ahGhH! Pls help! I've jst joined this forum and i need help on a Projectile Motion q!

dar|{anGel_

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)

thanks..! [t)]

dar|{anGel_

oh yes.. the answer i got was 259.75 metres... is that correct?

dar|{anGel_

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

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 :o)

dar|{anGel_

isnt there anyone online??? [8)]

russ_watters

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.

HallsofIvy

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!

dar|{anGel_

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 [:)] (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 [;)] )

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)