1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Dynamics: Impulse and Linear momentum (impulsive force exerted on catchers glove)

  1. Feb 29, 2012 #1
    1. The problem statement, all variables and given/known data

    A baseball player catching a ball can soften the impact by pulling
    his hand back. Assuming that a 5-oz ball reaches his glove at
    90 mi/h and that the player pulls his hand back during the impact
    at an average speed of 30 ft/s over a distance of 6 in., bringing the
    ball to a stop, determine the average impulsive force exerted on
    the player’s hand.

    2. Relevant equations

    mv1 + Imp1→2 = mv2
    v2 = v02 + 2a(x - x0
    x = x0 + v0t + 1/2at2

    3. The attempt at a solution

    (o ft/s)2 = (132 ft/s)2 + 2a(6/12 ft)
    a = -17421 ft/s2

    (6/12 ft) = 0 ft + 132 ft/st + 1/2(-17424 ft/s2)t2
    t = 0.007575s

    0.3125 lbs (132 ft/s) + F(0.007575s) = 0.3125 lbs (0 ft/s)
    F = 54446 lbs

    The answer is wayyyy less as it should be..
    76.9 lbs

    I just can't figure out how to get there..
     
  2. jcsd
  3. Feb 29, 2012 #2

    NascentOxygen

    User Avatar

    Staff: Mentor

    This is a quadratic; not correctly solved.
     
  4. Feb 29, 2012 #3
    oh sorry

    0 = -8712 ft/s2t2 + (135 ft/s)t + 0.5 ft

    t = [-(135 ft/s) + sqrt((135 ft/s)2) - (4*(-8712 ft/s2)*(0.5 ft))]/(2*(-8712 ft/s2))

    t = -0.003088 s


    t = [-(135 ft/s) - sqrt((135 ft/s)2) - (4*(-8712 ft/s2)*(0.5 ft))]/(2*(-8712 ft/s2))

    t = 0.0185841 s

    mv1 + Imp1→2 = mv2
    0.3125 lbs (132 ft/s) + F(0.0185841 s) = 0.3125 lbs (0 ft/s)
    F = 2219.639
    F = 2220 lbs

    I'm still getting the wrong answer.
     
  5. Feb 29, 2012 #4

    NascentOxygen

    User Avatar

    Staff: Mentor

    I believe one of these terms has the wrong sign.

    It is always helpful, when you know the correct answer, to provide it so others know their working is right.
     
  6. Mar 1, 2012 #5
    Oh I put = + 0.5 ft.. in my calculator I did -0.5 ft

    the answer for t is still the same.

    I'm obviously missing something in my approach..

     
  7. Mar 1, 2012 #6

    NascentOxygen

    User Avatar

    Staff: Mentor

    I've almost forgotten how to work in imperial measures.
    Isn't there a factor of 32.2 because F is in poundals https://www.physicsforums.com/images/icons/icon5.gif [Broken]
     
    Last edited by a moderator: May 5, 2017
  8. Mar 1, 2012 #7
    Ah of course... I'm so stupid. Yeah there is, mass in English units is in slugs not lbs. I should be dividing .3125 lbs by 32.2 ft/s2

    I'm closer... but still off by about 6.4 lbs.. I must be forgetting something..

    mv1 + Imp1→2 = mv2
    (0.3125 lbs/(32.2 ft/s2)) (132 ft/s) + F(0.0185841 s) = 0.3125 lbs (0 ft/s)
    F = 70.499 lbs
    F = 70.5 lbs

    The answer is 76.9

    This is frustrating, I feel like an idiot.
     
  9. Mar 1, 2012 #8
    I just realized in the second time I did the quadratic formula I was using 135 ft/s instead of 132 ft/s.

    Also used that for the linear momentum.. I'm redoing everything right now.
     
  10. Mar 1, 2012 #9

    NascentOxygen

    User Avatar

    Staff: Mentor

    Yes, I noticed your 135 mistake. Still doesn't quite get me to 76 though. I used 0.0183 sec
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Dynamics: Impulse and Linear momentum (impulsive force exerted on catchers glove)
Loading...