I am confused by the graph for Question number 7.  It seems to me that the scenario described and the graph do not match.  I understand that the graph is depicting the value of friction in order to maintain the object moving at a constant velocity.  But the sensor will always read the applied force.  So if the applied force is 5 N the sensor will also read 5N and the object will accelerate at a rate.  The sensor will not read 2N  Friction might be 2 N but not the sensor reading. In this case, the applied force cannot exceed 4 N so the X axis cannot be extended beyond the maximum static friction value.  Am I misunderstanding the problem??

Jacob, Thanks very much for pointing out our error. You are not misunderstanding the problem, we need to fix it, and we will be revising this shortly.

Jacob, We corrected the problem statement - and will be posting it later today - but, it will be moved to Dynamics 1D. Please check it out at this site:

https://njctl.org/materials/resources/dynamics-1d-ap-free-response-problems/attachments/

Thanks again for your comment.

John

The prompt on #3 is unclear about the load of ore's motion. Is it accelerating? Is it moving at a constant velocity? Is it being held in static equilibrium? And in what direction? The response in part (b) suggests that it is in static equilibrium but I would not know in the original prompt. Thanks for addressing this!

Question on #4a, when the string between the blocks are cut. Would not there be static friction parallel to mass2, up the ramp? Even if at that moment the string is cut, the static friction would keep the block stationary until its overcome and the kinetic friction kicks in?

Question #5c key, there is a missing mu, so that it should read: F(u sin theta + cos theta)

Nick, replying to your 3 comments:

#3 - I agree, and I made it clear that the miners are just holding the ore and not lifting it.

# 4b - the statement that there is no more friction force was in the wrong place - it applies to both parts of the question. I moved it to its proper place.

#5c - correct, change made.

New problem set is posted, thanks very much for your comments.  John

Thank you!

With regards to #3c (the two miners problem), I think the ore can be held perfectly horizontal with respect to the ground in the given force configuration. The question should be worded differently. Its not the ore being perfectly horizontal, it is the ropes being perfectly horizontal. In other words will the ore be in static equilibrium if you pull hard enough that the two ropes are perfectly horizontal to the ground. Am I correct ?

Vidanage, that was certainly our intent, but the wording was wrong. Thank you very much for your correction, and I updated the files.  John

Thank you, John for the reply and making the correction.