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B Common physics misconceptions

  1. Aug 17, 2016 #1
    Was wondering if any Physics teachers would care to share just one common misconception that arises when students encounter Newton's Laws of Motion. I'm a learner and want to watch out for some of the pitfalls.

    Appreciate your help,
    Beany
     
  2. jcsd
  3. Aug 18, 2016 #2

    haruspex

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  4. Aug 18, 2016 #3
  5. Aug 18, 2016 #4
    Judging by the number of recent posts here, there is often confusion about the 3rd law that goes like this: A person pushes on a block, the block exerts an equal and opposite force on the person, so why does the block move? The resolution comes with the understanding that the motion of the block is determined by the net force on the block; it's reaction force on the person is not relevant to is motion.
     
  6. Aug 18, 2016 #5

    phinds

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    If the block is heavy enough, it's the person that moves (at least to the length of their arms ... possibly farther if they are on a slippery surface)
     
  7. Aug 18, 2016 #6
    1. The direction of an acceleration and thus a net force.

    ex. A ball is thrown up. When it is at its maximum height and v = 0 is there a net force? Yes. Then of course what is happening while the ball is in the hand while throwing it up and then catching it.

    2. If an object is moving a net force must be acting on it. Not true of course. Ex. I apply a horizontal force with my finger to a block on a rough table and it moves at a constant velocity.

    If one goes through the entire process of applying the finger to the block and it does not move, to applying enough force with the finger so that it BEGINS to move. To moving at a constant velocity. To removing the finger and noticing that the block slows down and stops. To asking why the block does not return to its original position like presented in #1 (i.e. friction can be confusing)

    Then make the table perfectly smooth and ask the same questions as the above and realizing that the only time the block should move at a constant v is when the finger is not applying a force.

    3. Then the proper application of newtons laws to a free body diagram. Texts have different rules for making free body diagrams and it can become a mess after the student becomes use to a method meant for fairly simple situations. ( Tension and friction can be confusing)

    4. Circular motion and Newton's laws. Changing direction but not speed requires a net force.

    5. The whole idea that an applied force can be countered by another force that has no intentions.

    Ex. A box on a table. You push down on it with your finger. How does the table know to push back up... Intention based thinking.
     
  8. Aug 18, 2016 #7

    Andy Resnick

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    If I can only list one, I would say "confusing the concepts of 'force' and 'velocity' (e.g. 'a force causes an object to move')". If I could list another, it would be the confusion that force resides within, or is a property of, an object ('one object imparts a force onto another')
     
  9. Aug 18, 2016 #8
    So, if I've understood you then, we learners struggle initially with the concept of "net force"?
     
  10. Aug 18, 2016 #9
    Thanks ever so much. I'm particularly struck by number 5. Forces being applied without obvious intention. I've encountered this confusion myself. I suppose this is more to do with the meaning attached to the word as used by scientists compared to everyday usage.



     
  11. Aug 18, 2016 #10
    I think you mean 'massive' enough.
    I dont think weight is part of this expanation?
     
  12. Aug 18, 2016 #11

    phinds

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    I'm thinking of it sitting on a rough surface, so that its friction with the surface is what causes the human to move instead of the block, in my example. In this consideration, weight clearly does matter. With no weight, both objects always move, proportional to their weight and the force.
     
  13. Aug 18, 2016 #12
    I was thinking in a more general physics sense, such as in space outside the space station with a massive object or on ice with something massive...mass is the important property in general...not weight.
    misconceptions in this area arise by confusing 'weight' with mass
     
  14. Aug 18, 2016 #13
    The OP should, IMHO, not go overboard with this. My former science teacher, in an attempt to get people using mass instead of weight, actually used mass all the time, even when weight was the correct term. For example, she said something like "objects with more mass are heavier," when how heavy an object is has to do with weight. Whether this is to technical or not, instead of saying "mass is generally the correct term" as I viewed the post implied, just know the difference between mass and weight.

    Also, learn the "second part" of Newton's third law-- third law pairs must be the same type of force (i.e. gravitational, tension, contact, electric, etc.), which will show you that the normal force (a contact force) is not the third law pair of mg*cosθ (a gravitational force), though they are equal and opposite. There are many misconceptions about the third law, many of which have been mentioned. I'd pay extra close attention to all of them.
     
  15. Aug 18, 2016 #14
    I wonder why your former teacher would try to get you to use 'mass' instead of 'weight' ? they (as you know) are different things. Your teacher's wording is OK as part of her attempt to make physics interesting and understandable.
    I think you misunderstood my point. Newtons laws refer to masses not weights.
    I know about the third law.
    just know the difference between mass and weight
     
  16. Aug 18, 2016 #15
    I posted that because of this line
    Which is probably what my teacher was trying to get at.

    Also, the second paragraph was for the OP, it wasn't in response to you.
     
  17. Aug 18, 2016 #16

    sophiecentaur

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    This misconception arises, I think, because it is tempting to ask the 'how does it know?' question. When you push an object, there is always a transitional time, during which the two surfaces (your finger and the ball) deform until the (N3 pair of) forces reach a final value. The same sort of 'how does it know?" question is often asked about the currents and voltages around an electrical circuit. They are only there after a certain transitional delay, during which the various circuit elements respond to their individual situation
     
  18. Aug 18, 2016 #17

    haruspex

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    No, that would only result in the block accelerating more slowly than the person. phinds was describing the case where the person moves andthe block does not, which would be a consequence of weight and friction.
     
  19. Aug 18, 2016 #18
    have you checked with phinds that this is indeed what he meant. Too many misconceptions here
    presumably we agree that there is no misunderstanding about the meaning of weight and mass. Life would become nore complicated if friction had to be included in sorting out misconceptions..
     
  20. Aug 18, 2016 #19

    haruspex

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    Intentionality is a legal and psychological concept, not a physical one.
    If you want to think of objects having it, you could say a rigid body has the intention of not being penetrated by another object. This is actually useful. It tells you that the magnitude of the force is just sufficient to prevent penetration. In particular, it will be normal to the contact plane.
     
  21. Aug 18, 2016 #20

    haruspex

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    Did you read post #11?
     
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