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- May 17, 2013
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This problem says that a child is on a frictionless pond with a sled. She slides forward, hits a log, and stops. The first couple of problems in the passage deal with just that. Then she throws her hat to get back where she started and eventually slows down. The questions asks that given this, what's true.
A. Momentum is not conserved; kinetic energy is conserved
B. Momentum is not conserved; total energy is not conserved
C. Momentum is conserved; kinetic energy is not conserved
D. Momentum is conserved; total energy is conserved
The answer is B. "The sled slows down because of some external force acting on it (usually friction). Because there is an external force, the momentum of the child/sled system is no longer conserved. The slowing sled loses kinetic energy presumably to the heat generated by friction, so B is the best choice".
But didn't they just say in the passage description that the pond is frictionless? What would have prompted me to use this logic despite the fact that they say the pond is frictionless?
A. Momentum is not conserved; kinetic energy is conserved
B. Momentum is not conserved; total energy is not conserved
C. Momentum is conserved; kinetic energy is not conserved
D. Momentum is conserved; total energy is conserved
The answer is B. "The sled slows down because of some external force acting on it (usually friction). Because there is an external force, the momentum of the child/sled system is no longer conserved. The slowing sled loses kinetic energy presumably to the heat generated by friction, so B is the best choice".
But didn't they just say in the passage description that the pond is frictionless? What would have prompted me to use this logic despite the fact that they say the pond is frictionless?