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Force at constant speed?

  1. Dec 21, 2015 #1
    So force equals mass times acceleration. What happens then if we pushed a box horizontally without fiction at constant speed? The box keeps moving but acceleration equals zero? Is there still force?
    Thank you!
     
  2. jcsd
  3. Dec 21, 2015 #2

    DaveC426913

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    No force is required to keep the box moving.
    But the box has momentum. This momentum will be applied as a force if the box encounters an object.
     
  4. Dec 22, 2015 #3

    A.T.

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    No. The net force equals mass times acceleration.

    If the net force is not zero, velocity cannot be constant. The speed can be constant if you have a centripetal net force, perpendicular to velocity.
     
  5. Dec 28, 2015 #4
    B
    But i think he is not thinking that if there is constant speed (velocity) then there is no net force. any net force will result in acceleration (a=F/m)
     
  6. Dec 28, 2015 #5

    ZapperZ

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    Unless there is another equal but opposite force other than friction, then this will never happen. If velocity is a constant, there is no net force. If there is a net force, then velocity is not a constant.

    Zz.
     
  7. Dec 30, 2015 #6
    there is NO RESULTANT force...ther could be 2 (at least) forces acting...There is no resultant
     
  8. Dec 30, 2015 #7

    DaveC426913

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    The term normally used is net force.
     
  9. Dec 30, 2015 #8
    I am more familiar with RESULTANT... in my text books. But the meaning is the same. Makes no difference to the physics
     
  10. Dec 30, 2015 #9
    I have always heard of the resultant of vectors.... I suppose it is the net ?? resultant is a good word, should not be dismissed.
     
  11. Dec 30, 2015 #10

    davenn

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    have to agree with Dave and Zz
    net is the normally used term and it's only 3 letters to type :smile::wink:
     
  12. Dec 30, 2015 #11

    DaveC426913

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    Perhaps, although "normally" does not exclude the possibility that lychette's term is not common and accepted where he teaches. I don't know where, but I'm willing to grant that we may be guilty of provincialism. Or whatever the inverse of provincialism is.
     
  13. Dec 30, 2015 #12
    are you sure the word is 'net' and not 'nett'.(4 letters....does it matter!!!).... what exactly do you mean by 'normally'....do you mean ; here on physics forums or in text books ? .. Is there any confusion over 'resultant'.... what is used in 'your' text books.
    Do you ever refer to the 'net' of 3 vectors.... I would say the 'resultant' of 3 vectors... in reality the difference in meaning is zero
    Thinking about it,.... the 'net' of 3 vectors sounds plain wrong....it has to be 'resultant' ???
     
  14. Dec 30, 2015 #13

    DaveC426913

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    Common in the science world which mostly uses English as a common language.

    There's nothing wrong with the word resultant; it's just unfamiliar to many. We often forget that PF is global, not exclusively Western.
     
  15. Dec 30, 2015 #14
    surely one aim here should be to ensure that we all understand the same physics language in a communication. In this thread the difference between 'resultant' and 'net' (nett?)is unimportant...some reference to text books should be made...( i think it is part of the membership here) I would refer to 'Nelkon and Parker' as a start..in fact it came from a contribution by ''Davnn'' pointing out that only 3 (4) letters needed to be typed!!!!..nothing to do with physics knowledge... (is this a p;hysics or an english language forum?)....pathetic...sorry, subjective opinion
    A physics forum should be absolutely clear about the terms used in communications, I have seen pd confused with emf in recent posts amongst other sloppy usage.
    I will continue to contribute to these forums but it seems to be a process of finding fault rather than imparting knowledge.
     
  16. Dec 31, 2015 #15

    FactChecker

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    The situation in the OP is similar to pushing a car on a level surface. Once you get it going, you are mostly just keeping up with it and only pushing a little to overcome the friction that tries to slow it down. Essentially no pushing is necessary, once it is rolling at the desired speed.
     
  17. Dec 31, 2015 #16
    The push would have to be zero!

    The test for negligible friction is to see what happens when you stop pushing. If the object continues to move at a steady speed across a horizontal surface with no push, friction is negligible. Of course, friction can never be eliminated, but it can be reduced to the point that the measuring devices being used detect no significant change in speed during the time it's being measured.

    As to the discussion of resultant versus net force. In the US, introductory college and university level textbooks use resultant to refer to any vector sum. In the special case of force vectors the phrase "net force" is used to refer to the vector sum of the forces acting on an object. You could also call it the resultant force, but that's not as common. If referring to the sum of velocity vectors, or displacement vectors, or any other vector quantity, the term resultant could be used, but the term net would not.

    In grocery stores we commonly see "net weight" used on product labels. By law, it refers to what physicists would call the mass of the package contents, after subtracting off the mass of the packaging material.
     
  18. Jan 1, 2016 #17
    In Physics 'net' and 'resultant' are essentially the same. I, and many others, prefer the word 'resultant' when referring to forces because it is a specific science term. 'net' is a more everyday term and can cause confusion, especially students learning the basics.
    But now we are talking about semantics not physics...pity the poor student.
    I must admit that a site I greatly value...'hyperphysics'.. does use the word 'net' in its articles about newtons laws... I prefer 'resultant'..it is a great point for discussions for those of us who are not 'learners'.
     
    Last edited by a moderator: Jan 1, 2016
  19. Jan 1, 2016 #18
    'no pushing is necessary' ..I think you mean that no resultant/net force is nessary...A push is required if there is friction.
     
  20. Jan 1, 2016 #19

    FactChecker

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    Yes. "only pushing a little to overcome the friction"
     
  21. Jan 1, 2016 #20
    to equal friction ?????
     
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