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Homework Help: AhGhH! Pls help! I've jst joined this forum and i need help on a Projectile Motion q!

  1. Oct 15, 2003 #1
    umm... i'm not really used to using forums.. but whoever is online now and reading this... i would reaLLY appreciate if you could help me on this question.. this is fairly uRGent because its a bonus question which i want to hand in early tomorrow.

    i've already spent a lot of time on it.. and i think i have the answer, but i'm still not sure of my answer.. Please help or attempt to help me by tonight..

    Okay, here is the question:

    A golfer is standing at the top of a hill that slopes downward at a 45 degree angle. If he hits his golf ball with an initial velocity of 30.0m/s forward, what distance (d) down the slope from him does the ball first strike the hill?

    (sorry i can't draw the diagram here but if you dont understand the question i'll try to add a pic or sketch)

  2. jcsd
  3. Oct 15, 2003 #2
    oh yes.. the answer i got was 259.75 metres... is that correct?
  4. Oct 15, 2003 #3
    Full solution

    i'll show my work here.. although i must admit it's hard to do all these formulas on the computer..

    SOLUTION (i know this format is weird, but my physics teacher wants it in this format.. ps, i am doing this in components and i am not taking into consideration significant figures)

    Let "up" and "forward" be positive, and "time zero" be the instant the golf ball is hit. (also let A repressent acceleration, V be velocity, T be time, x be the x-components and y be the y-components)

    Horizontal Motion:
    Xo = 0 m
    Vox = 30.0 m/s
    Ax = O m/s2

    X = Xo + VoxT - 1/2 AxT2
    X = 30.0T

    Vertical Motion:
    Yo = 0 m
    Voy = 30.0 m/s
    Ay = -9.8 m/s2

    Y = Yo + VoyT - 1/2 AyT2
    Y = -4.9t2

    Since the hill is sloping downwards at an angle of 45 degrees, the X and Y component should be perpendicular, and the other angle should be 45 degree as well.

    Thus the X and Y components can be equated (is this right?):
    30.0T = -4.9T2
    T = 6.122s (must be positive)

    Therefore sub back into equation:
    X = 30.0T
    X = 183.67 m

    Y = -4.9T2
    Y = -183.67 m

    Therefore to find distance (d) of the hill, use pythagorean theorum:
    d2 = X2 + Y2
    d2 = (183.67)2 + (-183.67)
    d = 259.75 m

    *sigh* that was long.. blah.. can anyone tell me if that was right please??? thanks :eek:)
  5. Oct 15, 2003 #4
    isnt there anyone online???
  6. Oct 15, 2003 #5


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    Staff: Mentor

    Yep, you got the right answer - and just as a rule of thumb, even at night (when a lot of people are on) it usually takes an hour or two for a response.

    And its probably just a typo (you didn't use it): Voy=0.
  7. Oct 16, 2003 #6


    User Avatar
    Science Advisor

    I agree with Russ_Watters although I would not necessarily have interpreted "initial velocity of 30.0m/s forward" to mean exactly horizontal. Of course, given the information, that's the only interpretation you can make- but I would have worried about it!

    By the way, dar|{anGel_, please don't expect people to answer your questions instantly!
  8. Oct 20, 2003 #7
    THanks! :eek:)

    um.. heh.. heh.. heh.. well, as i said, i wAS new to the forum and i was desperate to get the answer because i was about to give up on it since i worked on it for so long, and it wasnt until i realized that the angle being 45 degrees meant that the X and Y would be equal that i figured it out...

    (i was doing to question according to a diagram given along with the question, which was reaLLY misleading, because it made the 45 degree angle really small, and therefore the X looked longer than the Y)

    .. of course later, i realized it was so easy.. coz all you had to do was equate the two equations to get the time.... i mean, the method was easier than i thought it would be because i assumed that it would require a lot more work (since it was a 'bonus' question given to my Gr. 12 Physics class, and i expected something reaLLY hard.. i guess if the angle was not 45 degrees it would be a lot harder.. but not impossible)

    well.. i diDnt expect "instantly".. just within.. like.. minutes.. since you guys are soOOOooo presumably smart (was it wrong to overestimate your abilities?)

    oh yes, and that was a typo.. the Voy was supposed to be zero.. (i copied and pasted it.. and forgot to change it)

    Anywayss... Thanks a lot, russ_watters and HallsofIvy!! i am completely and utterly humbled by your presence and superfluous amounts of knowledge in Physics :smile: (in other words, i'll just assume you guys are hard core Physics junkies with either too much time on your hands or are just really, really nice :wink: )

    if you noticed, im not exactly a Physics type person, but i like to blab on about nothing a lot (anyways, it looks like i'll become a permanent resident of your forum..i'll probably need it for the rest of my Physics-engrossed year)
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