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May23-07, 10:32 AM
P: 606

Quote Quote by HallsofIvy
a cannon fires a projectile (no air or wind) at 100 m/s at a 20 degrees angle north of west. it is fired down a "hill" (which is impossibly straight) the hill has a 20 degree angle north of east. find d (see picture.
You do understand, don't you, that we CAN'T "see picture" because you didn't post it! I assume that "d" is the distance from the point at which the projectile is fired to where it hits the ground. The real point of the problem is to find where it hit.

You say "at 20 degrees angle north of west". That sounds like you mean it is aimed 20 degrees to the north of due west but if that is true, we don't know the upward angle at which the projectile is fired.

I'm going to take it that the cannon is aimed "north-west" (that is "45 degrees north of west", and that it is pointed 20 degrees above the horizontal. Also that the hill slopes downward in the same direction as the cannon is aimed (which makes the problem MUCH easier) with a 20 degree downward slope.

Here's how I would do the problem: We can ignore the direction. Since the cannon is fired in the same direction as the hill slopes, just do it as a standard two dimensional problem- take (0,0) as the position of the cannon and the x axis in the direction the cannon is fired. First ignore the hill. Use the standard (parabolic) formula for a projectile launched at angle [theta] with initial speed v0:
x= v0 cos[theta] t, y= -(g/2) t2+ v0 sin[theta] t.
You can easily solve solve for t in terms of x and replace it in the formula for y to get y as a function of x. In a simple, "standard" problem, the projectile would hit where y= 0 so you would solve y= 0 for x.

Here, the ground slopes. Knowing that the ground slopes downward in the x direction, find the equation of the straight line:
it is y= (tan[theta])x where [theta] is the angle the line makes with the x axis (in this problem it is -20 degrees).

The projectile "hits" the ground when it crosses that line.
Solve the two equations y= f(x) for the projectiles motion and
y= mx for the ground simultaneously to find the point at which the projectile hits the ground. Once you know the coordinates of that point, you can find d, the distance between (0,0) and that point.
Someone now says that the penguins can fly because they will have developed to a level that their wings are larger than now, they have many feathers, and so, flying is possible...

Would you please tell me if that guy is correct or just crazie ?

Thank you,

>>> This should sound like a joke (lol) but this is what I read from a book about evolution, and I really need some your ideas about this <<<
Please help me...
I think I got the post that was meant for dduardo or penguino.