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!

What did I do wrong? 2 tests questions ( work and force questions )

  1. Nov 26, 2013 #1
    1. The problem statement, all variables and given/known data
    1 question: How much work must be done to launch a 100-kg object to a height of 2 x 10^3 kilometer above the Earth's surface?

    2.A person with the mass of 10 kg jumps up and down on her mattress with a spring constant of 4.3 nK.m. She does this by applying a sinusoidal force at a frequency of 1 Hz. Assuming no dampening force exists, if her amplitude is 3 cm, what is the maximum force she places on the mattress?

    by the way, these are the only one I completely missed. He took off the maximum amount : ( And I still have no idea how to solve them. So I'm just wondering.

    3. The attempt at a solution
    1 question ( my attempt ) - okay, first I thought I would use the Vesc equation, but I simply got velocity out of it and I didn't know how to go from velocity to work. Then I had the idea, I would simply use the fact work is independent of the path and work = the change in potential energy
    so mgh = work thus, 100kg x 9.8m/s^2 x 2x10^6 mThus, I got 1.96 x 10^9 joules. Now, I knew this was too simply to be right, but I am still confused why this is the case. Why can't I use the fact work equals the change in potential energy in this case?

    2 question: I used the equation A = (Fsmall o / m) /( w^2 - wsmall o ^2) and solved for w^2. then used the fact the max acceleration = Aw^2. Then to get the force. I simply used F=ma. and got 13N
    So, why is this wrong? I'm pretty sure, I converted to all the correct units. But, again, I thought this was too easy. Did I use an equation that wasn't applicable?

    Thanks in advance!
     
  2. jcsd
  3. Nov 26, 2013 #2
    P.E.=mgΔh is only valid for the approximation that $$Δh<<R_{earth}$$
    You need to use ##ΔU=-Gm1m2/R^2##
     
  4. Nov 26, 2013 #3

    rude man

    User Avatar
    Homework Helper
    Gold Member



    What kind of spring constant is that? Do you mean 4.3 N/m??
     
  5. Nov 26, 2013 #4
    If the amplitude is 3 cm, and the frequency is 1 Hz, then represent the displacement at time t by:

    x = 3 sin(2πt) (cm)
    With this representation of the displacement, what is the velocity v in cm/sec?
    What is the acceleration a in cm/sec2?
     
  6. Nov 28, 2013 #5
    Yup. sorry! kN/m kilonewtons per meter
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: What did I do wrong? 2 tests questions ( work and force questions )
  1. What did I do Wrong ? (Replies: 10)

  2. What did I do wrong? (Replies: 4)

  3. What did I do wrong? (Replies: 6)

Loading...