Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Problem help please

  1. Oct 16, 2008 #1
    A man can throw a ball a maximum horizontal
    distance of 138 m.
    The acceleration of gravity is 9.8 m/s2 .
    How far can he throw the same ball vertically upward with the same initial speed?
    Answer in units of m.

    I figured out the initial speed to be about 280 m/s. Am I right? How can I finish the problem?
     
  2. jcsd
  3. Oct 16, 2008 #2
    What steps did you take to determine the initial speed? I ran through the problem and got a much different answer.
     
  4. Oct 16, 2008 #3
    I used the equation range equals (initial velocity squared * sin of 2 theta) divided by gravity.
    range equals 180
    theta equals 45 (this is the angle that will achieve the farthest range)
    and gravity equals 9.8 (the negative sign is not needed)

    And I did the problem again, and I got about 1352
     
  5. Oct 16, 2008 #4
    A good idea would be to show your solution.
     
  6. Oct 16, 2008 #5
    You forgot to take the square root this time.
    Is the distance 138 or 180?
     
  7. Oct 16, 2008 #6
    range is equal to 180, and yes, thank you. I always forget to square root. So it's about 36.77 m/s. Now how should I proceed?
     
  8. Oct 16, 2008 #7
    Check the math on that one. It looks like you used 138 m for the range.
     
  9. Oct 16, 2008 #8
    I got 42 m/s. What now?
     
    Last edited: Oct 16, 2008
  10. Oct 16, 2008 #9
    Given the initial speed and the final speed (at max height, v = 0), use the formula
    Vf2 = Vi2+2ad

    Solving for d and substituting knowns:

    d = Vf2/(2 g) = 422/(2*9.8) = 90 m
     
  11. Oct 16, 2008 #10
    It says the answer is wrong! (PS it's one of those online hw)
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook