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Homework Help: Free Body Diagrams help

  1. Mar 23, 2005 #1
    Free Body Diagrams...help plz!!!

    hello hello,
    i'm just having a bit of difficulty concerning free body diagrams:

    how would i draw an FBD for the following...
    a) an object has reached terminal velocity. would this mean that the object has reached a constant velocity, therefore force gravity and air resistance are the same?

    b)an object is at the top of a parabolic trajectory. once again, does that mean that the object is simply at a constant velocity and should be drawn with both Fg and Fair resistance as being even. OR...is the object still following and therefore only draw Fg (force gravity)?

    c)a ball on a ramp is block from slidding down the ramp. i have the Fn and Fg of the FBD, but i'm just wondering if friction would come in hand in this diagram due to the block.

    Assistance greatly needed! :D
    thank you!
    Last edited: Mar 23, 2005
  2. jcsd
  3. Mar 23, 2005 #2
    a) yes, the object would have two opposing vectors, one for gravity and one for air resistance. the resulting vector is a constant velocity vector downwards.

    b) when you throw a tennis ball straight up in the air, wha happens at the top of the trajectory? think about what defines the top of the trajectory

    c) I think you mean its being blocked, like somethings holding it. In which case you have your gravity vector, and two normal forces holding the ball up. Theres two because the normal force from the ramp isnt equal to the force of gravity, the block provides another normal force to completely neutralize the force of gravity.
  4. Mar 23, 2005 #3

    Last edited: Mar 23, 2005
  5. Mar 23, 2005 #4
    Usually you neglect air resistance unless told to include it, but that force diagram is correct once the ball starts falling. At the peak of the trajectory, there is no motion, so no air resistance.
  6. Mar 23, 2005 #5

    so you're saying that the FBD of this should only contain Fg, therefore:

    \/ Fg


    so then, if an object was to rise in a parabolic trajectory, it would also only have Fg and no applied force or air resistance???

    ps) thanx for your help! :smile:
  7. Mar 23, 2005 #6
    In simple freefall, anything takes a parabolic trajectory. The only force acting in freefall is gravity. Gravity is also the only force acting when something is thrown upwards.

    The tennis ball example: When you throw a ball upwards, as soon as the ball leaves your hands, the only force acting on it is gravity, until it hits the ground.

    Air resistance is a much more copmlicated function which depends on the speed of the object, and is usually neglected for the purpose of teaching basic kinematics
  8. Mar 23, 2005 #7
    alright, i think i know where you're getting at. there's only one thing that's stumping me. if gravity is the only force acting upon an object which leaves your hands once thrown into the air, another force has to overcome the force of gravity to go up. so wouldn't that be applied force placed on the object in order to project it?
    Last edited: Mar 23, 2005
  9. Mar 23, 2005 #8
    A force creates a momentum in the direction. When you apply the force, you build up momentum on your hands, once you let go that momentum is no longer increasing. As soon as you let go of the ball, at no point is it speeding up. Force causes a change in momentum. The force of gravity is decreasing the momentum of the ball (momentum is mass times velocity).
  10. Mar 23, 2005 #9
    AH!! I see....
    so if i'm getting this, and please tell me i am, no matter where an object is once it's projected (rising or falling) it was be at a constant velocity and the only force acting upon it is gravity.
    thus, the free body diagram of this situation would simply be:

    \/ Fg

    ...throughout the entire time it is in the air...?
  11. Mar 24, 2005 #10
    Your free body diagram is correct, The object isnt at constant velocity though, because it is being accelerated by gravity. The net force on the object is gravity. If there is no net force, or the net force = 0, then the velocity is constant.

    Im sorry I can't explain this better to you.
  12. Mar 24, 2005 #11
    Yes, in free fall the only force is downward pointing force of Gravity. Whether it drops, is thrown upwards, follows a 2D parabola...

    Regarding your question about an object sliding down a ramp, there are usually three forces: downward pointing Gravitational force, force of Friction that is opposite to the direction of motion, and the Normal force that is perpendicular to the ramp surface.
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