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Stalled car net force hurry!

  1. Oct 10, 2011 #1
    1. The problem statement, all variables and given/known data
    A stalled car is being pushed up a hill by 3 people and it is moving at a constant speed the net forcce on the car is
    a in the opposite of the cars motion
    b in the same direction as the cars motion-wrong
    c zero
    d none of the above


    2. Relevant equations

    noone

    3. The attempt at a solution
    is it c zero because theres gravity and the normal force from the hill?
     
  2. jcsd
  3. Oct 10, 2011 #2

    SammyS

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    How do you know that these forces cancel each other completely?
     
  4. Oct 10, 2011 #3
    i don't?
     
  5. Oct 10, 2011 #4

    SammyS

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    You should. One (or two) of Newton's Laws tells you why.
    ...

    If they don't cancel, then the net force isn't zero.
     
  6. Oct 10, 2011 #5
    because there opposie and equal forces?
     
  7. Oct 10, 2011 #6

    DaveC426913

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    What do you know about the net force acting upon an object that is moving with constant velocity?
     
  8. Oct 10, 2011 #7

    SammyS

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    But how do you know they're opposite & equal. There's something in the problem that should tell you.

    (One of Newton's Laws)
     
  9. Oct 10, 2011 #8
    that its a constant speed?
     
  10. Oct 10, 2011 #9

    DaveC426913

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    And what does that tell you about the forces acting on it, specifically the net force?
     
  11. Oct 10, 2011 #10

    SammyS

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    Answer Dave's question by using Newton's Second Law.
     
  12. Oct 10, 2011 #11
    f=ma
    so the forces =0 because if there is constant speed theres no acceleration
     
  13. Oct 10, 2011 #12

    SammyS

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    We'll assume it's moving in a straight line, so constant sped means constant velocity.

    Yes, that's why you know the net force is zero ! The reason it's zero is that the forces cancel. Kind of a picky point here, but it could be important for understanding what's going on.
     
    Last edited: Oct 10, 2011
  14. Oct 10, 2011 #13
    okay thanks
     
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