F (#4) N (Student Had to Add This.)

Follow-Up:

Answer a new question based on the situation of everything AT REST so the f you see in the diagrams is static friction:

n2 (#1)

n (#3) f (#2)

f (#4) n (student had to add this.)

A. In the FBD on the right, n2 and Mg are not necessarily drawn to scale. I now want you to comment on their relative magnitudes: must n2 and Mg be equal in magnitude to each other? Must n2 and Mg be unequal in magnitude to each other?

B. State which of Newton’s Laws defends your response to part A.

You need to see through the above two questions and answer them together with one response. The only way to comment on the competing forces in a single diagram is to unleash Newton’s 2nd Law. This is supposed to be second-nature to you by now (and Newton’s Third Law is entirely off-topic for this application, and that’s now supposed to be second-nature as well. Newton’s Third Law was supposed to be crucial in the stage of making the diagrams above, and No one’s saying Newton’s Third Law is irrelevant in general. It’s irrelevant to what’s being in asked in task A.)

Define up as the +y axis:

∑Fy = May

n2 + (-fy) + (-ny) + (-Mg) = M(0)

→ yields: n2 = Mg + fy + ny

Question A’s answer is now proven for full credit, and this answer is intact even if one mistakenly replaces fy with fcosθ and ny with nsinθ. The true statement is actually n2 = Mg + ncosθ + fsinθ. (If you think this means n2 could be less than Mg due to n or f being inherently negative, then you’ve fallen behind and need to ask about this issue in class.)

A person who has made the above process second-nature never worries about what’s gonna be on the test. Such a person models the problem regardless of what’s asked. Then that person connects what’s in the written model to the question at hand. In other words:

· The application of ∑Fy = May correctly to yield n2 = Mg + fy + ny is done before Question A is even answered. Then its meaning is read, and then Question A is answered. People who don’t care about this spend their time on tests trying to brilliantly answer Question A, never spending their time building the very expression that lets the question answer itself.

· Would applying ∑Fy = May correctly to yield n2 = Mg + fy + ny be a waste of time even if it weren’t used in the question? You’re not supposed to think so.

· People who expect “hard” just because it’s “test” actually don’t know if these things are hard or easy. Without language intact, there’s no way of knowing. Specific elements of the language: raw force vectors, defined x and y axes, and x and y components of the diagram’s forces.

· If I grade something related to this topic, and someone thinks that hanging onto the paper for many minutes will somehow help them do better, that person is mistaken. Inability to do a FBD quickly and to use it efficiently comes from incomplete terminology, from incomplete definition awareness. That problem isn’t going to go away during quiz time. The people who walk in with complete definition awareness make the quizzing process simple and quick. The people who don’t are the people who end up writing off-topic things, and they waste a lot of time in the process of doing that. People in this situation could be given an hour on such a quiz, and it would only hurt them (and yield no points.)