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: Model Rocket question regarding height,time,acceleration

  1. Sep 1, 2008 #1
    A model rocket rises with constant acceleration to a height of 4.0 m, at which point its speed is 25.0 m/s.
    How long does it take for the rocket to reach this height? Ans: t= 0.32s
    What was the magnitude of the rockets acceleration? Ans a= 78 m/s^2

    "Find the height of the rocket 0.20 seconds after launch."
    I cant seem to get it right, I know its at least close to 1.5, because that was my original answer but the darned online system says "Not quite. Check through your calculations; you may have made a rounding error or used the wrong number of significant figures." the answer is supposed to be to two significant figures.

    "Find the speed of the rocket 0.20 seconds after launch."
    I havent attempted this part of the question yet because I think I need to use what I figure out from the previous question to work on this.

    All this stuff is not hard, but I cant seem to grasp the simple concepts that Im missing in figuring it out, any help and/or guidance would be really appreciated.
  2. jcsd
  3. Sep 2, 2008 #2


    User Avatar
    Homework Helper

    h = 1/2*a*t^2. Put the values of a and t and see how much you get.
  4. Sep 2, 2008 #3


    User Avatar
    Homework Helper

    The thing to keep in mind is what forces (hence accelerations) are acting on the object at different points in time. Since they are asking you for the V and x at a time during which the object was undergoing uniform acceleration starting from rest (V=0) then the equation for finding distance x (or h), is going to be simply
    x=1/2 a t2 because the other terms will be 0 because it started at 0 from rest.
    Likewise for finding speed you know starting from rest that V = a t

    Basically you can use the formulas that relate x, V and t according to what you know and what you need to figure out. Here is a link to various kinematic equations that you might find useful.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook