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: 2D kinematic

  1. May 4, 2010 #1
    A golf ball is hit and makes a 45o angle with the horizontal as it leaves the tee. It lands 200 m away. What was the ball’s initial speed?

    I though i know this well but i don't even know how to set this up
  2. jcsd
  3. May 4, 2010 #2
    You are supposed to at least make an attempt at the problem, even if you're just guessing.

    Try dividing up the problem into two dimensions: the horizontal and the vertical. Questions:
    a) What happens as the ball moves upwards (vertical)?
    b) What happens as the ball moves forward (horizontal)?

    Try answering these and see where they take you.
  4. May 4, 2010 #3
    as i said i dont know how to set it up....all i know is i need to find t in order to find the Vi on the horizontal part and the vertical part has g as acceleration and on the vertex the Vf is 0
  5. May 4, 2010 #4
    You're on the right track. I'm trying to help you set it up by asking you leading questions, so you should try answering the questions :wink: If you have to guess at the answers, that's ok :smile:

    I'll give you another hint: you don't need to find t. t is going to cancel out at the end, so don't worry about actually finding it yet. We need to set up equations for the horizontal and vertical directions first. These equations will include t as a variable.

    The questions in post #2 will help you set up the equations you need, so I want you to try and answer them first.
  6. May 4, 2010 #5

    is that even close?
  7. May 4, 2010 #6
    Looks close. You can break down your steps for me so I can tell you where any mistakes might be. It's also helpful (for both of us) if you include units :)

    Okay, so 200 m / (Vicos45) = t is correct for the horizontal.

    And your vertical part is correct too, assuming you meant sin(45), not sin(4).

    Now you can solve for Vi.
  8. May 4, 2010 #7
    o crap so for vertical i can assume the time for the ball falls down from the highest point is the same as when it is going up to the highest point?
  9. May 4, 2010 #8
    Yep. It's actually not an assumption, it's a fact. Because the acceleration is the same going up as it is coming down, the time is going to be the same both ways.

    And the total time (time for it to go up and come back down) is the same time it takes for the ball to travel horizontally 200 m. Does that make sense?
  10. May 4, 2010 #9
    yea i was not sure about that so i doubt myself thanks alot anyways
  11. May 4, 2010 #10
    You're welcome. If you aren't sure drawing a diagram is usually helpful.
  12. Feb 23, 2011 #11

    initial speed is zero..since it is at rest before hitting...after hitting it will move with speed 44.271 m/s
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook