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A homework question that i need help with

  1. Mar 8, 2007 #1
    1. The problem statement, all variables and given/known data

    hi, i was trying to figure out how to tackle this physics question, but im a little stuck. the question is:

    a tennis player, playing for a match hits a ball 2m above the groung. the ball leaves his racquet with a speed of 20m/s at an angle of 5 above the horizontal. the horizontal distance to the net is 7m and the net heigh is 1m does the ball clear the net?

    i've diagram to help me and im just stuck about where to start. thanks for helping me.

    2. Relevant equations

    i used v^2=u^2+2as and Vox=Vo*cos(angle) <- but i think im wrong :S

    3. The attempt at a solution
    i got a=-28.57m/s. but and i got that it didn't claer the net. im not sure wot i was doing really. :S
     
  2. jcsd
  3. Mar 8, 2007 #2
    Please don't "double-post" questions.
     
  4. Mar 8, 2007 #3
    sori i didn't mean too
     
  5. Mar 10, 2007 #4
    You say you have found a value for "a" but "a" is not a speed it is acceleration. At least if you are using "a" as it is found in your equation.

    [tex] v^2 = u^2 + 2as [/tex]

    Hint... the only acceleration in these sorts of problems is downward, ie. a = 9.81m/s^2. Friction from the air is generally ignored, unless it is specified as part of the problem.

    Questions like these need to be solved in components. That is... what the ball does in its vertical motion and then.. horizontal motion. The two are quite indepenent.
     
    Last edited: Mar 10, 2007
  6. Mar 10, 2007 #5
    ok thank you for help
     
  7. Mar 10, 2007 #6
    Calculate the horizontal and vertical components of the velocity using trig.

    In the x direction, where there is no acceleration, calculate how long it will take the ball to travel 7 m horizontally.

    Then you can solve for how high the ball will be at this time, by using the equation

    d = vit + 1/2at^2 in the y direction

    Based upon the value of d, u can determine whether the ball cleared the net or not.
     
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