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Understanding basic principles

  1. Jan 9, 2010 #1
    Hello, I have a few questions with some things I can’t seem to grasp.

    1. If objects *without* any force being applied to them move at a constant velocity, which might be zero, .... and objects being *acted* on by a force are always accelerating. What causes an object being acted on by some force to gain a constant velocity if its initial velocity is zero?

    Meaning that, am i right in thinking the above is impossible, as you can only begin to move forwards from an initial velocity of zero, with positive acceleration which only occurs once you overcome opposing forces. Then and only then can you lower your force to equal opposing forces to have a constant acceleration. So constant velocity from an initial velocity of zero cannot occur, some form of acceleration has to occur first.

    2. With regard to friction, from my understanding, friction is a force in the opposing direction or travel, that is some force to the strength of all forces acting perpendicular to the direction of travel. If this is so, if i have a horizontal surface, with a ball with an infinitely small force being applied to it to make it move to the right. How come that ball isn’t being pushed in the negative direction of travel by the force of friction? Which in this case will be a pretty large force of f = FrictionConstant*mg, in the negative direction of travel.

    3. I keep seeing graphs of friction as slopes that go up as the static friction is being reached, and then slowly back down as static friction is overcome, and dynamic friction becomes the opposing force. Why the slope down as we finally overcome static friction? Seeing as its a constant, wouldn’t it be more of a step down?

    Im sure that all of the above is a lack understanding on my part. And so as a final request, can anyone recommend a book on vector physics? (i believe is the term) That is, im interested in learning things like how changing the lengths marked in red of the following structure changes its state as to whether or not it collapses, and how removing things like the cross bar bit affects it.



    2pyusk2.jpg
     
  2. jcsd
  3. Jan 9, 2010 #2

    Doc Al

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    Right. In order for an object to begin moving (from 0 to some non-zero velocity) it must accelerate, which requires that a net force act on the object.

    Friction opposes slipping between surfaces.

    You cannot assume that f = μ*mg. That's true for kinetic friction, but not for static friction. Static friction is less than or equal to μ*mg. Furthermore, when you push a ball the amount of static friction required to prevent slipping (so the ball rolls) will always be less than the applied force (at least in the usual simplified model of friction).

    On the other hand, if you were to push a block (something that doesn't roll), then there would be a minimum force you'd have to exert in order to get it moving. (Which would equal μ*mg.)

    It would be pretty close to a step down, but rounded just a bit. See the diagram here: http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html" [Broken]

    What you're looking for is a book on engineering statics or vector mechanics (and strength of materials).
     
    Last edited by a moderator: May 4, 2017
  4. Jan 9, 2010 #3
    This applies to both kinetic and static friction, Kinetic friction is μ*mg and static is the same thing. The only difference is that the 'μ' will be a different value. Also Kinetic friction is less than Static friction, it wouldn't make sense that Kinetic be more, this would mean that if the static was 10N and the kinetic was 11N if you applied 10.5 the ball/block would still not move because it hasnt overcome the KInEtIc friction (kinetic means that the object is in motion)

    And in both cases, both rolling and not rolling, there would be a minimum force you need to exert. Although the force on the rolling ball would be alot less.
     
  5. Jan 9, 2010 #4

    russ_watters

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    That's all very oddly worded. The first sentence seems trivally true, but too wordy, implying you think there is something odd or profound about it. The second sentence is wrong - if you have equally opposing forces, you will have no acceleration (caveat: the inertia and acceleration result in an opposing force via f=ma). So I don't know what this "lower" thing you are talking about imeans.
    Sure - if you're at zero speed and want to get to 60, you have to accelerate. This again seems trivially true, so you must be seeing something to overcomplicate it in your mind.
     
  6. Jan 9, 2010 #5

    Doc Al

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    No. Static friction (as I stated, albeit quickly) does not simply equal μs*mg--it is whatever it needs to be, up to a maximum value of μs*mg.
     
    Last edited: Jan 9, 2010
  7. Jan 9, 2010 #6
    Thanks for your replies. My misunderstanding was that friction as a force was always there in its full form uMg, depsite the amount force acting against it. So would this mean that firction has to be calculated last? as you can only apply it once you know the normal force, and also the force opposing the friction, which are both sums of other forces.

    That last but was supposed to be constant velocity, not constant acceleration. Sorry abour that.
     
  8. Jan 10, 2010 #7

    jtbell

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    For static friction, basically yes. Keep in mind though, that there are situations in which you cannot calculate all the forces one after the other, but instead must set up a system of equations, e.g. two equations in two unknowns, and solve them simultaneously.
     
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