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Newton's Three Law's on Cars, Bicycles, Objects

  1. Oct 28, 2011 #1
    The problem statement, all variables and given/known data

    Indicate whether the following statements are always true or can be false.

    A) If two objects are under the influence of equal forces, they have the same acceleration.
    B) In order not to slow down, a motorcycle moving at a constant velocity needs a small net force applied.
    C) The net force which acts on an object which maintains a constant velocity is zero.
    D) If a net force acts on an object, the object's speed will change.
    E) During the collision of a large truck with a car, the car exerts an equal size force on the truck as the truck exerts on the car.
    F) If a net force acts on an object, the object's velocity will change.

    The attempt at a solution

    My thought process on this has been:

    A) F - The equation for force is F=ma. If two objects are under the influence of equal forces, they are still subject to mass. Meaning the acceleration is not ALWAYS the same.

    B) F - If something is going a constant velocity, the forces are in balance. It is in motion, overcoming any force of friction. Therefore, it should not need an applied force (as it would create acceleration)

    C) T - For something to have a constant velocity (be it moving or at rest) all the forces have to be in balance, making ƩFnet = 0

    D) F - If the forces are in balance, they're still acting on the object, but the speed of the object isn't changing.

    E) T - For every action, there is an equal and opposite reaction.

    F) F - Net force acting on an object could be zero, meaning the velocity will no change.

    Note: the above is showing as incorrect. There are flaws in my reasoning and I don't know what they are. I need help.

    My previous answers have been as follows:

    1 Incorrect. (Try 1) FFTTFT
    2 Incorrect. (Try 2) FTTTFT
    3 Incorrect. (Try 3) FFTTTT
    4 Incorrect. (Try 4) FFTTTF
    5 Incorrect. (Try 5) fffttf
    6 Incorrect. (Try 6) fftftf
    7 Incorrect. (Try 7) fttftf
    8 Incorrect. (Try 8) FFTFTF
    9 Incorrect. (Try 8) FTTFTF
    I only have a few tries left and this is the only question keeping me from completing my assignment. Any and all help would be extremely welcome.
    Last edited: Oct 28, 2011
  2. jcsd
  3. Oct 28, 2011 #2
    I believe F is true.

    Net force on an object implies the forces are imbalanced, in my eyes.

    The reason F would then be true and D is still false is that an object may be moving in uniform circular motion, so the magnitude of its velocity (speed) remains constant while direction changes.

    The rest of your reasoning seems right.

    So the new guess would be FFTFTT.
  4. Oct 28, 2011 #3
    Yes! It's correct! I got it! Oh, thank you so, so much!
  5. Oct 28, 2011 #4
    Whenever a series of questions asks about speed, then velocity under the same conditions, always be wary. Those can get tricky.
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