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High School Level Physics

  1. Sep 13, 2009 #1
    Hello everyone,

    this is my first post on the forum.
    I'm well...a dunce and not too bright, but I'm really interested in science and want to do well at it as well as at school. These are two questions from a textbook I borrowed from the library. I can't ask my teacher because it is not the prescribed text (i've done these and they have a reputation for being too easy) and he was one of the authors of the prescribed book itself.


    1. The problem statement, all variables and given/known data

    1) Cassie starts from rest at the top of a 3.2 m long
    playground slide and slides to the bottom with a constant
    acceleration. If she takes 2.4 s to reach the bottom,
    calculate:

    a her average speed
    b her average acceleration
    c her final speed
    d her speed when she is halfway down the slide.

    2) Vinh is investigating the bouncing ability of a golf ball and a
    tomato. He drops both objects from a height of 2.00 m
    and measures the rebound heights. He found that the golf
    ball rebounded to 1.50 m and the tomato just splattered
    without rebounding at all. Ignore air resistance when
    answering these questions.

    a What was the speed of the golf ball just before it
    landed?
    b What was the speed of the tomato just before it hit
    the ground?
    c Calculate the speed of the golf ball as it rebounded.
    d Which object had the greater change in speed as it landed? Calculate the speed change of this object.
    e Which of these objects experienced the greater
    change in velocity as it landed? Calculate the velocity
    change of this object.

    2. Relevant equations

    Maybe the equations of motion which can be derived
    from the definitions for velocity and
    acceleration.
    a = change in v over t
    By rearrangement, at = Δv, so that
    at = v − u, or v = u + at
    Also, v(subscript) av = x/t , or x = v(subscript)av(normal script)t
    Substituting for v(sub)av,
    x = (u + v)t
    and substituting v = u + at for v:
    x = (1/2)(u + u + at)t
    x = ut + at^2

    3. The attempt at a solution

    The first question 1a) I worked out to 1.33m/s (distance over time). This was the right answer.
    parts b,c,d I did not know what to do at all.

    as for question 2a:

    9.8 (gravity acceleration) x time^2 = 2
    s^2 = 0.204
    s=0.451
    9.8/0.451 = 21.73

    but the book answer was: 6.26 m/s

    as for question 2b

    I know that there was the experiment of dropping a bowling ball over that tower as well as a feather experiment so they would both have the same speed = 6.26m/s. this is correct if you allow for consequential errors.

    I didn't know anything for question part c
    whilst part d I knew would be the tomato and part e the ball
    I don't know more than that though.


    I hope someone has time to help me. I would really appreciate it. thank you
     
  2. jcsd
  3. Sep 13, 2009 #2

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Welcome to Physics Forums :smile:

    For 1c, it is useful to know that the average velocity is the average of the initial and final velocities.

    For 2a,

    Actually this should be

    (1/2) (9.8 m/s^2) t^2 = 2.00 m​

    where t is the time.

    -----

    p.s. for subscripts, try [noparse]vav[/noparse] for vav
     
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