Engineering Mechanics TA

Asked Why does a free-body diagram need the normal force even on a flat surface?

Answered

Because "flat" doesn't mean "nothing's happening there" — it means something in particular is happening: contact and no relative sinking. Students often skip the normal force on flat ground because "nothing is pushing up," but something is: the surface.…

Because "flat" doesn't mean "nothing's happening there" — it means something in particular is happening: contact and no relative sinking. Students often skip the normal force on flat ground because "nothing is pushing up," but something is: the surface. If the surface weren't there, the object would accelerate downward under gravity. Since it doesn't, the surface must be exerting an upward force on it. That upward contact force is the normal force. On a flat horizontal surface with no other vertical loads, the normal force happens to come out equal to the weight — which is exactly why people get sloppy and start treating it as "the weight" rather than a separate force with its own cause. But it's the surface's response to contact, not gravity's stand-in. Tilt the surface, or add a downward push on the object, and the normal force immediately stops equaling the weight — proof that they were never the same thing. Rule of thumb for your FBD: normal force exists whenever two bodies touch and press against each other, full stop — flat, inclined, curved, doesn't matter. Leave it off and your equilibrium equations will be missing a force that's actually there.

Asked When should I use the method of sections instead of the method of joints?

Answered

Use the method of sections when you want the force in just one specific member, or a small handful of them, without solving the whole truss.…

Use the method of sections when you want the force in just one specific member, or a small handful of them, without solving the whole truss. You make a cut through the truss that passes through the member(s) you care about, treat one side of the cut as a rigid body, and apply the three equilibrium equations directly to that section — often you can pick a moment point that eliminates everything except the member you're after. Method of joints, by contrast, is the systematic approach: you work through the truss joint by joint, solving two equilibrium equations at each pin, until you've found every member force. It's reliable but slow if all you need is one answer buried in the middle of the truss. So the practical rule from the course: if the problem asks for one member's force, cut through it with a section. If it's asking for all the member forces, grind through joint by joint. A common time-waster on exams is students defaulting to joints out of habit and solving half a dozen joints they didn't need just to reach the one member the question actually asked about — recognize that pattern and go straight to a section instead.

Asked What can I bring to the midterm?

Answered

For each midterm, you're allowed one letter-sized sheet of handwritten notes, plus a calculator. Phones are not allowed, and exams are closed book otherwise. For the final exam, the allowance is larger — three sheets of handwritten notes instead of one.…

For each midterm, you're allowed one letter-sized sheet of handwritten notes, plus a calculator. Phones are not allowed, and exams are closed book otherwise. For the final exam, the allowance is larger — three sheets of handwritten notes instead of one. If you're prepping your sheet, it's worth being deliberate about what goes on it (key formulas, sign convention reminders, a checklist for free-body diagrams) rather than trying to cram in worked examples — you won't have room to hunt through those under time pressure anyway.

Asked I keep getting friction problems wrong. What am I missing?

Answered

The classic mistake is using F = μN as if it's always true — it isn't. Static friction is an *inequality*, not an equation: the actual friction force is at most μₛN.…

The classic mistake is using F = μN as if it's always true — it isn't. Static friction is an *inequality*, not an equation: the actual friction force is at most μₛN. Until the body is actually on the verge of slipping, friction takes whatever value equilibrium requires, which is often *less* than μₛN. So the workflow should be: 1. Draw the free-body diagram (surprise — always the first step). 2. Write your equilibrium equations with friction as an unknown, F. 3. Solve for F from equilibrium. 4. *Only then* check: is F ≤ μₛN? If yes, the body is in equilibrium and F is whatever you calculated. If the equilibrium value you'd need exceeds μₛN, the body can't stay put — it slips, and now (and only now) does F = μₖN (kinetic friction) actually apply. The error to watch for is jumping straight to F = μN in step 2, as if friction is always maxed out. That's only true right at the point of impending slip — which is a specific condition a problem has to actually specify or imply (like "find the minimum angle before sliding begins" or "the block is on the verge of tipping/slipping"). If the problem is just asking whether something stays in equilibrium under a given load, you generally solve for F first and compare it against μₛN afterward. A good gut-check: if your answer for F comes out *larger* than μₛN, that's not an error in your arithmetic — it's telling you the assumption of equilibrium was wrong, and the body actually accelerates or slips. If you want, describe one of the specific problems you're stuck on and I can walk through where in that four-step process it's going sideways.

Asked What happens if I miss an exam?

Answered

If you miss an exam, contact Dr. Raman before the exam if at all possible. Documented illness, emergencies, and university-sanctioned travel are excused and get a makeup exam.…

If you miss an exam, contact Dr. Raman before the exam if at all possible. Documented illness, emergencies, and university-sanctioned travel are excused and get a makeup exam. Oversleeping is explicitly not an excused reason — so that one won't get you a makeup. If something urgent is happening close to exam time, reach out as soon as you can (email [email protected]) rather than waiting until after. That's what "contact before the exam where possible" is getting at — it's not a strict requirement that you predict the emergency, but the sooner you notify her, the smoother it goes. Anything beyond that — like what specific documentation counts as "documented illness" or how a makeup gets scheduled — isn't spelled out in what I have, so that would be a question for Dr. Raman directly.