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Physics - Mechanics

  1. Oct 15, 2003 #1
    Ok so I'm havin just a bit of trouble with this problem...

    ok here it is.

    A snowball rolls off a barn roof that slopes downward at an angle of 40 Degrees. The edge of the roof is 14.0 m above the ground, and the snowball has a speed of 7.00 m/s as it rolls off the roof.

    A)How far from the edge of the barn does the snowball strike the ground if it doesn't srike anything else while falling?

    B)A man 1.9 m tall is standing 4.0 m from the edge of the barn. Will he be hit by the snowball?

    Part A: ok this was easy I got this:

    tan (40 degrees) = X/15

    X ≈ 11.75 m

    (X = distance from the edge of the barn to where the snowball strikes the ground)

    Part B: I'm stumped here I'm not even sure what approach to take...

    I'm guessing I have to caculate the path of the snowball. And see if it hits him. (I'm not sure how to do this i'm guessing using (1/2at^2+Vit+Xi)

    (i = initial)


    Any help would be appreciated thanks. :)
     
  2. jcsd
  3. Oct 15, 2003 #2

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    Apparently it wasn't easy enough. You are assuming the snowball continued in a straight line after leaving the roof. It doesn't happen that way. Things FALL!

    Do you know the formulas for velocity and distance when objects have acceleration (in this case acceleration due to gravity)?
    The snowball will hit about 10.4 meters from the barn. Can you see how to get that?
    Give it a try and get back to us.

    b) Use the same formulas you did in a) to determine the height of the snowball when the distance from the barn is 4.0 m. Does the snowball pass over the man or hit him?
     
  4. Oct 16, 2003 #3
    err...yea that was careless of me well ok so after lookin at it again I got this.

    Vx = (Vo)(cos θ), Vx ≈ 5.36 m/s
    Vy = (Vo)(sin θ), Vy ≈ 4.50 m/s

    Ok now I need to find the horizontal distance

    Vo = 7 m/s
    Xo = 14 m
    a = -9.8 m/s^2

    Y = Xo + Vot + (1/2)(a)(t)^2
    t ≈ 2.55 s

    V(t) = at + vo
    V(t) ≈ -17.98 m/s

    5.36m ≈ X/2.55s

    x ≈ 13.67 m

    Thats what I got i'm not all that sure if i'm right.

    And so for part B I think I have to use this equation but well...man i', still kind of confused about which equation(s) to plug in the variables 1.9 m and 4.0 m to determine if he gets hit.

    Y = Xo + Vot + (1/2)(a)(t)^2

    (hey thanks for helpin me get started HallsofIVY)

    Any help is appreciated.
     
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