# Co_ops__Selected_Answers__P1112_S09 - Physics 1112 Spring...

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1 Physics 1112, Spring 2009, Answers to Selected Co-op Problems Week 1 (1d) d /4; (2a) 20037 L; (2b) 0.26 Week 2 (1a) a G and b G are in the same direction; (1b) b G = 0; (1c) a G and b G are perpendicular; (1d) a G and b G are perpendicular; (3) v 0 x = v x = x v = 24.2 m/s; v 0 y = 14 m/s; v y = 14 m/s; y v = 0; Δ x = 69.3 m; Δ y = 0; a x = 0; a y = 9.8 m/s 2 ; t 1 = 1.43 s; t 2 = 2 t 1 = 2.86 s; (4a) v 0 sin θ ; (4b) L tan θ − gL 2 /(2 v 0 2 cos 2 θ ). You can show from this expression that a meaningful solution (height 0) requires that L the range of the projectile. Week 3 (1b) the canoe’s displacements (wrt water) are equal in magnitude and opposite in direction and the bottle’s displacement is zero; (1c) 1.0 h; (1d) the canoe’s displacements (wrt water) both have magnitude 3.5 km, the river speed (wrt bank) is 1.5 km/h, and the canoe speed in a still lake is 3.5 km/h. (2F) on the upper book: downward gravitational force due to Earth and upward contact force due to lower book, with equal magnitudes. on the lower book: downward gravitational force due to Earth, downward contact force due to upper book, and upward contact force due to table. (2I) upward contact force exerted by the lower book on the upper book and the downward contact force exerted by the upper book on the lower book make up a Newton’s third law pair. Week 4 (2b) vertical wall, mg/(tan θ ); slanted wall, mg/(sin θ ) Week 5 (1a) The friction is static from P = 0 to P = 75 N. The friction is kinetic for P > 75 N. (b) static, 0.25; kinetic, 0.17. (2b) F = (M+m)g tan α Week 7 (1a) –0.90 J (1b) 0.46 J (1c) 1.04 m/s (1d) slides back (2e) 5R/2 Week 8 (1a) unstable at r = 4 fm, neutral for 0 < r < 2 fm; (1b) attractive for 2 < r < 4 fm, repulsive for r > 4 fm; (1c) r = 3 fm, 3000 N; (1e) 5 fm; (1f) 3 pJ; (1g) 2 pJ (B) cart B: greater mass smaller acceleration smaller average velocity; (C) F o = m a = m Δ v / Δ t; (C1) m| Δ v | = F o Δ t; since Δ t A is smaller, m| Δ v A | is smaller; (C2) impulse for A is smaller; (C3) see C1; (C4) final momentum of A is smaller; (D) work done is the same: same force and same displacement; KEs are equal; (E) nothing wrong with 1 other than being a bit vague; 2 is incorrect because the final speeds are not inversely proportional to the masses; 3 is incorrect

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