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Homework Help: Projectile motion, missed two weeks, fell behind, no book, Please help! Thank you!

  1. Mar 19, 2012 #1
    1. The problem statement, all variables and given/known data
    Hi guys, Im not one to ask for help but i missed ALOT of school because I got sick and I was wondering if anyone could solve three problems for me and help me out!
    Thanks in advance !
    Problem 1-
    A plane is dropping a supply of coconuts to survivors of the USS Minnow, who are stranded on a desert island. If the plane is 2km in the air and is traveling at 200 m/s, How far in front of the island should the supplies be dropped?
    Problem 2- A watermelon is thrown off the 10th (40 meters above the ground) floor of a building. It is launched with a speed of 30 m/s at an angle of 40 degrees above the horizontal. How far away does it land?
    Problem 3- A football is kicked at 25 m/s at an angle of 50 Degrees.
    How fast is the football traveling when it strikes the ground and what direction is it traveling.
    show work please.
    and once again, I'm very grateful to anyone who will help me out! Im trying to do better in this class and I just happened to get sick:/

    2. Relevant equations
    All can be found here - http://tutor4physics.com/projectilemotion.htm
    And on other sites, but none seem to copy + Paste
    Equation of Trajectory (Path of projectile)

    3. The attempt at a solution
    AND I have read the FAQ and the statement, however, I am really in a bad position here, I have attemped these but I know that it is not right, and Posting it would just probably make me look dumb and contribute nothing. I know how projectile motion works and I have done about 80% of my Homework, but these 3 problems are stumping me.

    For the Watermelon Problem All i know is the X and Y Componants
    Which are Cos(40Deg) * 30 M/s and Sin(40Deg) * 30 m/s
    And that A=Change in V / T
    - 9.8m/s^2= -30 /t
    -9.8m/s^2 * t = -30
    T = 3.06
    And then I know you must use the 40m To make another vector to complete the problem.
    For the Football Problem I know.
    How Long it is in the air- 3.9 Seconds
    Because Ttotal=38.3m/s / 9.8 m/s
    How high it goes= 18.7m
    How far away does it land 62.8 m
    because Range= (25M/S)^2 * Sin2(50) /
    9.8 M/s^2

    But for the plane problem I am totally lost
    Last edited: Mar 19, 2012
  2. jcsd
  3. Mar 19, 2012 #2
    Re: Projectile motion, missed two weeks, fell behind, no book, Please help! Thank you

  4. Mar 19, 2012 #3
    Re: Projectile motion, missed two weeks, fell behind, no book, Please help! Thank you

    The key to all projectile motion problems is to remember that the x component of the velocity will remain unchanged from start to finish, and the y component of the velocity will decrease according to gravitational acceleration. Therefore, for any problem, you should first begin by determining the initial x and y velocities, and then you will have two independent problems to solve--one in x, and one in y.

    For 1, the y velocity begins at 0. The plane is 2km up, so first of all, you should try to figure out how long it will take to hit the ground. Once that's done, try to determine what the horizontal velocity is, and then you should have enough information to determine how far forward the coconuts will go.

    For 2, you've already determined the watermelon's initial vertical velocity. For a moment, just pretend the watermelon was thrown straight up into the air, instead of at an angle. Again, try to figure out how long it takes to go up into the air, fall back to its starting point, and then fall an additional 40m.

    3 is exactly the same as 2, except the football only goes down to 0, not any further.

    In all cases, time is the parameter that links the x and y equations together. Find that, and you should be able to compute all the other missing quantities off of it. The equations at http://physics.bu.edu/~redner/211-sp06/class01/equations.html might prove useful--they seem a bit more straightforwardly written than the ones on the page you linked. For each problem, write down each variable that you know, and determine which equation has only one unknown quantity left in it.
  5. Mar 20, 2012 #4
    Re: Projectile motion, missed two weeks, fell behind, no book, Please help! Thank you

    There are 2 component of the flight of the coconuts.
    1. Horizontal component, no acceleration
    2. Vertical component with acceleration due to gravity.

    2000=1/2(10 t²) taking a=10m/s² and they dropped, with no initial vertical velocity.
    400=t² => t=20sec

    H=ut+1/2(at²) where a=0
    H=20(200)= 4km.
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