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Finding velocity and displacement with gravity

  1. Jul 14, 2009 #1
    need some help figuring these out!


    A stone is dropped from a bridge and falls freely under the influence of gravity.
    a) Calculate its velocity after 1.2s

    b) Calculate is displacement after 1.2s
     
  2. jcsd
  3. Jul 14, 2009 #2

    LowlyPion

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    Welcome to PF.

    How would you think to start looking at the problem.

    What does gravity do to its velocity?
     
  4. Jul 14, 2009 #3
    thank you, well since gravity is a form of uniform acceleration, my teacher said i could apply the equation for uniform acceleration to problems involving falling objects. instead of using the variable a for acceleration, i can use the variable g to represent the acceleration due to gravity. with that said i can assume that are resistance remains 9.81 m/s2 throughout the fall.

    however i am having problems putting it all together.
     
  5. Jul 14, 2009 #4

    LowlyPion

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    Last edited by a moderator: May 4, 2017
  6. Jul 14, 2009 #5
    ok thank you so much!
     
  7. Jul 14, 2009 #6
    A stone is dropped from a bridge and falls freely under the influence of gravity.
    a) Calculate its velocity after 1.2s

    v(ave) = (vi + vf)/t
    v(ave)= (0m/s+ ?)/ 1.2s

    what would my final velocity be?


    b) Calculate is displacement after 1.2s
     
  8. Jul 14, 2009 #7

    LowlyPion

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    What is gravity?

    How does that affect velocity?
     
  9. Jul 14, 2009 #8
    im not sure what you mean...isnt gravity the acceleration throughout the fall?
    would that be my final velocity?
     
  10. Jul 14, 2009 #9

    LowlyPion

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    Acceleration increases the velocity 9.8 m/s every second.

    After 1 second it's going 9.8 m/s.
    After 2 seconds its going 19.6 m/s.

    So what do you figure its velocity would be after just 1.2 seconds?
     
  11. Jul 14, 2009 #10
    v(ave) = (vi + vf)/t
    v(ave)= (0m/s+11.76)/ 1.2s
    v(ave)= 11.76/ 1.2s
    v(ave)= 9.8
    ?
     
  12. Jul 14, 2009 #11

    LowlyPion

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    No.

    V = g*t
     
  13. Jul 14, 2009 #12
    v(ave)=9.8*1.2
    v(ave)=11.76

    ?
    im confused
     
  14. Jul 14, 2009 #13

    LowlyPion

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    No. They want V for the answer. Not V average. I'm not sure why you are insisting on trying to use V average here at all.
     
  15. Jul 14, 2009 #14
    Since acceleration is the amount that the velocity changes each second, then
    velocity = initial velocity + acceleration*time

    displacement is the integral of velocity with respect to time, so
    displacement = initial velocity*time + 1/2 * acceleration * time * time

    If you don't know about integrals yet thats ok, but its like the changing velocity always being multipied by time. Its sort of complicated, but that's why there is a 1/2 in front of the acceleration * time, because its like multiplying the average velocity by time.
     
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