Newton's Third Law hot air balloon

  1. 1. The problem statement, all variables and given/known data

    A physics student in a hot air balloon ascends vertically at a constant speed. Consider the following four forces. that arise in this question.
    F1 = the weight of the hot air balloon
    F2 = the weight of the student
    F3 = the force of the student pulling on the earth
    F4 = the force of the hot air balloon pulling on the student
    Which two of these forces form an "action-reaction" pair that obeys Newton's third law?
    a)F1 and F2
    b)F2 and F3
    c)F1 and F3
    d)F2 and F4
    e)F3 and F4

    3. The attempt at a solution

    I think it's d) because gravity is pulling the student downwards while the hot air balloon is exerting the same force in the opposite direction, keeping the student in the basket. Is this a correct assumption?
     
  2. jcsd
  3. PhanthomJay

    PhanthomJay 6,284
    Science Advisor
    Homework Helper
    Gold Member

    No, this is not the 3rd law. Look up its definition in regard to the 'action-reaction' pair. Essentially, if A exerts a force on B, then B exerts an equal but opposite force on A.
     
  4. Do you mean the question does not pertain to the third law or my answer?
     
  5. PhanthomJay

    PhanthomJay 6,284
    Science Advisor
    Homework Helper
    Gold Member

    I meant your answer is incorrect because what you have described is an application of newton's first law , with 2 forces acting on the same object and summing to zero. Newton's 3rd law applies to the force pairs acting on different objects. One of the multiple choices describes the correct application of Newton 3.
     
  6. So, would it then be b)?
     
  7. PhanthomJay

    PhanthomJay 6,284
    Science Advisor
    Homework Helper
    Gold Member

    Please explain why this is your choice.
     
  8. because the earth is pulling on the student while is is pushing away from the earth at an equal and opposite direction?
     
  9. PhanthomJay

    PhanthomJay 6,284
    Science Advisor
    Homework Helper
    Gold Member

    Your answer (b) is correct, but your explanation is a little bit off. The earth is pulling downward on the student with a force equal to his weight, while the student is pulling upward on the earth with a force equal to his weight.
     
  10. Ah, I see. Thanks for your help PhanthomJay!!
     
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?