Unformatted text preview: 1994 Physics C Solutions Distribution
of Points Mech i.
(a) 3 points For the correct expression for the energy stored in a spring: 1 _
U - 31m” 1 point
For substituting the correct values:
1 U - ~2—(200 N/rn)(0.4 in)2 I point For the correct answer (including units): U - 16 .l I point
(b) 3 points For applying conservation of energy 1 point Kinetic energy of ball/block combination 2 energy stored in spring For a correct expression for kinetic energy 1 point 1 2 5-le u U ”U1 :2 J 2U / M m 4/2 (16 J) /(2.1 kg) For the correct answer: iu1 .. 3.9m/s lpoint
(c) 3 points For applying conservation of momentum 1 point For a correct expression for momentum 1 point mv - My; 2) an Mu1 / m a (2.1 kg)(3.9 m /s) /(O.1 kg) For the correct answer: 1) u 81.9m/s lpoint
(d) 2 points For indicating that the maximum compression of the spring will be less than 0.4 meters. 1 point For a correct explanation referring to the motion of the blocks or their kinetic energy 1 point Some examples: The 0.8-kg block will begin to move before the spring can compress as much, due to the force that the spring exerts on it
The kinetic energy of the center of mass of the system in this case is non~zero, so some of the initial energy must remain as kinetic energy. Less energy is available as potential
energy, so the spring compression is less. ...
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