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

No resultant force means no external force?

  1. Mar 7, 2009 #1
    Hmm. I'm quite confused with this:
    Does no resultant force means no external force?
    i.e. if there is an external force, there is a resultant force?


    Like say, sand falls onto a moving conveyor belt at the rate of 5.00 kg/s. The conveyor belt is supported by frictionless rollers and moves at a constant speed of 0.750m/s under the action of a constant horizontal external force Fext supplied by the motor that drives the belt.

    so the friction force is the force required for the sand to move at same speed as the belt right? then can i say there is no resultant force acting on the sand?


    anyway i duno if this is considered as h/w qns.. sorry.
     
  2. jcsd
  3. Mar 7, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Not at all. Consider a book sitting on your desk. The net force is zero, yet both gravity and the desk exert forces (external forces) on the book.


    Note that the sand must be accelerated from zero speed to the speed of the belt. That requires a force.
     
  4. Mar 7, 2009 #3
    yah, so that force is the friction force?
     
  5. Mar 7, 2009 #4

    Doc Al

    User Avatar

    Staff: Mentor

    The conveyor belt exerts a horizontal force on the sand, if that's what you mean by "friction force".
     
  6. Mar 7, 2009 #5
    hmm. okay!

    then since they are moving at same speed can i say there is no resultant force on the system?
     
  7. Mar 7, 2009 #6

    Doc Al

    User Avatar

    Staff: Mentor

    What system?
     
  8. Mar 7, 2009 #7
    the system of sand and belt?
     
  9. Mar 7, 2009 #8

    Doc Al

    User Avatar

    Staff: Mentor

    It's unclear what you mean by "sand and belt" as your system. All the sand?

    Focus on a section of the belt alone. Since its speed remains fixed, the net force on it must be zero.

    The falling sand is being accelerated by the belt, so there must be a net force on that sand during its acceleration.
     
  10. Mar 7, 2009 #9
    Oh. okay sorry, i was actually referring to the sand on belt and the belt itself. for this, is no net force?
     
  11. Mar 8, 2009 #10
    Strictly speaking, the belt DOES slow down a bit as the sand particles fall on it. As the belt exerts a force to speed up the sand, the sand exerts an equal and opposite force on the belt slowing it down.
     
  12. Mar 8, 2009 #11

    Doc Al

    User Avatar

    Staff: Mentor

    Since the belt maintains a constant speed, the net force on it is zero. Since the sand exerts a force on the belt, the mechanism must exert an equal and opposite force on the belt to maintain its constant speed.
     
  13. Mar 8, 2009 #12
    Ya. But i think, the belt should slow down for a very small interval of time before getting 'reinforcements' from the motor to maintain the speed.
     
  14. Mar 8, 2009 #13

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    First lesson:

    NEVER, EVER, break the PREMISE given in the exercise!

    You are GIVEN that the velocity is constant, so that is what you are to assume for the remainder of the exercise.


    Besides, STRICTLY speaking, you'd have to take into account relativistic and quantum mechanical effects, at least after you have included the gravitational effects from the planets impinging upon the scenario.

    How do you know that these would be insignificant compared to the error source you happened to mention?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook