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Homework Help: Free Body Diagram questions?

  1. Feb 27, 2015 #1
    See attached photographs.

    For the car problem, I think the drag should be smaller than the forward acceleration but I'm not sure.


    For the Rocket question, I am pretty sure I am correct but I'm not sure why. The drag and the weight would both be in the down position. free%20body%20diagram_zpsitlj6aot.png
  2. jcsd
  3. Feb 27, 2015 #2
    For the car problem, what is the drag on an object at rest?

    For the rocket, check to see if the sum of all your vectors makes sense, if they were all placed head to tail.
  4. Feb 27, 2015 #3
    Thanks for your reply. Would the drag start from rest? Therefore making the drag larger than the acceleration since it starts upon movement from rest?

    And for the rocket question, do you mean they have to be equal lengths? I only have one more chance on that one so I want to be sure since I already wasted tries.
  5. Feb 27, 2015 #4
    Drag is velocity dependent, not acceleration dependent. Think about what is it that physically causes drag on a car.
    Also, remember that the way the car will ultimately move is going to be the direction of the net vector. So always check that it makes sense. If you're saying that your largest horizontal component of force is drag then you are telling me that the car will be pushed backwards (from rest) due to drag. Does that make sense?

    For the rocket, just do the vector addition and you will understand. If the rocket is ultimately supposed to move upward then what do you expect the net force vector should look like?
    Does that expectation match what you get when you add all 3 of your vectors?
  6. Feb 27, 2015 #5
    I see what you mean. So forward would indeed be larger since that is the direction it is bearing, correct?

    For the vector addition, are you are saying that I should add the lengths of the weight vector and drag to get the total size of the thrust vector?
  7. Feb 27, 2015 #6
    That's right, the forward acceleration of the car would have to be greater. However, you're still missing something. What would cause drag on a car at rest?
    If a rock is at rest on the ground does it have drag? Does a car parked in a garage have drag? Why would that answer change if the car experiences instantaneous acceleration, but still has zero velocity?

    For the rocket I'm not saying add the 2 downward vectors, I'm saying add all 3 vectors.
    If you don't know how to graphically represent vector addition please look it up.
  8. Feb 28, 2015 #7
    Thank you for your help, and I figured out how to do the rocket one.

    The sum of the weight and drag vectors have to be less than the length of the thrust vector; the trust vector (biggest vector) points up and the other two vectors point in the downward direction.
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