E)98m/s

15.A neutral water molecule (H₂O) in its vapor state has an electric dipole moment of magnitude  6.2 ́ 10Cm. How far apart are the molecule’s center of positive and negative charge? (A)1.9pm (B)2.9pm (C)3.9pm (D)4.9pm (E)5.9pm

16.In a mass spectrometer, the initally stationary ion is accelerated by the electric field due to a potential V. The ion enters a separator chamber in which a uniform magnetic field B is perpendicular to the path of the ion. The magnetic field causes the ion to move in a semicircle, striking a photographic plate at distance x from the entry slit. Suppose that B 80mT and V 1000 Volts, and ion of charge q +1.60 ́ C strikes the plate at x 1.63m, what is the mass of the ion? (A)185.7u (B)204.9u (C)224.6u (D)241.5u (E)267.8u

17.Consider a parallel-plate capacitor of plate area A and plate separation. A potential difference Vo is applied between the plates. The battery is then disconnected, and a dielectric slab of thickness b and dielectric constant k is placed between the plates. Assuming A 100cm² , d 1.00cm, Vo 90V, b 0.8cm, k 2.00. What is the capacitance with the slab in place? ( ) (A)14.8pF (B)16.8pF (C)17.9pF (D)20.9pF (E)21.9pF 

18.A 200g top spinning at 20 rev/s makes an angle of 30° to the vertical and precesses at a rate of 1 rev per 10.0s. If its CM (Center of Mass) is 4.0cm from its tip along its symmetry axis, what is the moment of inertia of the top? (A) (B) (C) (D) (E)

20.A thermodynamic system undergoes a process in which its internal energy decreases by 500J. If at the same time, 220J of work is done on the system, what is the thermal energy transferred to or from it? (A)280J (B)280J (C)720J (D)720J (E)1220J

21.A dipole for which p m is in a uniform field E 7 10⁴ N /C  . What is the external work needed to rotate the dipole from alignment with the field to angle 60° to the field? ( cos60 ^o 0.5;sin 60^o 0.9   ) (A)(B)  (C)   (D) (E)

22.A 5A current flows in a circular loop of radius 2cm. The axis of the loop is at 30° to a uniform field 0.06T. What is the magnitude of the torque on the loop? (A) (B) (C) (D) (E)

23.A 15pF parallel-plate capacitor is connected to a 50V battery. The area of each plate is 80cm² . What is the energy density in the fiele? ( ) (A)  (B) (C)  (D) (E)

24.What minimum frequency of light is needed to eject electrons from a metal whose work function is 4.3 ) (A)  (B) (C) (D)   (E)

25.Through how many volts of potential difference must an electron be accelerated to achieve a wavelength of 0.28nm? ( ́ ) (A)12V (B)16V (C)19V (D)31V (E)34V