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Homework Help: Newton's Laws- 2 Questions- Help Please

  1. Mar 17, 2005 #1
    Newton's Laws-- 2 Questions-- Help Please

    Hey everyone
    I have two questions and any help on them would be greatly appreciated.
    Thanks a lot in advance.
    Problem #1
    A person steps onto a dock off a boat that is tied securely and close to the dock. In another case, a person steps off a boat not attached to the dock. Use Newton's laws to explain what happens in each case.

    Problem #2 (Attached)

    Attached Files:

  2. jcsd
  3. Mar 17, 2005 #2
    Try useing Newtons third Law with question 1.

    PS. There seam to be some problems with you attachment.
  4. Mar 17, 2005 #3
    What's wrong with the attachment? I see it fine.

    The problem with question #1 is that there are so many situations where you can apply Newton's laws that it gets a bit confusing and I'm having difficulty finding a certain focus.

    For Problem #2 what I can think of is that what is drawn is an internal force whereas, according to Newton's first law, an unbalanced, external net force is needed inorder for the boat to move. Am I correct?
    I need more hints & help. Thanks
  5. Mar 18, 2005 #4
    For Problem # 1, for the case that the boat is not tied to the dock, the person may end up landing in water since the jumping force of the person on the boat will push the boat backwards; the equal and opposite force of the boat (according the Newton's 3rd law) on the human will propel that person toward the dock, but since the boat moves backwards, the person can fall into the water.

    As for Problem # 2, the reason the boat doesnt move is because the fan blows against the sail. The boat does not move forward because there is an equal and opposite force on the fan (Newton's 3rd law).

    Any help?
  6. Mar 18, 2005 #5
    You just answered your own question

    usualy when they use the person on the boat analogy, they are just trying to find a virtualy friction proof surface, but the person will most likely not land on the dock, though it is possible, the force needed is tremendous. because say you push with 50N the boat will only push back with merely a fraction of that on you

    also for the other one, the fan would be needed to face the other direction, or put it in the water, and use it as a propellor
  7. Mar 20, 2005 #6
    Thanks for the reply. But I still don't have clear understanding on why the boat doesn't move in the first place (for Problem #2).
  8. Mar 20, 2005 #7
    And can anyone please think of a third modification? I can't get anything. Any help would be appreciated :confused:
  9. Mar 20, 2005 #8


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    You're correct that internal forces cancel. In this case, the jet of air blowing against the sail tries to move the boat to the left. But, as someone pointed out, just having a propeller like that is enough to propel the boat, and pointing it to the left will make the boat want to go to the right. I can't quite see the "action-reaction" pair here...it has to do with why propellers shooting air backwards allow a craft to move forward in the first place. My guess is that the propeller pushes left on the air, and so the air pushes right on the propeller, making the boat want to move right. But then the jet of air encounters the sail and pushes left on it, making the boat want to go left. The two forces (air on fan, air on sail) are equal and opposite, and so they cancel. The boat goes nowhere. What you do you guys think of my analysis?
  10. Mar 21, 2005 #9
    to shorten your point - the fan pushes, say +5 on the sail, whereas, in order to keep the fan on the boat, the boat pushes -5 on the sail - thus [tex]5 + -5 = 0[/tex]
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