TBR Physics Ch. 4

[Synapsis]

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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?

 

[Patassa]

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?

Because they told you it "eventually slows down". They just redefined the system in this question. The MCAT does this as well, not every question will be directly testing you on the passage set up.

 

[Synapsis]

Because they told you it "eventually slows down". They just redefined the system in this question. The MCAT does this as well, not every question will be directly testing you on the passage set up.

Okay then, since there's nothing else that could have slowed it down, I'm just forced to assume there's friction then? Got it.

 

[sakabato93]

Okay then, since there's nothing else that could have slowed it down, I'm just forced to assume there's friction then? Got it.

Well, its not necessarily true that it had to be friction that slowed the child down. What really matters is the fact that momentum and kinetic energy were LOST (i.e. slowing down) due to an external force acting upon the system. Energy and momentum are only conserved when external forces are NOT acting upon the system. Friction is a common external force, but it isn't the only possible choice.

 

[rguil15]

Okay then, since there's nothing else that could have slowed it down, I'm just forced to assume there's friction then? Got it.

No, friction is just the most likely culprit, it could have been a strong gust of wind, that wouldn't change the answer.