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Homework Help: First Post. Kinematics

  1. May 16, 2004 #1
    Hello everyone, i'm new to this forum. I am kinda having trouble with a physics problem. Could someone please help me out with 3 problems. Thanks.

    A rocket is fired vertically with an upward acceleration of 27 m/s2. After 21 s, the engine shuts off and the rocket continues as a free particle until it reaches the ground.
    (a) What is the highest point the rocket reaches?
    km
    (b) What is the total time the rocket is in the air?
    s
    (c) What is the speed of the rocket just before it hits the ground?
    m/s
    ===================================================
    Two stones are dropped from the edge of a 200-m cliff, the second stone 1.3 s after the first.
    (a) How long after the second stone is dropped will the separation between the two stones be 35 m?
    s
    (b) At this point, how far below the clifftop is the second stone?
    m

    ====================================================
    A particle undergoes three successive displacements in a plane, as follows: 2.0 m southwest, 5.0 m east, and 8.0 m in a direction 60.0° north of east. Choose the y axis pointing north and the x axis pointing east.
    (a) Find the components of the first displacement.
    m east and m north
    Find the components of the second displacement.
    m east and m north
    Find the components of the third displacement.
    m east and m north
    (b) Find the components of the resultant displacement.
    m east and m north
    (c) Find the magnitude and direction of the resultant displacement.
    m at an angle of ° North of East
    (d) Find the displacement that would be required to bring the particle back to the starting point.
    m i + m j.

    I got some fromt the last one correct but am having trouble with the whole concept.
     
  2. jcsd
  3. May 16, 2004 #2
    Ok so apparently i am suppose to show work which i have done on the problems. I haven't done much work on them except draw the pictures etc... For the first problem i know i'm suppose to use the two main kinematics problems which are: v = v0 + at and x = x0+v0t+.5at^2

    For the last problem i got the letters a) and c) but i think i am miscalculating the angle on b) and and completely lost on how to do the other parts.
     
  4. May 16, 2004 #3

    Doc Al

    User Avatar

    Staff: Mentor

    Time to get to work! You'll need those kinematic equations. (Here's another that may come in handy: V^2 = V0^2 + 2ax )

    For the first problem (the rocket) do it in pieces. How fast is it going after 21 sec? How high is it? After 21 sec, it's just a projectile under gravity.

    Give it a try.
     
  5. May 16, 2004 #4
    Ok so i plugged the numbers into the first kinematic problem to get the speed at 21 sec which was 500 m/s. After that i used the 3rd equation listed in the quote above to get how high it went and i got the wrong answer from what it is suppose to be. Anyone know what i am doing wrong?
     
  6. May 16, 2004 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Check that one over. If you don't find your mistake, post your calculation.
     
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