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Homework Help: 2-Dimensional Motion and 3-D

  1. Sep 21, 2007 #1
    Hi just wanted to do an introductory before i ask for anyone's appreciated help. My name is Sonny, freshman in college and currently working on motion. My homework is on online so the computer system is frustrating sometimes. I need your help because I'm confused on why my answer doesn't work, but I have a list of problems to be answered though.

    (1)The position of an electron is given by r = 5.00t i - 5.00t2 j + 4.00 k, with t in seconds and r in meters.

    (a) What is the electron's velocity v(t)?
    5 m/s i + -10 t m/s j + 0 m/s k
    (b) What is v in unit-vector notation at t = 5.00 s?
    5 m/s i + -50 m/s j + 0 m/s k
    (c) What is the magnitude of v at t = 5.00 s?
    50.2 m/s

    I need help on this question. All of the above are correct
    (d) What angle does v make with the positive direction of the x axis at t = 5.00 s?
    ??° (from the +x axis)

    I tried and i got 84.3 degrees

    (2)A baseball leaves a pitcher's hand horizontally at a speed of 164 km/h. The distance to the batter is 18.3 m. Neglect air resistance.

    (a) How long does it take for the ball to travel the first half of that distance?
    (b) How long does it take for the ball to travel the second half of that distance?
    (c) How far does the ball fall under gravity during the first half?

    I need help on this.

    (d) How far does the ball fall under gravity during the second half?

    I tried and i got -0.5, 0.5, and -1.

    (3)In Fig. 4-49, a stone is projected at a cliff of height h with an initial speed of 42.0 m/s directed 60.0° above the horizontal. The stone strikes at A, 5.00 s after launching.

    (a) Find the height, h, of the cliff.
    (b) Find the speed of the stone just before impact at A.
    (c) Find the maximum height H reached above the ground.

    I need help on this completely and I don't know when to incorporate the h

    (4)You throw a ball with a speed of 15 m/s at an angle of 40.0° above the horizontal directly toward a wall (Fig. 4-35). The wall is 22.0 m from the release point of the ball.

    (a) How long does the ball take to reach the wall?

    I need help on this.
    (b) How far above the release point does the ball hit the wall?
    ?? m
    (c) What are the horizontal and vertical components of its velocity as it hits the wall?
    11.5m/s (horizontal)
    ??m/s (vertical)

    For (b) I got .53meters and for (c) Vertical I got 9.56m/s

    (5)A cat rides a merry-go-round while turning with uniform circular motion. At time t1 = 2.00 s, the cat's velocity is v1 = (1.50) i + (4.00 m/s) j, measured on a horizontal xy coordinate system. At time t2 = 8.00 s, a half-revolution later, its velocity is v2 = (-1.50 m/s) i + (-4.00 m/s) j.

    (a) What is the magnitude of the cat's centripetal acceleration?
    (b) What is the magnitude of the cat's average acceleration during the time interval t2 - t1?

    for (a) i got 1.17 and for (b) i got 3.04
    Last edited: Sep 21, 2007
  2. jcsd
  3. Sep 21, 2007 #2


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    Gold Member

    Which can't be right. Just sketch your vector, where is it pointing?
    I guess that you forgot to include the minus sign of the y component.
  4. Sep 22, 2007 #3
    i = x component and j = y component (i,j) so (5,-50). x-axis is the t and y-axis is the v so from the horizontal just find the angle using tangent? If i use tangent would the sign of the y component really matter?
  5. Sep 23, 2007 #4
    Cool i got the answer. Now about the other problems still need help for them. I need to turn these in at midnight
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