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I have a question.

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by , 28th September 2012 at 11:06 PM (272 Views)
So I took my physics test today, and I didn't know how to do one of the problems, can anyone here help? The unit was projectile motion (no air resistance yet, this is only 10th grade :P).

I'll try my best at explaining, but the basic problem is that you have a guy who kicked a soccer ball and tried to score a goal. You were given the initial velocity of the ball and the angle he kicked it at. The goalie caught the ball. You were given the height he caught the ball at from the ground (see horrible picture):
[SPOILER]
[IMG]http://i1283.photobucket.com/albums/a545/notnuts/physics_zps38a6fdf1.jpg[/IMG][/SPOILER]

You had to find the ball's velocity [I]just[/I] before the goalie caught it (I could do that) but you also had to find the angle the ball made with the horizontal axis just before he caught it. This angle (horrible picture again, sorry):
[SPOILER][IMG]http://i1283.photobucket.com/albums/a545/notnuts/physics2_zps83d108b4.jpg[/IMG][/SPOILER]

Any smart physics people out there who know what I'm talking about, how in the world do you do it?
I [I]guessed[/I] an answer by finding the x and y components of the final velocity vector and using the inverse tangent function to find the complement of that angle, then subtracting from 90 degrees, but I don't know if that's right. I got an answer that looked reasonable though (can't remember any numbers, sorry)

And I'm scared now because I have a 90.0% in physics, and I really want it to stay as an A so I hope I did well on the test. :(
Attached Thumbnails Attached Thumbnails blogs/61549/attachments/78059-physics.jpg   blogs/61549/attachments/78060-physics2_zps83d108b4.jpg  

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  1. Silktree's Avatar
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    I guessed an answer by finding the x and y components of the final velocity vector and using the inverse tangent function to find the complement of that angle, then subtracting from 90 degrees, but I don't know if that's right. I got an answer that looked reasonable though (can't remember any numbers, sorry)
    That should be right, as long as you divided the x component by the y component. You could have calculated the angle directly (not its complement) by dividing the y component by the x component.

    Why are you unsure about the solution?
  2. Shadows's Avatar
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    @Silktree; Cool, thanks for the help :) I wasn't too sure because we never learned how to do it in class, and because I didn't know if by doing that you assumed the trajectory was a triangle instead of a parabola (since I used trig functions and complements of angles which you usually use on triangles in math class...) But in the end I was running out of time, and I wanted partial credit at least. :P
  3. Silktree's Avatar
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    It seems that your teacher didn't do that good a job teaching this unit, then. Even though the trajectory is indeed a parabola, the fact that we're dealing with instantaneous velocity means that we can look at the motion over a short time interval as if it were linear.
  4. Shadows's Avatar
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    [QUOTE=Silktree;bt258014]It seems that your teacher didn't do that good a job teaching this unit, then. Even though the trajectory is indeed a parabola, the fact that we're dealing with [i]instantaneous[/i] velocity means that we can look at the motion over a short time interval as if it were linear.[/QUOTE]

    Oh, I get it! Our class does go kind of fast, so we don't really get enough time to properly understand everything. :P
    Thanks so much! :)