Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Net force zero = velocity > zero ?

  1. May 15, 2005 #1
    net force zero = velocity > zero ??

    can the net force on an object be zero yet the velocity not be zero?
    I say yes but under what conditions and must any restrictions be placed on the magnitude or direction of the velocity?
     
  2. jcsd
  3. May 15, 2005 #2
    Yes. The condition is that there is no force applied on the object. Then the velocity will be constant which can be non-zero. Magnitude and direction do not have any restrictions (assuming that there's nothing it can collide with etc.).
     
  4. May 15, 2005 #3

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    You may want to reread and understand Newton's First Law, which explicitly indicates that for an object with no net force, it will either be stationary, or move with a constant velocity. This is the beginnings of the concept of no prefered inertial reference frame.

    Zz.
     
  5. May 15, 2005 #4

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    It's the essence of the reinterpretation of the first postulate in terms of forces (defined in the second one).The principle of inertia applies,iff the vector sum of all forces acting on the body/system is zero.

    Daniel.
     
  6. May 15, 2005 #5
    Thank you but.....how can the velocity be constant and the external force be net zero? Would not friction cause an object to slow? Would the conditions have to be in space? I am a novice so go easy. Thanks
     
  7. May 15, 2005 #6

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    When there's friction, there's no longer the condition where no net force is acting on the object. Friction IS a form of an external force acting on that moving object.

    Zz.
     
  8. May 15, 2005 #7
    if there is friction there is force. the space would have to be empty.
     
  9. May 15, 2005 #8
    OK guys.....give me an example where this can occur....its easy to say that the net force is zero and velocity can be something other than zero but I have not seen any examples yet.
    How about a space ship that is accelerating and then stops accelerating....the net force is removed and yet the ship will continue moving at the last speed before removal of the force....sound good???
     
  10. May 15, 2005 #9

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    Pretty much so.However,the principle of inertia is still valid.

    Daniel.

    EDIT:Yes,it does.
     
  11. May 15, 2005 #10
    Basic stuff - Check out Newton's first law of Motion. You need a force to STOP a body moving, or to start it moving. You most certainly do not need a force to keep something moving. Think about the Voyager space probes for example.
     
  12. May 15, 2005 #11

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Don't feel too dumb, though.
    Precisely because it is very difficult to totally remove forces like friction and air resistance from acting upon an object, the "natural" idea is to assume that objects tend towards a state of rest (the Aristotelian view). The inertia concept (as developed by Galileo, and later Newton) is subtler/more difficult to understand because it seems to go counter to our day-to-day experiences.
     
  13. May 15, 2005 #12
    Fnet = sum of Forces (x,y,z). I use the Free Body Diagram to illustrate Fnet and anet. The forces acting on an object are drawn on the FBD (Fgrav, Ffriction, Fapplied, Tension, Fnormal, Fair, Fspring). Then the forces are added vectorally to find the resultant or net force. I encourage my students to view anet as the acceleration that you see or feel (if you are part of the system). Reference frames, as always, are critical here.
     
  14. Jun 4, 2005 #13
    Choggo, I guess you aren´t thinking in terms of NET force, but, instead of this, in terms of the force that YOU, as an extern agent, is applying on the object. Of course that YOU are aplying a force, you are "feeling" the force you are aplying on the object, BUT, otherwise, the 1st Law says that the NET force NEED to be zero for that the actual state of rest or uniform movement (both having a=0) remains like that. Your force on the body exists, and it will always exist in real cases because the friction, the gravity or any other sort of oposite force, but so as to know the behavior of the velocity of the body (if it´s changing or not) you need to calculate the NET force. If it's zero so velocity is constant. I hope I have helped you.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Net force zero = velocity > zero ?
  1. Zero Force Still Hurts (Replies: 5)

  2. Zero-G reaction forces (Replies: 36)

  3. Zero force (Replies: 4)

Loading...