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Homework Help: Discussion Questions in University physics by young

  1. Aug 11, 2012 #1
    Hello I took a general physics course 2 years ago, and now I am trying to refresh a bit on my knowledge of physics so I started solving discussion questions+problems in University physics by young
    My problem is I can't verify my answers nor can I look for the correct answer if I don't know it, since it isn't included in solution manual, I mean the normal exercises are, but the more discussion questions aren't
    1. The problem statement, all variables and given/known data

    Like for instance this question
    Q5.18) You often hear people say that "friction always opposes motion" Give at least one example where a) static friction causes motion, b) kinetic friction causes motion

    2. Relevant equations

    3. The attempt at a solution

    Well for static friction I would think the the friction between our feet and the ground causes walking i guess? I mean when you push your feet on the ground the reaction force would be considered a static friction force, but I don't know about kinetic friction. I also made this not for this question specifically it's just that the book no matter how many times I re read it, I don't find answers to these question, so how exactly am I supposed to answer them?
    Also there is this question that says can a body be in equilibrium when only one force act on it I think no but I can't be 100% sure cause let say you put an metal in an air free container in middle of space and position a positively charged particle exactly above it the way I think of it since electrons would be going in orbits wont the object affect the individual protons in the atom I know the atom is by itself neutral but wouldn't there be a coloumb repulsive force between that charged object and the protons in the metal even though it wont move?
    Last edited: Aug 11, 2012
  2. jcsd
  3. Aug 11, 2012 #2

    Simon Bridge

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    Imagine climbing a slippery hill ... you work hard and you make it. What kind of friction between your feet and the hill?

    You've also seen those problems where a block is dragged across the top of another, longer, block. What sort of friction between the blocks? Direction of the friction force on the longer block?
  4. Aug 11, 2012 #3
    Well the friction between those two blocks is static since they are on top of each other they don't move with respect to each other, when I climb a slippery hill isn't it also static friction?
    Ermm I don't know exactly what you mean by climbing a slippery hill though.
  5. Aug 11, 2012 #4

    Simon Bridge

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    Oh no - in the example, one block is sliding over the other one and, as a result, the other one slides over the ground.

    You never tried climbing a slippery slope and succeeded?
    You take a dozen flailing steps to advance just one step up the hill.
    The experience is like trying to climb the down escalator only no steps.

    OK, how about this one ... pulling a tablecloth out from under the plates?
    You can leave the plates behind if you are very fast. If you are slow, the plates come with the cloth. But, in between, the cloth moves faster than the plates but they both move. Give it a go.
  6. Aug 11, 2012 #5
    Well ok I guess kinetic friction is under place, but I still need a hint on how to solve such question, rereading my lectures notes and the book doesn't help much. Am I doing something wrong?, or are those questions meant to be answered via googling and such?

    also about the second question I assume they wanted me to say Sigma (F) = m*a, so if there is only one force it can't be zero, but sometimes I don't know if there is a "trick" like maybe the example I suggested etc...
  7. Aug 11, 2012 #6


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    When a car is driven away from the lights in a normal way, it is the static friction between the tyres and the road that enables the motion.
    When a car is driven away by a hoon - with the driving wheels spinning - we have kinetic friction causing the motion.
  8. Aug 11, 2012 #7
    yes well I made my topic title for a more general question is, what is a good guide line to answer such questions, How to study for them etc? I don't feel those discussion questions make a good self study materials.
  9. Aug 11, 2012 #8

    Simon Bridge

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    Those are "discussion" question - so you need to discuss the topic.
    It's not like the problem-solving questions where you need to be rigorous. PeterO's answer was spot on.

    To do well you have to start by understanding that questions like this, in an assessment, want you to show your understanding of a concept. In an exercise they want to develop your ideas about a concept. So identify the concept ... this can take a bit of thinking around the topic.

    Once you have, imagine you are trying to explain the concept, in terms of the question, to someone else. Imagine you are answering the question posed by someone else on PF for eg. You have to put it in simpler terms than you'd use to exaplain it to yourself, but you don't have to prove anything with any rigor.

    It's a really good skill to get.

    In exams, these questions are usually worth the most since they are hard to write a detailed marking schedule for: pretty much anything reasonable gets full marks.
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