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Simple Newton's Law Situations

  1. Sep 28, 2016 #1
    #1 A bird strikes the windshield of an airplane and makes a big mess. Which of the following statements is correct?
    A) The force that the airplane hits on the bird is larger than the force that the bird hits on the windshield.
    B) The force that the bird hits on the windshield is larger than the force that the airplane hits on the bird.
    C) The two forces are equal in magnitude.
    D) Which of these two forces is bigger depends on how fast the bird strikes on the airplane and on the size of the bird.

    # 2 Suppose you are at a stop light and realize that an important music CD sale ends in ten minutes. Naturally, you speed off very rapidly. To you, some papers on the dashboard fly straight backwards. To an observer on the ground these papers
    A) remain where they were.
    B) move forward rapidly.
    C) move backwards rapidly.
    D) fall straight down.


    The attempt at a solution:
    For the #1, I originally thought the two forces are equal in magnitude however I don't believe that's correct now. My second inclination is D. I know that F=Mass*Acceleration.


    For #2, I originally thought that the paper would fall straight down from the observer's perspective. That is incorrect though. The questions are worded badly as my teacher speaks broken English sadly.

    If anyone could help me visualize these, or give some insight I would really appreciate it. I know they are probably simple questions, but I am having trouble visualizing them. Thank you! (This is the first time I've used this website!)
     
  2. jcsd
  3. Sep 28, 2016 #2

    PhanthomJay

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    For the first one, consider all 3 Newton laws and which one applies. For the second one, it is given that the papers fly straight backwards in the drivers perspective .

    Welcome to PF!
     
  4. Sep 28, 2016 #3
    So if the papers fly straight backwards in the drivers perspective, would they also fly straight backwards to an observer outside of the car?
     
  5. Sep 28, 2016 #4

    PhanthomJay

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    Explain why you think this,
     
  6. Sep 28, 2016 #5
    I am standing outside. I see a car wiz by and paper fly out. Hmm. Actually they would be moving forward in my perspective because they are still moving from being in the car!
    Am I Right? I think I just got that one haha
     
  7. Sep 28, 2016 #6

    PhanthomJay

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    But the car was stopped at a light as the driver hit the pedal.
     
  8. Sep 28, 2016 #7

    jbriggs444

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    The papers do not fly out of the car. The problem statement says that from the car point of view they move straight back, not from side to side. Recall that you are not watching a car whiz by. You are watching a car accelerating from a stand-still.
     
  9. Sep 28, 2016 #8
    So If the papers don't fly out of the car, and I am watching from outside the car, to me the papers don't move. Am I correct?
    What is the difference between choice A and D?
     
  10. Sep 28, 2016 #9

    PhanthomJay

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    Yeah. Choice d says they fall straight down. But the problem states that they move straight backwards with respect to the driver. So vertically, they are initially sort of floating (flying)in the air. Ultimately they must fall down from both perspectives due to gravity and land perhaps on the back seat and move with the car, but at the first instant you don't notice that.
     
  11. Sep 28, 2016 #10
    Ok I understand. Thank you!
    Now for the first one, I think I can rule out A + B because regardless, I believe the forces would be equal, but opposite. When I did this question before, I think I selected C however it was wrong. I believe D is the correct answer but am not positive. Any ideas?
     
  12. Sep 29, 2016 #11

    jbriggs444

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    So you are ruling out choices A and B because C is right and ruling out C because C is wrong?
     
  13. Sep 29, 2016 #12
    Well, Honestly I don't know how do figure it out. And I believe that I selected C during class today and it was incorrect. I'm trying to figure out if that's true or not. Sorry I'm a little confused! Haha
     
  14. Sep 29, 2016 #13

    jbriggs444

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    Newton's third law says equal and opposite, right?
     
  15. Sep 29, 2016 #14
    Correct!
     
  16. Sep 29, 2016 #15
    Would you say D is correct? :eek:
    Or is C? Maybe I choose D originally! I can't remember.
     
  17. Sep 29, 2016 #16

    jbriggs444

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    When in doubt, choose the answer you know is right. Not the answer that you think the test giver thinks is right.
     
  18. Sep 29, 2016 #17
    I don't know which is right though sadly. :(
     
  19. Sep 29, 2016 #18

    Ray Vickson

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    Try to think it through, then.

    For an observer standing on the ground, he sees the papers initially at rest. Why would they suddenly fly backwards when the car starts to accelerate? The only forces on the papers are gravity and friction with the dashboard. There is nothing that pushes the papers backward.

    But, to give a plausible answer that is not A, B, C or D: friction between the dashboard and the papers might cause the papers to be dragged slightly forward before finally breaking free of the dashboard and falling down. So, there might be a small---definitely not a "rapid"--forward motion.
     
  20. Sep 30, 2016 #19
    Thank you, honestly I love the explanation. For the second one, I think the answer is A) remain where they were.
    Now the first problem, I am still rather lost. :/
     
  21. Sep 30, 2016 #20

    PhanthomJay

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    but you answered it already in post 14 when you agreed with post 13!
     
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