1. Limited time only! Sign up for a free 30min personal 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!

Centripetal Acceleration check my reasoning

  1. Sep 13, 2009 #1
    1. The problem statement, all variables and given/known data

    ok well then...

    we did a lab were you have a hanging mass attached to a stirng that went through a straw and was attached to a rubber stopper. The lab was to find the mass of the rubber stopper once you know the velocity. So I was woundering if this looks correct

    2. Relevant equations

    f = ma
    m1 = mass of stopper
    m2 = mass of hanging mass

    radial acceleration = r^-1 v^2

    3. The attempt at a solution

    net force radial = m (acceleration radial) = force of gravity on mass m2
    net force radial = (m1 + m2) (acceleration radial) = m2 g
    net force radial = m1(acceleration radial) + m2 (acceleration radial) = m2 g
    net force radial = m1(acceleration radial) = m2 g - m2 (acceleration radial)

    divided both sides by acceleration radial

    m1 = (acceleration radial)^-1 ( m2 g - m2 (acceleration radial) )

    at which point I plugged in the value I found for the radius
    the value I found for the period
    and found the radial acceleration
     
  2. jcsd
  3. Sep 13, 2009 #2
    ok hmmm

    m2 g = m1 (acceleration radial)

    so then if this is the correct equation my question... is how do you know when to consider the masses as a system and when not to?

    net force = mass times acceleration

    by deffintion
    it dosen\'t mean this necessarily

    net force = net mass times acceleration

    when do I know when I should use the net mass and when not to as I have gotten points taken off before because i didn\'t use the net mass and am not sure when to consider the masses as one system and when not to...

    like how come this is wrong

    net force radial = (m1 + m2) (acceleration radial) = force of gravity on m2

    and this is right

    net force radial = m1 (acceleration radial) = force of gravity on m2

    I need help on when to make this distinction between the masses and when not to as I don\'t know when i should...

    THANKS!!!
    This has been eating me away and I need to know this or the whole problem is wrong so it\'s important that I know when to make this distinction and when not to. So can you tell me when I should and when I shouldn\'t???
     
  4. Sep 13, 2009 #3
    M1g = Msv^2/r

    M1 was your known mass hanging. g is the acceleration due to gravity. Ms is the mass of your rubber stopper. v is the speed of your rubber stopper and r was the length of the string from the straw to the rubber stopper (the radius of the circle the rubber stopper was making around the straw)

    I guess you counted how many times it took to make a certain number of revolutions in a certain amount of time. 1 rev. =2 x pi x r. Divide the number of revolutions by the time in seconds it took to make those revolutions and you got v. At this point you know everything but Ms so put all the numbers in and solve for Ms
     
  5. Sep 14, 2009 #4
    can you answer my question in #3
     
  6. Sep 14, 2009 #5
    Could you please give me your data:

    Mass of the object hanging from the string.
    radius of the circle the stopper was making while holding the mass at rest below the straw.
    How many times the stopper went around and in what amount of time.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook