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Homework Help: Physics HW Question

  1. Feb 7, 2005 #1
    Alright i have two problems on my homework sheet that i dont understand completely. they deal with two dimensional motion.

    # 4 says. Consider an object fired from the ground with an initial vertical velocity component of 4.5 m/s and an initial horizontal velocity component of 2.4 m/s. find a) the speed and b) the angle above the ground at which it was fired. Also, give its velocity at c) its highest point and d) when it returns to ground level.

    i think i figured out a and b. A) being 5.1 m/s and b) being 62 degrees. C) and D) are giving me problems. im night sure how exactly to find the velocity at the highest point i want to think its zero but i dont think it is. im thinking to solve this i might use the trig functions which i was given that are COS=Ax/A and SIN=Ay/A.
    which would give me c)2.4m/s d)4.5m/s but i dont think thats right either. if someone could point me in the right direction that would be great.

    the other question i had was on this problem.

    a projectile is fired horizontally at 14m/s from a cliff top. it hits the ground 3.5s later. a) find its vertical velocity componet at ground level (would this be the final velocity?) b) find the velocity (speed and direction of travel) at which the prjectile hits the ground ( the whole hitting the ground thing confuses me) and c is just sketching it.

    i made a list of the componets for the x and y axis. with time at 3.5s for both and initial velocity as 14m/s on the x axis and 0m/s on the y axis. im not sure if i put these in the right places. than i put acceleration as 0m/s^2 for the x axis and 9.80 for the y. im thinking to solve for vertical velocity at ground level, i would solve for final velocity and then use the x and y's to find speed and direction.

    it this idea right? or could you point me in the right direction?

    thank you,
  2. jcsd
  3. Feb 7, 2005 #2


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    Homework Helper

    For both questions, keep in mind that the gravitational force only has a vertical component, so it doesn't affect the horizontal component of velocity at all. Also, for the first question, what do you know about the vertical component of velocity at the highest point? And what about the potential energy when the particle returns to it's original height as compared to the potential energy at the beginning?
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