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

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

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

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)

28. As shown in the figure, a wire and a 10-Ω resistor are used to form a circuit in the shape of a square, 20 cm by 20 cm. A uniform but nonsteady magnetic field is directed into the plane of the circuit. The magnitude of the magnetic field is decreased from 1.50 T to 0.50 T in a time interval of 63 ms. The average induced current and its direction through the resistor, in this time interval, are closest to  (A) 63 mA, from b to a. (B)38 mA, from b to a. (C) 63 mA, from a to b. (D) 38 mA, from a to b. (E) 95 mA, from a to b.

29. A transverse wave traveling along a string transports energy at a rate r. If we want to double this rate, we could (A) increase the amplitude of the wave by a factor of 8. (B) increase the amplitude of the wave by a factor of 4. (C) increase the amplitude of the wave by a factor of 2. (D) increase the amplitude by a factor of √2. (E) increase the amplitude by a factor of √8.

30. A tube open at one end and closed at the other end produces sound having a fundamental frequency of 350 Hz. If you now open the closed end, the fundamental frequency becomes (A) 87.5 Hz. (B) 175 Hz. (C) 350 Hz. (D) 700 Hz. (E) 1400 Hz.

31. A solid nonconducting sphere of radius R carries a charge Q distributed uniformly throughout its volume. At a certain distance r1 (r1 ＜ R) from the center of the sphere, the electric field has magnitude E. If the same charge Q were distributed uniformly throughout a sphere of radius 2R, the magnitude of the electric field at the same distance r1 from the center would be equal to (A) E/8. (B) E/2.(C) 2E.(D) 8E. (E) E.