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!

Two objects in free fall

  1. Mar 30, 2016 #1
    1. The problem statement, all variables and given/known data

    A stone is dropped from rest from the roof of a 100-m high building; 2.00 s after that, a second stone is thrown straight down with initial speed v0, and the second stone passes the first stone 21.6 m above ground. (a) When are the stones at the same height? (b) What was the initial speed, v0, of the second stone, in m/s? (c) What are the speeds of the two stones, in m/s, just before they hit the ground?

    h= 100m
    h(meet)= 26.1m
    V1i= 0m/s
    2. Relevant equations
    Xf=Xi +Vt +0.5at^2

    3. The attempt at a solution
    (a). Xf1= -4.9t^2
    Xf2 = Vo(t-2) -4.9(t-2)^2
    -73.1 = -4.9t^2

    (b). -73.1 = Vo(1.86) -4.9(1.86)^2
    Vo = 30.19 m/s

    (c). Vf = Vi + at
    Vf = 0 -9.8(3.86)
    Vf = 37.83 m/s

    Vf= 30.19 -9.8(1.86)
    Vf= 48.42m/s

    Am I right? Not sure if I am
     
  2. jcsd
  3. Mar 30, 2016 #2

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    You don't give the time, although the next entry seems to have used the correct time.

    You calculated the speed of the both stones at 21.6 m above the ground.
     
    Last edited: Mar 30, 2016
  4. Mar 30, 2016 #3
    I could be wrong, but I found slightly different values. For time, t = 4s, and initial speed V0 = 29,4. Are you sure about your results?
     
  5. Mar 30, 2016 #4

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    I think you're right.

    I didn't even think to check the distances OP came up with.

    100 - 21.6 = 78.4, not 73.1

    If it's supposed to be 26.1,
    then 100 - 26.1 = 73.9, not 73.1
     
  6. Mar 30, 2016 #5

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Something isn't adding up here:
     
  7. Mar 30, 2016 #6
    @Ab17 double check your computation in (a).
     
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: Two objects in free fall
  1. Object in free fall (Replies: 5)

  2. Free fall two objects (Replies: 3)

Loading...