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How are you supposed to "Study" physics?

  1. Sep 24, 2014 #1
    I feel like in my math courses, my chemistry courses, my computer science courses, I have a solid way of studying and mastering the material. For instance, with math; the secret to my mastery I've found in calc 1/calc 2 is to first clarify my understanding of the theory perfectly, and then do an insane amount of problems. I'm talking about 15-20 double sided blank pages of problems a day. This system has served me well and gotten me great math grades, but I have always been coming up short in physics. I have tried many different systems, from perfecting concept understanding and then doing a lot of problems, or trying to memorize and understand every step of as many solutions as possible in hopes I get one like that on the exam, and none have really succeeded.

    I find my problem is that I can spend a massive amount of time doing practice question after practice question (With the aid of a soln manual) hoping it'll just stick when I do enough, but I still get to the point where I can flip the page in the textbook, read a question I just did 30 of on the same concept, and just get it all wrong.

    I'm in engineering so I'm trying to really get good at problem solving, which I feel is likely my weakest skill. I always strive for clarity in my conceptual understanding, but I find myself applying the same rote method based processes that have done me well in math, mainly because I don't know what else to do.

    This is not to say I'm "bad" or doing bad in physics, I just want to develop that ability to, even with a lacking concept and 1000 practice questions of the same format, perfectly break down a problem accurately and intuitively and get the right answer in the end.

    By the way I "do" follow a read carefully -> think -> picture/fbd - > solve algebraicly -> substitute - > check answer process, but the questions I get wrong on exams tend to go wrong right from the start (I panic and am uncomfortable because I haven't seen the question before and end up drawing a wrong picture/doing a wrong step/adding a wrong force/ not adding a force/ thinking of it wrong/ setting up my axes wrong and screwing up my algebra / going down the path of one way i think will get me a solution but I end up with something completely wrong

    in short, how do i get good at problem solving and fissix in genral
  2. jcsd
  3. Sep 24, 2014 #2


    Staff: Mentor

    The usual problem for people first learning physics is that for each specific problem there's a "formula for that" and teachers teach it like a cookbook. Physics majors on the other hand learn to derive the specific formula by looking at the problem in a more general way noticing certain symmetries and using them effectively.

    Alternatively you could develop a list of conditions for when the formula is valid so youll know when to apply it.

    So perhaps if you do that at the start of each problem it will begin to stick.
  4. Sep 24, 2014 #3
    There's so many things that I do methodically and tediously in hopes of getting better. I always try to draw some kind of pic, do forces, write out what i have and what i need, but in the end it just slows me down on an exam. I need that intuition A+ physics students get when they look at a question they have never seen before and know exactly what to do
  5. Sep 24, 2014 #4


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    The equations should just be an aide to getting a specific answer to a specific question. If you don't develop a thorough understanding of the underlying concepts but focus on applying specific equations to specific problems, you are likely to have exactly the problem you are having. This may be obvious to you, and not a helpful suggestion, but it's all I can think of --- focus on understanding the concepts FIRST, not on doing lots of problems before you have mastered the core concepts.

    Of course, that begs the question of how do you master the core concept without doing practice problems. I'd say do SOME practice problems as part of the effort to understand the concept but focus on the concept more than the specific problem.

    Once you get that then drawing the right figure will come naturally and be an aide to solving specific problems, not a hindrance.
  6. Sep 24, 2014 #5


    Staff: Mentor

    I'd practice deriving the formulas needed from first principles even if the study guide doesn't do it. They'll say under these conditions this formula applies then try to understand how those conditions took the general formula and created the specific one.

    Perhaps if you post a problem that caused you some distress where you didn't know how to proceed,
  7. Sep 24, 2014 #6

    Sure. Here's the latest one.

    A 10k.0-kg block rests on a 5.0kg bracket, as shown below. The 5kg bracket sits on a frictionless surface, the coefficients of friction between the 10kg block and the bracket on which it rests are u_s = 0.40 and u_k = 0.30.

    a) what is the maximum force F that can be applied if the 10kg block is not to slide on the bracket?

    b) What is the corresponding acceleration of the 5kg bracket?

    What had me stuck forever is the fact that we were taught to do a FBD of each, and to find the maximum force that can be applied without the block slipping you would do a F_staticmax - F_tension = 0 and solve. But here, we are "supposed" to just be okay with setting F_Staticmax - F_Tension = ma, implying that the actual force of static friction is somehow greater than the tension creating it. I can't get over the fact that because the box is not moving relative to the bracket, you should find F_staticmax by solving u_s * mg = F, then applying that force to the entire system of mblock+mbracket to find the acceleration, but instead you "go backwards" and the acceleration dictates the force.

    It's not very intuitive. Nothing in the textbook went over a scenario like this, and even my teacher couldn't explain it properly (He drew the FBDs wrong as well, there's supposed to be a 2F force on the bottom bracket but I I would have never been able to figure this out myself). I still don't understand what exactly the force is that is making up for the difference in f_staticmax and the tension since the only other force I can think of having anything to do with the block is the reaction pair of the f_staticmax from the bracket)
  8. Sep 26, 2014 #7


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    F_staticmax - F_tension = 0 corresponds to the block not accelerating in the lab frame. If the block does not slide, it will accelerate, together with the bracket, and you have to take that into account.

    You can solve this in the frame of the bracket, but then you get an additional "fictional" force from the non-inertial frame, with the same conclusion.
  9. Sep 26, 2014 #8
    What I don't understand is why you can't just set F_staticmax - F_tension = 0 and solve for F_tension, then apply that force to the whole system with F = (M1+M2)*a

    What I dont understand is why the fstatic max is cut "short" when you examine the systems acceleration. We're lookign for the max force youre able to apply without it budging, so why would the force that will cause the box to budge off the bracket be any different than the force applied to the system (Since taut rope, T = F).
  10. Sep 27, 2014 #9


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    It doesn't sound to me like you're truly focusing on understanding the material. Why? Because of your reliance on doing "an insane amount of problems." Yes, you need to put effort into the class if you want to succeed, but what you describe sounds like a lot of wasted effort to me.

    You would probably benefit from acquiring new learning techniques. Does your school offer support services of this nature? I'm not referring to tutoring for the course, but a place where you can learn how to learn.
  11. Sep 27, 2014 #10
    Do you understand the problem now? I couldn't tell from your description if you were happy with it now, or not.

    Put your working down *step by step* and we might see where you are going wrong. Here's my step by step reasoning, which may be wrong - I'm rusty on this stuff!

    M - mass block
    m - mass bracket

    1. Applying the Force F by the pulley is exactly the same as applying the force F by pushing the 10kg block.
    2. F = usgM = 0.4 * 10 * 10 = 40N
    3. F = (m+M)a, therefore a = 40/15 = 2.667 ms-2

    OK - that might be rubbish, but others should easily be able to point out where I'm going wrong(!)
  12. Sep 27, 2014 #11

    i know how to do the problem and what you have to do, I just don't see why you the maximum force that accelerates the system without the block slipping is different than the force it takes to budge the block
  13. Sep 27, 2014 #12


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    See post 7. There is a net force on the block, so you cannot assume the forces on it sum to zero.
  14. Sep 28, 2014 #13
    Neither do I. Isn't it the same force?
  15. Sep 28, 2014 #14
    Isn't that just the force F in the left horizontal direction? (The only other force on block is the force of gravity downward: Mg)
  16. Sep 28, 2014 #15


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    Staff: Mentor

    This is the only horizontal force on the block beside friction, sure, but friction plus tension don't add up to zero because we know the block is accelerating.
  17. Sep 29, 2014 #16
    Th original problem statement says nothing about tension in the rope! Can't we just assume the rope transmits all the force F?
  18. Sep 30, 2014 #17


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    What do you mean with "all the force"?
    Tension in the rope is F, sure.
  19. Sep 30, 2014 #18


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    This is a homework problem, Mentor's please see report.
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