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Can someone please help me with these problems? New chapter start. 3-d motion

  1. Sep 17, 2006 #1
    I am trying to study for an exam, but I got some practice questions wrong that I'd like to understand to study better. Can someone help me? Thanks!

    (I don't just want the answer please, I really want to understand this stuff. If you have time, please explain? I'd like to check myself and figure it out.)

    1. Two balls ( red and blue) released at the same time from the same height above ground. Which ball has the highest final speed?

    <img src="http://img103.imageshack.us/img103/1997/2ballsmg3.gif">

    My answer: both balls hit the ground at the same speed

    choices:

    a. The blue ball
    b. The red ball
    c. Both balls hit the ground at the same time
    d. More information is needed to answer the question



    2. The figure below shows 2 balls ( red and blue) released at the same time from the same height above ground. Which ball has the highest initial velocity?

    (same graph as #1)

    My answer: both balls have the same initial velocity

    choices:

    a. The blue ball
    b. The red ball
    c. The initial velocity is the same for both balls
    d. More information is needed to answer the question


    5. The figure below shows 2 balls ( red and blue) released from the same height at the same time with no initial vertical velocities. Which ball will hit the ground with the highest speed.

    <img src="http://img177.imageshack.us/img177/8498/2ballsbuy0.gif">

    My answer: both balls hit the ground at the same speed

    choices:

    a. The two balls will hit the ground with the same speed
    b. The blue ball
    c. The red ball
    d. More information is needed to answer the question


    7. A soccer ball is kicked from the ground and travels in the air as a projectile. Ignoring air resistance, which graph correctly represents the vertical velocity of the ball as a function of time?

    <img src="http://img158.imageshack.us/img158/9913/pic1sr6.jpg">

    My answer: A

    choices:
    a. A
    b. B
    c. C


    10. A ball A is thrown off a cliff with an initial horizontal velocity. At the same time ball B is dropped with no initial velocity from the same height. Which ball has the largest speed upon hitting the ground?

    My answer: the speeds are the same

    choices:

    a. A
    b. B
    c. The speeds are the same
    d. Can't tell with the given information.


    22. At the highest point of its trajectory a projectile has

    a. zero acceleration and a constant horizontal velocity
    b. zero vertical velocity and an acceleration equal to g
    c. zero vertical velocity and a zero acceleration

    My answer: A

    choices:
    a. zero acceleration and a constant horizontal velocity
    b. zero vertical velocity and an acceleration equal to g
    c. zero vertical velocity and a zero acceleration


    23. If a ball is thrown at a 45 degree angle

    a. it will reach the highest point it can ever reach
    b. it will travel 100 m along the x axis
    c. its initial horizontal and vertical velocities will be equal to each other

    My answer: A
    choices:

    If a ball is thrown at a 45 degree angle

    a. it will reach the highest point it can ever reach
    b. it will travel 100 m along the x axis
    c. its initial horizontal and vertical velocities will be equal to each other

    31. A ball A is thrown at an angle with an initial vertical velocity of 5 m/s. At the same time a second ball B is thrown from the same height toward the ground ( vertically) with an initial velocity of -5 m/s. The two balls will hit the ground

    a. at the same location but at different times
    b. at the same time but at different locations
    c. with the same vertical velocity but at different locations and different times

    My answer: B

    choices:
    a. at the same location but at different times
    b. at the same time but at different locations
    c. with the same vertical velocity but at different locations and different times
     
  2. jcsd
  3. Sep 17, 2006 #2

    berkeman

    User Avatar

    Staff: Mentor

    -1- The two balls have a different horizontal speed, but same vertical speed. Velocity is the vector sum of the two components, vertical and horizontal. Notice how some questions are clear about the "vertical velocities", but this question is not. Still, since they omit "vertical component" in the question, I think you should account for the total velocity, and change your answer.

    -2- (see #1)

    -5- (see #1)

    -7- Image won't display for me

    -10- (see #1)

    -22- Gravitational acceleration does not stop just because the vertical velocity does. Gravity is constant.

    -23- (see #22)

    -31- Nope. If the first ball is thrown up and out with the vertical velocity component Vv = 5m/s, how fast will it be going vertically when it passes your hand on the way back down?
     
    Last edited: Sep 17, 2006
  4. Sep 17, 2006 #3
    With your help (thank you very much!) my new answers are:

    1. a -- the blue ball has the faster speed

    2. b -- the red ball has the highest inital velocity

    5. b -- the blue ball hits the ground with the highest speed

    10. a -- ball A

    22. b

    23. c

    31. a

    did i get it, or am i still way off? :(
     
  5. Sep 17, 2006 #4

    berkeman

    User Avatar

    Staff: Mentor

    -2- Look again. Which ball travels the farthest horizontally?

    -23- c is correct, but I also think a is correct with no air resistance. I'm not sure about that, though. If you only can choose one, c is the best bet.

    -31- Still not correct. Read my hint again please.

    Getting closer -- good work. Welcome to PF, BTW. Interesting handle also. You have other kitty friends in PF (not necessarily me however). Check out this recent thread with lots of kitty folks:

    https://www.physicsforums.com/showthread.php?t=130472
     
  6. Sep 17, 2006 #5
    hehe, thanks for the link, the welcome, and the help! I have a test on Wednesday -- have been out for a death in the family, so I'm trying to catch up without exhausting myself. I really really appreciate the guidance! I look forward to talking with you again, soon! Have a great night!
     
  7. Sep 17, 2006 #6

    berkeman

    User Avatar

    Staff: Mentor

    Condolences on the family death, sythetickitty. Hey, I just realized that there's a typo in your new PF handle. Should you be synthetickitty? Or is that on purpose?
     
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