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Newton's Laws Conceptual Problem

  1. Oct 19, 2014 #1
    1. The problem statement, all variables and given/known data
    "Normally, jet engines push air out the back of the engine, resulting in forward thrust, but commercial aircraft often have thrust reversers that can change the direction of the ejected air, sending it forward."

    "When might thrust reversers be useful in practice? (Chose all that apply)
    a) If the ejected air is directed forward then thrust force is backward. This might be desirable to conserve the speed of the plane.
    b) If the ejected air is directed downward, the thrust force is up. Jets can take off vertically without needing a runway this way.
    c) If the ejected air is directed forward then thrust force is backward. This might be desirable to accelerate the plane.
    d) If the ejected air is directed forward then thrust force is backward. This might be desirable to slow the plane down. "

    3. The attempt at a solution
    A - No. If thrust is moving a plane forward, then it reversing the direction of the thrust should slow it down rather than conserve the speed
    B - Yes. The thrust should allow the jet to rise even though it's not on a runway
    C - Yes. This will cause the plane to change velocity, which'll change acceleration
    D - Yes. If a plan. If thrust is moving the plane forward, then reversing the thrust will cause the plane to slow down.

    My Answer: B,C,D, but the website told me I was wrong and didn't provide me any feedback. What am I doing wrong?
     
  2. jcsd
  3. Oct 19, 2014 #2

    gneill

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    Staff: Mentor

    The problem writer probably wasn't as clever as you when considering C. In some situations the desire might well be to slow the plane's forward speed (a negative acceleration is still an acceleration). The problem writer may have (mistakenly) considered the phrase "desirable to accelerate the plane" to mean only speed it up in the forward direction. This is an error in the problem, not an error on your part. So, it's likely that your "C = True" answer is your culprit. Your other choices look okay to me.
     
  4. Oct 19, 2014 #3

    dlgoff

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    Science Advisor
    Gold Member

    I would agree with "C = True" if it weren't for this (bold by me). "... but commercial aircraft often have thrust ..."

    Actually using it while in flight would probably have similar results as:

    _503780_reverse_thrust5.gif

    http://news.bbc.co.uk/2/hi/science/nature/503780.stm
     
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