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Free fall acceleration of rocket

  1. Sep 13, 2009 #1
    can someone please help me?!?!

    I thought I had the answer to this problem, but it doesn't match the book's answer. I can't figure out what I'm doing wrong. This is the problem:

    A rocket is launched straight up with constant acceleration. 4 seconds after liftoff, a bolt falls off the rocket and hits the ground 6 seconds later. What was the rocket's acceleration?

    First, I used the free fall acceleration of the bolt, -9.80m/s2 and 6 seconds that if fell for to find its position when it fell off the rocket, which I found to be 176.4m. This would also be the height of the rocket when the bolt fell of. Since the rocket travelled for 4 sec to reach this height, I used this height and time to find the acceleration of the rocket, 22.05m/s2.
    The book answer however is 5.5 m/s2.
    What am I doing wrong?
     
  2. jcsd
  3. Sep 13, 2009 #2
    Re: can someone please help me?!?!

    Hello rich.You took the initial velocity of the bolt to be zero.It is not zero it is the upward velocity that the rocket has at the instant the bolt drops.
     
  4. Sep 13, 2009 #3
    Re: can someone please help me?!?!

    then how do i find the initial velocity?
     
  5. Sep 13, 2009 #4
    Re: can someone please help me?!?!

    You can write it in symbol form.
     
  6. Sep 13, 2009 #5
    Re: can someone please help me?!?!

    i can but symbols won't due me any good. i need a definite numerical answer to the question. i know i can write a bunch of equations but i have no numbers to put into them
     
  7. Sep 13, 2009 #6
    Re: can someone please help me?!?!

    Consider the rocket having risen for 4 seconds
    u=0, t=4, a=a(unknown)
    you can write v=4a and s=8a(equations of motion)
    Now consider the bolt
    u=4a, a=9.8,t=6, s=8a(this is the displacement not the distance travelled because the bolt rises a bit before it comes to momentary rest and then falls)
    Using the values above write down the equation connecting s,u,a(9.8 and not to be confused with a for the rocket)) and t,plug in the numbers and solve.Make sure you get the minus sign in front of ut ie the displacement and acceleration are down whereas u is up.Good luck with it.
     
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