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Homework Help: Physics Final-Multiple Problems-Help

  1. May 1, 2009 #1
    Physics Final---Multiple Problems---Help

    Hey everyone, I have my physics final on Monday and I'm working through a practice test right now that is giving me a ton of trouble. If you can help with any/all of the problems i'd appreciate it.


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

    1)A 74 kg bungee jumper is tied to a 39 m cord (unstretched). She leaps (from rest) off a
    tall bridge (much taller than 39 m), so that she falls freely for 39 m before the cord begins
    to stretch. The stretched cord stops her briefly above the water before yanking her back
    upward. What is the magnitude of the impulse exerted on the bungee jumper while the
    cord stretches during the downward motion?

    Answer: 2046 kg m^2


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

    10)A weight of Mg = 50 N is attached to the free end of a light string wrapped around a
    pulley of radius R = 0.25 m and mass m = 3 kg. The pulley is free to rotate frictionlessly
    in a vertical plane about the horizontal axis passing through its center, but remember that
    the pulley is a solid disk and is not massless. The weight is released from rest 5 m above
    the floor. The speed of the weight when it reaches the floor is:

    Answer 8.7 m/s

    I tried using the formula T = Fperpd = (I)(alpha) I wanted to find alpha, change it to linear acceleration and then plug that acceleration into a standard kinematic to find Vfinal. No such luck.


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

    11) A car of mass 1000 kg moves on a circular track of radius 100 m with a speed of 50
    m/s. What is the magnitude of its angular momentum (in kg m2/s) relative to the center of the race track?

    Answer: 5 x 10^6

    I tried using the formula Angular Momentum = (I)(angular velcoity) and manipulating some of the variables around. I couldn't make it work.


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

    12) Two 1 kg particles are connected by a string 1 m long. They are set spinning about
    their center of mass with an angular velocity of 2 rad/s in outer space, where external
    forces are negligible. The tension in the string is:

    Answer: 2N


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

    13) A disk of radius R and mass M has a moment of inertia given by I = ½ MR2 for
    rotations about an axis through the center of the disk. If a second disk of twice the mass
    and half the radius is added as shown in the figure, what is the new moment of inertia of
    the combined disks?

    Answer: 3/4 MR^2


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

    A spherical asteroid of mass Mast = 1016kg and radius Rast = 10 km (moment of inertia
    Iast = 4×1023kg m2) is slowly rotating in free space at an angular velocity of ωi =
    +6.28×10−4rad/s (one revolution every 10,000 seconds) around an axis through its center
    of mass. A small comet of mass mc = 1011kg and traveling at vi = 100,000 m/s hits the
    asteroid at a glancing blow, in a direction directly opposing the motion of that part of the
    asteroid, as described in the figure. If the comet sticks to the edge of the asteroid (embeds
    itself in it), what is the final angular velocity of the composite object?

    Answer: +3.78 × 10−4 rad/s (same rotation direction)


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

    A mass m = 0.1 kg is attached to a massless spring of constant k = 10 kg/s2. The other
    end of the spring is attached to a pivot point about which the spring is free to rotate
    frictionlessly on a tabletop. The mass also moves frictionlessly on the tabletop. The
    unstretched spring length is r = 20 cm. The mass is set to rotate at a constant angular
    velocity, which stretches the spring to a new length R = 25 cm, but the spring does not
    oscillate. What is the angular momentum of the mass about the pivot point?

    Answer: 0.028 kg m2/s


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

    A 3 kg air-track frictionless glider is attached to each end of the track by two coil
    springs. It takes a horizontal force of 0.9 N to displace the glider to a new equilibrium
    position, x = 0.3 m. What is the period of oscillations about the equilibrium point?

    Answer: 6.28 s


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

    The Earth has a mass mE = 6×1024 kg, and orbits around the Sun (mass mS=2×1030kg)
    along an approximately circular orbit of radius R=1.5×1011m. What is the total
    mechanical energy (K+U) of the Earth, in reference to the Sun? Take U=0 at infinity.
    (Remember: 1 year = 3.16×107s, G = 6.67×10-11 N m2/kg2; Hint: the circumference of a
    circle is 2πR)

    Answer: −2.7×1033 J
    Last edited: May 1, 2009
  2. jcsd
  3. May 3, 2009 #2
    Re: Physics Final---Multiple Problems---Help

    Perhaps too late, but better late than never.

    In the first assignment. What is the momentum of the jumper when when the rope begins to tighten? Since you know that the jumper eventually stops moving at the bottom, you can solve the impulse.

    In the second assignment you will save alot of work by simply using the rule of conservation of energy. What forms of energy are transformed into what?

    Work on those, and see where you end up.
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