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!

[Mechanics] Solving equations with 2 variables

  1. Nov 8, 2008 #1
    Hello there, I am having a lot of trouble solving a question with which I cannot even begin, I have tried making 2 equations and equation, but I just end up with some number = 0, which is of course wrong, here is the question.


    A stone is dropped from the top of a tower. One second later another stone is thrown vertically downwards from the same point with a velocity of 14ms^-1. If they hit the ground together find the height of the tower.

    I've managed to get that..

    First rock

    s = ?
    u = 0
    v = ?
    a = 9.8
    t = ?

    Second rock

    s = ?
    u = 14
    v = ?
    a = 9.8
    t = ?

    Now where do I go from here? I've tried making 2 equations using the same base equation i.e. v^2 = u^2 + 2as with the different values and then equating them, which doesn't work.

    Any help would be much appreciated, thanks.
  2. jcsd
  3. Nov 8, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    You haven't really shown much. Where did you get the formula v^2+ u^2+ 2as? "Conservation of energy".

    I would just use the dynamics equations: If an object falls under gravity with initial velocity v, then s= -4.9t2+ vt is the distance it has fallen. The first rock has fallen, at time t, a distance h= -4.9t2 and the second rock has has fallen a distance h= -4.9(t-1)2- 14(t-1). Those are negative because it is downward. The second equation has t-1 because the rock was thrown one second after the first rock so the time it has been falling is 1 less. Since the height, h, and the time, t, are the same for those two rocks, you can solve the two equations for h and t. h is what you are asked.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: [Mechanics] Solving equations with 2 variables