Unformatted text preview: oduces the same acceleration as the spring force on the 2.0 kg block in part (a); hence the spring force is greater than in part (a) so the extension is greater. Applying Newton's 2nd law to show that the new spring extension is 0.040 m. Notes: A partial justification worth a single point may note that the spring is pulling on a larger mass than before, or may note that the force exerted by the spring is larger than before (without explaining why this force is larger). Students who answered part (c) by saying that the acceleration is greater could earn 2 points here by noting that the force exerted by the spring on block B must be larger in order to give the larger mass a greater acceleration. (e) 3 points For indicating that, after block A impacts the wall, mechanical energy is conserved For correctly applying conservation of energy, equating the energy immediately after block A hits the wall to the energy when the spring is at maximum compression K before + Ubefore = K after + Uafter 1 point 1 point 1 point 2 points 1 1 m u 2 + 0 = 0 + kx 2 2 B 2 For the correct solution
x = mBu = k x = 0.16 m
2 1 point
2 (8.0 kg )(0.50 m s)
80 N m 2008 The College Board. All rights reserved. Visit the College Board on the Web: www.collegeboard.com. AP PHYSICS B 2008 SCORING GUIDELINES
15 points total (a) 4 points For using Ohm's law to find the total resistance of the circuit e I = Rtotal 1 point Distribution of points 16 V 4.0 A = 4.0 For a correct expression for the total resistance that has the internal resistance of the power supply in series with the wire's resistance Rinternal + Rwire = Rtotal
0.50 W + Rwire = 4.0 W Rwire = 3.5 W An alternate solution is to use a circuit equation to determine the voltage across the wire for the first point, and then Ohm's law to determine its resistance for the second point. Vwire = Vterm = e - Vint = 16 V - (4.0 A )(0.50 W ) = 14 V Rwire = V I = 14 V 4.0 A = 3.5 W Using the expression for the resistance of a wire in terms of its dimensions Rwire = rL A L = Rwire A r For correct substitutions 1 point L = (3.5 W) 3.5 10 ( -9 m 2 ) (1....
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