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Homework Help: Trebuchet Analysis Questions

  1. Mar 11, 2013 #1
    I have a few questions regarding concepts of a trebuchet. I have figured out 3/4 of these questions, and I am looking to ask what you guys think the answers are, or if you can explain the concepts because I do not know if I have even answered the questions right. And the trebuchet is a simple fulcrum with a fixed mass at one end, and a sling at the other end.

    No equations needed, its concepts. But I guess we can use:

    Torque=Moment of Inertia x Angular Acceleration
    Torque= F x r
    N=mg + Component of Throwing Arm Force
    I= mr^2

    Questions and my Attempts:
    a. How does the normal force of the floor on the Trebuchet change during a throw?

    When the counterweight is added and allowed to pull the short arm down towards the base of the trebuchet, the Normal force on the trebuchet increases because of the downward component of the swing force. Because there is more force being applied onto the floor, there has to be more normal force as it is the opposite. When the counterweight with the shorter arm begins to travel upwards to finish the swing, normal force decreases as force is being applied upwards, thus negating part of the force of gravity, which decreases normal force of the floor on the trebuchet.

    b. How and why does the torque on the throwing arm change during a throw?

    Since Torque= Inertia x angular acceleration, as the moment of Inertia remains the same and the angular acceleration remains constant until the max angular velocity. This means the torque on the throwing arm is constant until max angular velocity is achieved, which is before the countermass swings upwards. Then torque becomes 0 as max velocity is achieved at the lowest mass point (release point of projectile), and then increases in the opposite direction as the force of gravity pulls the counterweight downwards and creates acceleration until the trebuchet is stationary at the vertical position.

    c. Is the horizontal momentum of the trebuchet-projectile system conserved throughout the process of rotation and after the throw, before the projectile lands?

    No, this system does not conserve momentum due to friction of the system. Since it is not frictionless, our system loses velocity, therefore, momentum decreases. So the system does not contain the same momentum after the throw and during the throw as it did from the beginning. The projectile also loses momentum from air resistance, it loses velocity. So no, horizontal momentum is not conserved.

    d. Explain why the two methods of calculating net torque might not be in agreement.

    No clue :P
    Last edited: Mar 12, 2013
  2. jcsd
  3. Mar 12, 2013 #2


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    Gold Member

    The way this forum works is that you post your attempts/solutions first, preferably quoting any equations you deem relevant. Pls define your variables.
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