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Homework Help: Physics Review Packet, some problems i'm stuck on

  1. May 1, 2010 #1
    Just a few questions from a review packet i'm stuck on, any help would be appreciated!

    1) The moment of inertia about its axis is given as .5 mr^2. If this cylinder rolls without slipping the ratio of it's rotational kinetic energy to it's translational kinetic energy is
    a. 2:1
    b. 1:1
    c. 1:2 >My guess<
    d. 1:3

    I'm assuming Rotational Kinetic Energy = (.5mr^2)(w^2) has something to do with this

    2) An object’s angular momentum changes by 20 kg,m2/s in 4.0 s. What magnitude average torque acted on this object?
    A. 2.5 N’m >my guess<
    B. 5.0 N-m
    C. 40 N-m
    D. 80 N’m

    I'm not sure what equation i'm supposed to use

    3) A solid cylinder of mass 10 kg is pivoted about a frictionless axis thought the center O. A rope wrapped around the outer radius R1 = 1.0 m, exerts a force F1 = 5.0 N to the right. A second rope wrapped around another section of radius R2 = 0.50 m exerts a force F2 = 6.0 N downward. (See Fig. 8-1.) What is the angular acceleration of the cylinder?
    (It's like a circle inside a circle)
    A) 1.0 rad/s2
    B) 0.60 rad/s2 >my guess<
    C) 0.40 rad/s2
    D) 0.80 rad/s2

    I did angular acceleration = ((R1F1)-(R2F2))/mr1^2 and the closest i got was to .6

    4) A solid sphere of mass 1.0 kg and radius 0.010 m starts from rest and rolls without slipping down a 1.0-m high inclined plane. What is the speed of the sphere when it reaches the bottom of the inclined plane?
    A) 3.7 m/s >My guess<
    B) 4.4 m/s
    C) 5.6 m/s
    D) 6.3 m/s

    my final equation was gh=(7/10)v^2

    5)A railroad car with a mass of 200 kg is going at 10 m/s. A 70 kg stunt man drops directly down on it from a distance of 4 m above. What is the speed of the railroad car after?
    a. 4.7 m/s
    b. 7.4 m/s >my guess<
    c. 2.8 m/s
    d. 10 m/s

    Totally lost

    6) A projectile is launched with an initial velocity of 60 m/s at an agnle of 30 degrees above the horizontal. How far will it go?
    a. 318 m
    b. 160 m >my guess<
    c. 152 m
    d. 184 m

    I feel like this should be simple, and I'm using 2d kinematics but the answers I get don't match up
    Last edited: May 1, 2010
  2. jcsd
  3. May 1, 2010 #2
    1) What is the translational kinetic energy? What is v in terms of w?

    2) Similar to Newton's Second Law, [tex] \vec{F}_{net} = \frac{d\vec{p}}{dt} [/tex] , What does [tex] \tau_{net} = [/tex] ?

    3) I don't really understand the picture.

    4) Right.

    5) Conservation of Momentum.

    6) Show your work.
    Last edited: May 1, 2010
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