複選題
21. An object hangs from a spring balance. The balance indicates 30 N in air and 20 N when the object is submerged in water. What mass of the object would weigh 20N by the balance when the object is submerged in a liquid with a density that is half that of water? Take g = 10 m/s². (AB)20N (AC)25N(AD)30N(AE)35N (BC)40N (BD)一律送分

17. Liquid metal of mass m kg at 500 K is mixed with the same liquid metal of same mass at 1500 K. Given the specific heat of the metal is c cal/K, find the change in entropy (in cal/K) of the system. Take In 2 = 0.70, In 3 = 1.10, In 4 =1.40, In 5 = 1.61. (A)+2.0 mc(B) +0.6mc(C)-1.0 mc(D) +0.3 mc(E) -0.21 mc

18. A 5.23-g pebble is dropped from a 300-m building. If it reaches a terminal velocity of 40 m/s, and the rest of the energy is converted to heating the coin, what is the change in temperature (in °C) of the coin? (The specific heat of pebble is 400 J/kg.°C Take g = 10 m/s2. )(A)8.5 (B) 2.8 (C) 5.5 (D)21 (E)0.5

19. While temperature is kept constant, if consider only translational and rotational motion, what is the total energy of 1 liter of nitrogen gas with pressure of I atm (take 1 atm=105 Pa)? (A)0.1kJ (B)0.15kJ(C)0.2kJ(D)0.25 kJ(E) 0.35 kJ

20. n moles of ideal gas of pressure P and volume V undergoes an irreversible expansion with final pressure is P/3 and volume 3V. With R is the universal gas constant, find the entropy exchange of the process.(A)0(B)nR ln3 (C)3 nR (D)nR ln6 (E)6 nR

22. Water is streaming downward from a faucet opening with an area of . It lcaves the faucet with a specd of 5.0 m/s. The cross-sectional area of the stream 0.55 m below the faucet is how many : (Take g = 10 m/s2. ) (A)1.5(B)2.0 (C)2.5 (D)3.0(E) 3.5

23. A police crime lab is trying to determine whether someone was murdered or died as a result of an accident. He was struck in the temple by a 4.00 kg sculpture that is alleged to have fallen off a bookcase. The sculpture presumably fell a distance of 1.80 m and the corner that struck him had an area of 0.240 cm2. If the time for the sculpture to stop was 1.00 ms, take g = 10.0 m/s, the pressure on his temple, in N/m2, was (A)1.00x104.(B) 1.50x 105.(C) 1.50x 106(D) 1.00 x 109. (E) 1.25 x 109

24. A venturi tube may be used as the inlet to an automobile carburetor. If the 2.0-cm diameter horizontal pipe narrows to a 1.0-cm diameter, what is the pressure drop in the constricted section for an airflow of 2.0 m/s in the 2.0-cm section? (p= 1.2 kg/m3.) (A)70Pa(B)85Pa(C)95Pa (D)20 Pa (E) 36 Pa

25. Determine the minimum area of a flat ice floe 1.0 meter thick if it is to support a 2 000-kg car above seawater. ( = 920 kg/m3, = 1 020 kg/m3.) (A)20m2(B)40m2 (C)60m2(D)80m2 (E) 100m2

26. At what rate is thermal energy being generated in the 2R-resistor when ε = 12 V and R = 3.0 Ω? (A)12 W (B) 24 W (C) 6.0 W (D)3.0 W (E)1.5 W

27. A charged capacitor stores energy U, Without connecting this capacitor to anything, dielectric having dielectric constant K is now inserted between the plates of the capacitor, completely filling the space between them. How much energy does the capacitor now store? (A)2KU (B)KU (C)U (D)U/K (E)U/(2K)