## idku one year ago math-based-physics question.

1. dan815

o goody

2. idku

Julie throws a ball to her friend Sarah. The ball leaves Julie's hand a distance 1.5 meters above the ground with an initial speed of 11 m/s at an angle 44 degrees; with respect to the horizontal. Sarah catches the ball 1.5 meters above the ground. ------------------------------------------------------------------------- 2) What is the vertical component of the ball’s velocity right before Sarah catches it? 3) What is the time the ball is in the air?

3. idku

I attempted (for question # 2) $$11\sin(44)\approx7.6$$m/s but it doesn't work when I enter it in my pracice thingy.

4. idku

The image would be this: http://i.imgur.com/TLF8qrM.png

5. dan815

yep okay

6. dan815

a complete parabollic motion completed in this throw okayq

7. dan815

so the same vertical speed it started out with is the same speed it ended with

8. dan815

|dw:1441330388180:dw|

9. idku

Yes, so I tried 11sin(44) but it didn't work....

10. idku

(well, I entered 7.64 approximation)

11. dan815

its downward now so

12. dan815

it will be negative -11*sin(44)

13. idku

Oh, cool. Thanks, and how do I figure question #3?

14. idku

3) What is the time the ball is in the air?

15. dan815

okay now we dont really care about the horizontal component of the veloicty that is not going to determine how long the ball will stay up in the air

16. dan815

What we need to worry about is the veloicty in Y and the acceleration in Y

17. dan815

now lets break the problem into 2 symmetric parts, the time it takes the ball to go up, and then the time it takes the ball once its all the way up to fall back down will be the same

18. dan815

this is something you should prove to yourself when u have time

19. dan815

so what we will do is figure out the time he takes from |dw:1441330648992:dw|

20. idku

well, considering any vertex of (h,k) the vertical distance from (h,k) to (h,k+a) and the vertical distance from (h,k) to (h,k-a) are same. ik that:)

21. dan815

okay

22. dan815

what is the speed at the top of the arc?

23. dan815

the velocity / speed of the ball

24. idku

I am just very unfamiliar with the formulas I have to apply at each time.

25. dan815

no formulas needed

26. idku

lol (?)

27. dan815

what happens to the velocity at the very top

28. idku

it is 0

29. dan815

right

30. dan815

so we started with 11sin(44)m/s and went to 0 m/s now acecleration tells you how velocity changes wrt to time\ we have a constant acceleration of 9.8m/s^2 "9.8 m/s /s" we change 9.8m/s(speed) /s(per second)

31. dan815

so if we went from / 11 sin(44) m/s to 0 m/s how many second would that have took if every second we lose 9.8m/s

32. idku

11sin(44)/9.81 seconds

33. dan815

good :)

34. dan815

now that is the time for half the motion right

35. dan815

so multiply that time by 2 and ur done

36. idku

I am really not that good to do logical conclusions like this on my own though. I think that practicing formulas at first and then comprehending to an extent of a logic would be better for me..... Yes, multiply that by 2, so we got 2sin(44) (m/s) ------------ 9.81 (m/s^2) and our units are seconds as well....

37. idku

times 2, because we go from max vel to 0 velocity twice.

38. dan815

2 * 11 *sin(44)/9.81

39. idku

1.56 seconds

40. dan815

probably right

41. idku

Wolfram is always right:D

42. dan815

:)

43. idku

And how would I briefly go about the distance between the two girls?

44. dan815

okay so we know the time it was up in the air

45. dan815

now for that same time, it was constatly moving horizontally with that horizontal speed given 11 cos(44)

46. idku

distance = time 5 velocity ?

47. idku

oh, 5 was supposed to be ×

48. dan815

yes thats right

49. dan815

by definition speed is the change in distance per time unit m/s

50. idku

distance = 1.56 • 11cos(44) <-- that is our horizontal velocity ^ | the time for vertical and horizontal is same.....

51. dan815

yes

52. idku

1.56s • 11cos(44)(m/s) and units work too

53. idku

12.34

54. dan815

oh btw, you know how to you said youd rather pratice formulas to solve this, you should forget that way of thinking, it might even help you getting marks as far as highschool goes... but you will just be memorizing formulas and not understanding

55. Jhannybean

$\Delta x = v_{i_x}t + \frac{1}{2}gt^2$ can this work too? just wondering.

56. idku

a=0 in horizontal force

57. idku

because no gravity

58. dan815

IF you want to do good in college, and get further in physics and other related stuff, you are better of understanding and trying to do this more logically

59. idku

so just v•t

60. dan815

eventually those people that are memorizing wont be able to solve the tougher problems

61. idku

I am making sense out of it.... my prof is a good person but by no means can he be a teacher ... -:( tnx for helping me with it. I am kind of bad it (comparing to other subjects), but tnx!

62. Jhannybean

dan is absolutely right

63. dan815

dont worry it takes a while to get comfortable in physics

64. dan815

i had one highschool teacher that gave us good advice, he said make sure you can picture the problem

65. dan815

always do your best to picture the problems, really understand the question, understanding the questions is sometimes the most difficult part

66. idku

Jhanny it is the teacher, I promise....

67. idku

yes, understanding is best, because it is the only way to put it into memory

68. Jhannybean

Many people do not like their physics professors, but its really in understanding what youre learning thats weighing you down, not the professor

69. idku

I will have some more questions, and entire night to work on (waiting for friday 3pm after physics to chill). I have to ask more and do it..... sorry that I am stopping the conversation like this.

70. idku

No, he doesn't explain he just derives formulas from formulas....

71. idku

good luck, bye, apologize.

72. Jhannybean

np :)