9. Consider N gas molecules in a cubic box of lateral size L at temperature T. Neglect the interactions between the particles and let m be the mass of a molecule, the average nunber of particles hitting one of the bounding wals per unit time is (k_B is the Boltzmann constant)
(A)
(B)
(C)
(D)
(E) none of the above.

5.Which expression cannot represent a solution of the wave equation? (A)(B)(C) (D)(E) all expressions can represent a solution of the wave equation.

6. The key concepts behind Kepler's 2nd law for planetary motion (the line joining the sun to a planet sweeps out equal areas in equal times) include (A)energy conser ervation,(B)momentum conservation, (C)angular momentum conservation, (D) both (a)(b), (E) both (a)(c).

7. A rocket has a mass of 2 x 108 kg. It expels gas at the rate of 2 x 10t kg/s with an exhaust speed of 1000 m/s relative to the rocket. The initial acceleration of the rocket at the launch pad (neglect gravity) is closest to (A) 1 m/s2,(B)10m/s2, (C)100 m/s2,(D) 1000m/s2,(E)100000 m/s2.

8. Water (mass density ρ) fows at speed ui； through a garden hose with cross sectional area ao and emerges at pressure ps from a nozzle of cross sectional area a0/2. The pressure at the faucet is (A)(B) (C)(D)(E) none of the above.

10. N is the number of particles in the systen, V is the volume of the system, T is the tem- perature of the system. Which of the following is a possible expression of the entropy of a macroscopic system? (A) NIn(N/V),(B)NkBET, (C)NkBT/V, (D) NInV+N, (E) none of the above.

11. Which quantity changes during the free expansion of an ideal gas? (A)energy,(B)entropy, (C)temperature, (D) above all, .(E) none of the above.

12. When an external electric feld is applied to a dielecttic, (A)the electric dipoles align antiparallel to the external feld, the eloctric feld in the interior becomes smaller than external electric field,(B) the electric dipoles align parallel to the external field, the electric field in the interior becomes smaller than external electric feld, (C) the electric dipoles align antiparallel to the external feld, the electric feld in the interior becomes greater than external electtic feld, (D) the electric dipoles align parallel to the external feld, the electric field in the interior becomes greater than external electric field, (E) none of the above.

13. A long, straight wire carries a constant current I. A metal rod of length I moves at constant velocity on rails of negliglble resistance that terminate in a resistor R, as shown in the figire. The induced current in the resistor is (A)(B)(C) (D) (E) none of the above.

14. A solenoid with N turns has a length I and a redius r. For what current will the energy density within the solenoid be u? (A)(B)(C)(D) (E) nome of the above. 

15. Consider refraction of a wave, what changes as wave enters from medium I to mediurn II with different index of refraction? (i) wavelength changes (ii) frequency changes (iii) amplitude changes (iv) wave speed changes (A)(i) (iii),(B)(i)(ii) (iv), (C)(ii) (iv), (D) (ii)(iii)(iv), (E) none of the above.