4. The free-fall acceleration on the surface of the Moon is about one-sixth that on the surface of the Earth. If the radius of the Moon is about 0.250R_E, the ratio of their average densities, ρ_Moon/ρ_Earth is (A)2/3 (B)1/2 (C)1/3 (D)1/6 (E)1/4

5. A solid wheel with mass M, radius R, and rotational inertia MR²/2, rolls without sliding on a horizontal surface. A horizontal force F is applied to the axle and the center of mass has an acceleration a. The magnitudes of the applied force F and the frictional force f of the surface, respectively, are: (A)F = Ma, f = 0 (B)F = Ma, f = Ma/2 (C)F = 2Ma, f = Ma (D)F = 2Ma, f = Ma/2 (E)F = 3Ma/2, f = Ma/2

4. The free-fall acceleration on the surface of the Moon is about one-sixth that on the surface of the Earth. If the radius of the Moon is about 0.250R_E, the ratio of their average densities, ρ_Moon/ρ_Earth is
(A)2/3
(B)1/2
(C)1/3
(D)1/6
(E)1/4

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