Arturo O. answered • 10/25/17
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The numbers given in the problem statement are not compatible. If the block remains at rest when the 30 N force is applied to the left, the friction force must equal 30 N to the right. However,
μs(mg) = 0.30(10)(9.8) N = 29.4 N ≠ 30 N
If the block was moving at a constant velocity (which is not stated in the problem), then the kinetic friction must balance the applied force. However,
μk(mg) = 0.25(10)(9.8) N = 24.5 N ≠ 30 N
There has to be something else given in the problem, otherwise these numbers are incompatible. Would you restate the problem VERBATIM as given?
Arturo O.
If the block is accelerating, then the opposing force is the kinetic friction
μk(mg) = 0.25(10)(9.8) N = 24.5 N.
As currently worded, the problem does not state whether the block remains at rest, is moving at constant velocity, or is accelerating.
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10/25/17
Arturo O.
Apparently, they want you to assume the block is moving, so calculate the opposing kinetic friction, which gives 24.5 N, and then rounded up gives answer (4). But the problem is poorly worded (not your fault; it is poorly worded in the book or online material that you are using).
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10/25/17
Anita T.
Ok. Just to clarify, we can only even assume it's moving because the applied force (30 N) is greater than the static friction force (29.4 N). Once we know that, we can move forward with figuring out what the opposing force (kinetic friction force) is (i.e. if the static friction force was greater than 30N, the block would stay at rest).
Thank you
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10/25/17
Arturo O.
You are welcome, Anita.
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10/25/17
Anita T.
10/25/17