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!

Homework Help: Designing a toy rocket to be launched by a spring

  1. Apr 9, 2005 #1
    You're designing a toy rocket to be launched by a spring. The launching apparatus has room for a spring that can be compressed 14 cm, and the rocket's mass is 65 g. If the rocket is to reach 35 m altitude, what should be the spring constant?

    [tex]U_o = 1/2*kx^2[/tex]
    [tex]K_o = 0[/tex]
    well work is being stored so K = 0 and U has the spring energy right?

    [tex]U_f = mgh[/tex]
    [tex]K_f = 0[/tex]

    U_f = mgh because of the gravitational potention energy right? and K_f = 0 because it's at rest right when it reaches 35m? I'm not sure if K_f = 0 or not because i dont really know if the rocket is at it's max height when it reaches 35m.

    well, setting them equal to each other...
    [tex]1/2*kx^2 = mgh[/tex] and solving for k, i get 3.1 but the book gets 2.3 kN/M. what am i doing wrong?
  2. jcsd
  3. Apr 9, 2005 #2


    User Avatar
    Science Advisor

    Your work is correct except for UNITS. Recalculate ALL quantities in MKS (meter, kg, sec) units, and you'll obtain the book answer.

    Last edited: Apr 9, 2005
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook