When a question asks how much force is needed to get an object moving is the answer "at least the value of the static force" or would it be greater than the static force? I thought it would be the latter, but some example problems seem to indicate otherwise. I thought if the force exerted equaled the maximum static there would no net force so there would be no movement.
An example of this is 2.12a in the Berk Review physics book. Their answer choice is at least 12 for a 15lb pack with coefficient for static friction being .8. I would've thought it would be some value greater than 12. Thanks.
The point of this question is not only the topic, but also test logic. It's one of those
best, but not perfect examples.
The solution in the book states, "If she is to move the backpack, then the force she uses must be greater than the largest possible value of the static friction." So you are correct in your physics. The mathematical solution also shows that same thing, that F
applied > 12lbs.
BUT, the answer choices are as follows:
A) Less than 12 lbs.
B) At least 12 lbs.
C) At least 15 lbs.
D) More than 15 lbs.
So here is where you have to shift into test-taking mode. The absolute correct answer is not a choice. So, you have to choose the
best answer. Choice B is the best of the four choices.
I'm not sure which version you have, because in my older one, the answer stops there. In my newer one, there is the following line following the question:
"Often on the MCAT, you have to choose the best answer. Given that the answer must be slightly greater than 12 lbs., the closest answer is at least 12 lbs. The force cannot go much beyond 12 lbs., because once the backpack breaks free, the resistive force will decrease, due to reduced friction. Static friction exceeds kinetic friction, so choices C and D cannot be correct. Choice A is too small, so only choice B remains."
