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!

Projected motion/trajectory question [EASY]

  1. Jan 31, 2017 #1
    1. The problem statement, all variables and given/known data
    The rock 'A' is dropped 100m above a river. After 2 seconds, rock 'B' is thrown down from the same height. What initial velocity must be given to rock 'B' so that both rocks hit the river at the exact same time? [No air resistance]

    2. Relevant equations
    What is the answer?

    3. The attempt at a solution
    http://imgur.com/a/59fkU


    Rock A: v1=0 During a 2s drop it travels 19.6m, -4.9(2)^2+100
    So the equation for rock A is d=-4.9(t)^2+80.4
    When will rock A hit the river? isolate t and I get ~4.0507 seconds

    Rock B: v1=? equation is easy to set up: d=-4.9(t)^2+v1(t)+100 (starts 19.6m behind the dropped rock)
    Now we sub in the time rock A hits the river: (4.9(4.0507)^2-100) * (4.0507)^-1=v1
    This gives me an answer of ~-4.849m/s


     
    Last edited: Jan 31, 2017
  2. jcsd
  3. Jan 31, 2017 #2

    gneill

    User Avatar

    Staff: Mentor

    That's relevant equations, not relevant questions. What common kinematic equations might apply to this problem?

    Your attempt at solution image is barely readable if at all. Please type in your attempt.
     
  4. Jan 31, 2017 #3
    Done
     
  5. Jan 31, 2017 #4

    gneill

    User Avatar

    Staff: Mentor

    I think you forgot the velocity that rock A had attained by the time it reached the 2 second mark when you wrote its equation. I think you'll find that the time remaining for rock A to hit the water from the 2 second point is much less than 4 seconds.
     
  6. Jan 31, 2017 #5
    You're right, thanks. The velocity then turns out to be ~27.38m/s instead of 4.849m/s
     
  7. Jan 31, 2017 #6

    gneill

    User Avatar

    Staff: Mentor

    That certainly looks better! :smile:
     
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: Projected motion/trajectory question [EASY]
  1. Easy Motion Question? (Replies: 4)

Loading...