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!

Calculating Speed of a Ball

  1. Feb 4, 2014 #1

    B18

    User Avatar

    1. The problem statement, all variables and given/known data
    With what speed (in feet/second) must a ball be thrown vertically upward in order to rise to a height of 85.0 ft, neglecting air resistance?


    2. Relevant equations
    g=9.8 m/(s^2)
    speed=distance/Δt

    3. The attempt at a solution
    I started off by setting speed=85.0ft/Δt
    I understand I need to find the time duration.
    I converted 85 ft to 25.908m.
    Now I was thinking of multiplying 9.8 m/(s^2) by (1/25.908m) but that leaves me with seconds squared.

    Am I missing a general formula for this problem? It seems like I don't have enough information at this point.

    Thank you to anyone who can offer me any suggestions.
     
  2. jcsd
  3. Feb 4, 2014 #2
    You need the equations for uniform accelerated motion.
    speed=distance/t works only for uniform motion, motion with constant speed.
     
  4. Feb 4, 2014 #3

    B18

    User Avatar

    So if i were to use a kinematic equation such as X(f)=V(ave)t+x(i) to find t how can I solve for t without knowing the velocity?
     
  5. Feb 4, 2014 #4

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Try one of the SUVAT equations for motion subject to constant acceleration..

    http://en.wikipedia.org/wiki/Equations_of_motion#SUVAT_equations

    When I was at school it was worth memorising them as they help with all sorts of problems of this general type. Some 35 years later I can still remember equations 1,2 and 4.
     
  6. Feb 4, 2014 #5
    You cannot.

    But there is another kinematic equation.
     
  7. Feb 4, 2014 #6

    B18

    User Avatar

    X(f)=X(i)+V(i)t+(1/2)a(t^2)
    Im going to say this one with initial velocity set to 0 because in this problem that is the case.
     
  8. Feb 4, 2014 #7

    B18

    User Avatar

    Figured it out. Used the kinematic equation above and set a=9.8m/s^2 thank you.
     
  9. Feb 5, 2014 #8

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    just for completness.. The final velocity will be zero not the initial velocity.
     
  10. Feb 5, 2014 #9

    B18

    User Avatar

    So to ensure I understand this.. How does this look CWatters
    Can I use V(f)^2=V(i)^2+2a(Xf-Xi)
    solve for V(i)^2
    V(i)^2=0-2(-9.8)(25.91)=22.54m/s
    So in this problem a would be equal to -9.8 not positive??
     
  11. Feb 6, 2014 #10

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Yes. The initial velocity is in the opposite direction to the acceleration due to gravity - so velocity and acceleration will have different signs.
     
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: Calculating Speed of a Ball
  1. Speed of a ball (Replies: 7)

  2. Speed of ball (Replies: 4)

Loading...