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The motion of the mass starting from its highest point t=1.3

  1. Jan 3, 2016 #1
    1. The problem statement, all variables and given/known data
    (a) A large spring has one end attached to the floor and a platform attached to its upper end. On the platform is placed a mass of 30 kg. The mass of the platform itself is 10kg. The spring-platform-mass system is then set into vertical oscillations with a period of 1.5 s. Determine

    (i) the stiffness (spring constant) of the spring
    (ii) the amplitude of the motion if the maximum speed of the mass is 1 ms-1.

    (b) (i) If the motion of the mass (in part (a)) is timed starting from its highest point, calculate how far it has travelled in 1.3 s

    2. Relevant equations
    x= A cos (wt)

    3. The attempt at a solution
    The answer must be x= 0.88m

    I've solved part a) already
    k= 705.6 N/m
    w= 4.2 rad/s
    A= 0.24 m

    but I'm struggling in part b)
    Can someone help me using the formula
    x= A cos (wt)

    PS: This is a past exam paper, I'm using it to study and practice for my exam.
  2. jcsd
  3. Jan 3, 2016 #2


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    Did you think about plugging ##t = 1.3s## into that equation?
  4. Jan 3, 2016 #3
    yes. I get a different answer.
  5. Jan 3, 2016 #4


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    Try plugging in ##t = 0s## and ##t=1.5s## and see if that gives you any ideas.
  6. Jan 6, 2016 #5


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    What is x at t=0? And what is x at t=1.3 s? What is the displacement during that time?
    By the way, have you set the calculator to radians?
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