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Kinematics questions

  1. Nov 3, 2008 #1
    1. The problem statement, all variables and given/known data

    A ball is thrown off the top of a 50 m cliff at an angle of 30 degrees with a velocity of 22 m/s.
    How long is the ball in the air in s?

    How far from the base of the cliff does the ball land in m?

    What is the maximum height reached by the ball in m?

    2. Relevant equations
    so i tryed to find the velocitys in both axis but, i think i did it wrong because i got that answer wrong (im useing an online homework program thingy called web assign). please help me im so lost.
  2. jcsd
  3. Nov 3, 2008 #2


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    Homework Helper

    Here are some formulas.

    First separate your velocity into x,y components and then you can apply the relationships found there.
  4. Nov 3, 2008 #3
    thank you i rember going over most of thows equations in class but our teacher said so far all we need in the Xf equation and the Vf equation. what he has shown us in class to get the velocitys in both dirrections is to use trig. well i do that but i get negative numbers, why, what am i doing wrong?
  5. Nov 4, 2008 #4
    final velocity in the vertical direction should be negative.

    final velocity in the horizontal direction should be the same as initial velocity, since gravity does not act in the horizontal direction: mg[cos90]=0.

    i haven't worked it all out yet, but initial velocity in the Y direction should be 22 m/s * sin30 = 11 m/s. that velocity is going to fight against gravity, lose, and end up negative.

    initial and final velocity in the x direction should be 22 m/s * cos30 = 19.1 m/s.

    figure out how long it will take for the object to hit the valley below that cliff. that time can be used in the position function for the x direction, which is easy since horizontal motion has zero acceleration -- the equation is just 19.1 meters per second multiplied by the flight time.
  6. Nov 4, 2008 #5
    thank you now this might be dumb but how did you get 11m/s and 19.1m/s. when i put the veloctiy times the sin of 30 i got a negative number, what did i do wrong?
  7. Nov 4, 2008 #6
    I think you are using your calculator wrong... you need to have the setting of "degrees," not "radians" (or you need to convert your angle from degrees to radians).
  8. Nov 4, 2008 #7
    thank you very much i asked my teacher and for sum reason when i changed it from radians to degrees it worked (i did that at home and got the same answer, i think teachers are magical or somthing, lol). thank i now know how to do this it was just a matter of my calculator acting funny, i new how to do but i second geussed myself do to my calculator. thank you all.
  9. Nov 4, 2008 #8
    i need help on the 3rd problem. so i have the time in air (4.508) and the length of the base of the cliff (85.891). now what do i do to find the max hight.
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