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Net Force (Pushing Boxes)

  1. Mar 7, 2014 #1
    So, I am having some trouble with understanding force concepts. I do understand that Newton's 3rd law can be described as "describing the same interaction with the same magnitude of force, but in opposite directions on each object." So, I have a concern with a question that I have on my homework.

    "A student pushes horizontally on two boxes. The boxes are moving to the right at a constant velocity. Box 1 has more mass than Box 2. There is friction between the boxes and the table."

    The first question in regards to the situation, was...

    "For a situation in which the boxes would be slowing down, which of the following statements is true about the magnitude of the forces?"

    Out of the multiple choices I had, it was clear to me that the following answer was true:

    "The magnitude of the force that box 1 exerts on box 2 is equal to the force that box 2 exerts on box 1." - This is due to Newton's 3rd law.

    However, I have a question in regards to net force. The next question was asking, in the same situation where the boxes are slowing down, which box has the larger net force and which direction is the net force in? I am confused on the difference between net force and the magnitude of the force. If the magnitude of the force is the same between both objects in the interaction, what exactly is the net force and how is it different than the magnitude of the force?


    EDIT: Box 1 is behind Box 2
     
  2. jcsd
  3. Mar 7, 2014 #2
    Net force just involves all of the forces acting on an object. For the first question you answered, you're just dealing with the forces that the boxes exert on each other, but the net force for each box may be different if other forces are involved. So, with this in mind, what would the net forces be on each box?
     
  4. Mar 7, 2014 #3
    Ok, that makes sense. Well, I wasn't given any specific values for anything. However, based on what you just told me, wouldn't the net force on Box 1 be larger than the net force on Box 2 due to it's larger mass? This net force would be pointing to the right, which is the direction the boxes were traveling, correct?
     
  5. Mar 7, 2014 #4

    SammyS

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    The details of the problem are not all that clear -- at least not to me.

    Does the student push on one of the boxes, which in turn pushes the other? If so, which box is pushed directly by the student?

    Otherwise , what is the situation here?
     
  6. Mar 7, 2014 #5
    Box 1 is in contact with, but behind, Box 2. So, the student has contact with Box 1. The student pushes the boxes (they were already together when they were pushed).
     
  7. Mar 7, 2014 #6

    SammyS

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    What direction is the acceleration of Box 1?

    What does Newton's 2nd Law say about net force and acceleration?
     
  8. Mar 7, 2014 #7
    The direction of acceleration of Box 1 is to the right (which I am going to make an assumption is the positive direction, I should've made that assumption in my original post).

    Newton's 2nd Law states that net force and acceleration must be in the same direction.
     
  9. Mar 7, 2014 #8

    SammyS

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    Aren't the boxes slowing down as they move to the right?
     
  10. Mar 7, 2014 #9
    Oh, correct. So there is a negative acceleration, my apologies.
     
  11. Mar 8, 2014 #10

    adjacent

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    Isn't the student pushing the box?If so,it will not slow down
     
  12. Mar 8, 2014 #11
    The problem says there's friction on the table too though, so in this case the friction would have to be greater than the applied force.
     
  13. Mar 8, 2014 #12
    The problem does state that the student is pushing the boxes, however it doesn't state whether he continues to do so. It just asks, in the event that the boxes are slowing down when moving to the right, which net force is greater and in which direction is that net force pointing.

    When working the problem, I just assumed that the student stopped applying force to the boxes, which resulted in the slowing down.
     
  14. Mar 8, 2014 #13

    haruspex

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    We are not told what set the boxes in motion in the first place. The boxes may have been moving already when the student started pushing, so may be slowing down despite a constant push.
     
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