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Very confusing and very basic

  1. Feb 5, 2007 #1
    i just want to know what does these problems mean exactly:
    first:
    an experimental vehicle slows down and comes to a halt with an acceleration whose manitude is 9.80m/s². after reversing direction in a negligible amount of time, the vehicle speeds up with an acceleration of 9.80 m/s². other than being horizontal.
    what doesn the red sentence mean
    second:
    two objects are thrown vertically upward , first one, a bit later, the other.is it possible that both reach the same maximum height at the same instant?
    q: do we consider that there is the same intial velocity or different initial velocity?
     
  2. jcsd
  3. Feb 5, 2007 #2
    Uhh, I guess you have a flying car? If there is non horizontal acceleration, then then the car must not be held on the plane. I think that must be a typo or mistake.

    For the second one, you aren't supposed to assume anything because you are supposed to think of the type of problem that might let this happen. You could do this logically or mathematically.
     
  4. Feb 6, 2007 #3

    andrevdh

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    1. Neither do I know, but if the acceleration of the vehicle is in the return direction throughout the motion it will explain its behaviour. This is exactly the behaviour we get when throwing an object vertically upwards.

    2. It is definitely not possible if they have the same initial velocity, so yes, one need to consider them to be different.
     
  5. Feb 6, 2007 #4
    thanks but could you please try to explain to me the first exercise
     
  6. Feb 7, 2007 #5

    andrevdh

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    I get the impression that you got your hands on a British handbook. OXFORD explains the term "other than" - except - or -different or different from - which means it is not horizontal, and therefore vertical as I expected, since this is exactly how an object that is thrown upwards behave. Even the acceleration is that of the accepted gravitational acceleration.

    Throughtout the motion of an object thrown vertically upwards we find that its acceleration is downwards. As a result of this we find that on its way upwards it decelerates. When it reaches the top it momentarily stops and then continue accelerating downwards.
     
  7. Feb 7, 2007 #6
    I would phrase that last part a little differently because there is no continue to accelerate, as if it stopped accelerating when it stopped. The object will always be accelerating from the force of gravity, it's the velocity/position that have the turning points.

    I still have no idea what the first part is talking about. Actually, for the second part, I am pretty sure it would be possible to throw an object up with another one some time after, and still have them reach the max height at the same time, but only with nonconservative forces (drag)! More over, the drag would probably have to be nonlinear, such as a quadratic drag or have a non-constant coefficient for drag.
     
  8. Feb 8, 2007 #7

    andrevdh

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    For the second one to catch up with the first one it would definitely need to be thrown up with a larger initial speed. Since we have energy conservation it means that their maximum heights will therefore differ.
     
  9. Feb 8, 2007 #8
    Yes, that is exactly my point. If there is energy conservation then the scenario will definitely not happen, but nonconservative forces change this entirely. For example, a golf ball with top spin creates lift that would keep it at some max height for longer (and could even give it a new max height), so an object thrown later could easily reach the same max at the same time.
     
  10. Feb 9, 2007 #9

    andrevdh

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    The spin would need to be phenomenal to overcome the object's weight, but I agree it is a possibility, especially since they do not mention by how much later the second one is thrown.
     
  11. Feb 9, 2007 #10
    This means the vehicle is traveling perpendicular to horizontal and away from the earth, the acceleration making it come to a halt is the acceleration of gravity, it then reverses and falls to earth at that rate until terminal velocity is reached. the words in red are just giving you a clue in case you dont know the value of the acceleration of gravity, I guess.

    Definately not! If the second one is thrown up at a faster rate to catch up with the first one, it will take longer for gravity to oppose the extra force, thus it will reach a greater height. The answer is NO.

    Either way, you wont get them to reach the same height at the same time.
     
    Last edited: Feb 9, 2007
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