Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: KE and forces and comparisons

  1. Oct 18, 2011 #1
    During a throw a shot is initially swung around in a circle reaching a speed of about 3.5 m/s?
    during a throw a shot is initially swung around in a circle reaching a speed of about 3.5 m/s. it is then accelerated more or less in a straight line over a distance of about 1.7 m, leaving the hand at roughly 14 m/s. The shot is 7.26 kg.

    what is the average force exerted on the shot during this latter part of the launch? compare this to the measured peak force of 600 N. Is that reasonable?

    I think you use the equation F=W/d where W=667 J and d=1.7 m
    this gives you 392.4 N.
    Did I do this correct?
    How does this compare to the peak of 600 N and is this reasonable?
  2. jcsd
  3. Oct 18, 2011 #2


    User Avatar
    Homework Helper

    Why do you think W = 667 J? Not saying you are wrong, just wanting to see how you calculated that.
  4. Oct 18, 2011 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Welcome to PH.

    That all looks reasonable.
  5. Oct 19, 2011 #4
    I found the KE in the first part by KE=1/2mV2
    this gives you 44.4675 J.

    Then i found the KE of part 2 using the same equation which is 711.48 J.

    Then I found ΔKE=KEf-KEi which gives you 667 J.
  6. Oct 19, 2011 #5


    User Avatar
    Homework Helper

    That's fine, just wanted to check you were calculating correctly.

    I think the question is drawing yo to the idea that the thrower [putter] will not supply a constant force, but one which increases from zero to a peak, then drops back to zero. That will partly be affected by the angle of your arm, and ability to apply force with your elbow bent by various amounts.
    If the force increased uniformly, then reduced uniformly, the peak force would be double the average. How does that compare here?
  7. Oct 20, 2011 #6
    This is almost 200 N more than the peak of 600 N, so this answer is not reasonable. Does that make sense?
  8. Oct 20, 2011 #7


    User Avatar
    Homework Helper

    Not really, a human may be able to increase the force quickly and maintain his peak effort for a time before dropping off. I am not an exercise physiologist, so I don't really know how the human body can exert a force.
    I do know that if the object you are throwing gains sufficient speed I can no longer apply a force, since it is already moving as fast as I can move my hand.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook