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Homework Help: Angular acceleration of an arm

  1. Oct 16, 2013 #1
    1. The problem statement, all variables and given/known data

    Assume that a 1.20-kg ball is thrown solely by the action of the forearm, which rotates about the elbow joint under the action of the triceps muscle, the figure. The ball is accelerated uniformly from rest to 9.5m/s in 0.38s , at which point it is released. Assume that the forearm has a mass of 3.7 kg and rotates like a uniform rod about an axis at its end.

    Part A: Calculate the angular acceleration of the arm.

    Part B:Calculate the force required of the triceps muscle.

    2. Relevant equations
    i believe angular acceleration=Δω/Δt

    3. The attempt at a solution

    really lost on this problem physic is not my strong suit.
  2. jcsd
  3. Oct 16, 2013 #2
    Well, for part A, you pretty much solved it already. Just plug in the 9.5m/s for Δω, and .38s for Δt. For part B, remember that F=ma. You calculuated the angular acceleration, and you are given mass, once again, just plug it all in.
  4. Oct 16, 2013 #3
    Is there any diagram associated with your questions which may contain useful information? This question is not solvable without the length of arm.
  5. Oct 16, 2013 #4
    if you arent solving for torque, why is the length of the arm necessary?
  6. Oct 16, 2013 #5
  7. Oct 16, 2013 #6
    The unit for Δω is rad/s. 9.5m/s should be speed.
  8. Oct 16, 2013 #7
    Torque never come to my mind when i mention the length of the arm.

    since v=rω, it is important to have r in order to solve for ω.
  9. Oct 16, 2013 #8
    For part A, you can start from the equation "angular acceleration=Δω/Δt". (Note the word angular, acceleration is different from angular acceleration)

    By using v=rω, substitute into α =Δω/Δt and you will get the answer.

    For part b, it is already answered by Periapsis, except that you should use acceleration instead of angular acceleration in your "F=ma".
  10. Oct 16, 2013 #9
    so i substitute the equation for α in v=rω?
  11. Oct 16, 2013 #10
    what would r be?
  12. Oct 16, 2013 #11
    Originally, you have α=Δω/Δt. Now use ω=v/r to replace the ω in your original equation.

    r will be the length of arm. What do you think is the length of arm, according to the diagram provided?
  13. Oct 16, 2013 #12
    wouldn't it be 31?
  14. Oct 16, 2013 #13
    or .o31 m?
  15. Oct 16, 2013 #14
    it's neither of them. Since this has to do with unit conversion, I suggest you revise it from your textbook. You will find yourself solving problems with ease once you have strong command of the basics.
  16. Oct 16, 2013 #15
    oh dang my bad it was .31!
  17. Oct 16, 2013 #16
    Now you get it.:smile:
  18. Oct 16, 2013 #17
    so now ill just plug in the acceleration i found and the mass
  19. Oct 16, 2013 #18
    but which mass would i use?
  20. Oct 16, 2013 #19
    F=ma says that an object with mass m will experience acceleration a under force F.

    The acceleration you obtained belong to which object? Arm? Ball?

    Which mass should you use then?
  21. Oct 16, 2013 #20
    the mass of the arm? correct
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