Homework Help: Newton's Third Law hot air balloon

1. Oct 5, 2008

Mitchtwitchita

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. Oct 5, 2008

PhanthomJay

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.

3. Oct 5, 2008

Mitchtwitchita

Do you mean the question does not pertain to the third law or my answer?

4. Oct 6, 2008

PhanthomJay

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.

5. Oct 6, 2008

Mitchtwitchita

So, would it then be b)?

6. Oct 6, 2008

PhanthomJay

Please explain why this is your choice.

7. Oct 6, 2008

Mitchtwitchita

because the earth is pulling on the student while is is pushing away from the earth at an equal and opposite direction?

8. Oct 6, 2008

PhanthomJay

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.

9. Oct 6, 2008

Mitchtwitchita

Ah, I see. Thanks for your help PhanthomJay!!