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Homework Help: Projectile physics help

  1. Dec 9, 2012 #1
    1. The problem statement, all variables and given/known data

    A 5-kg projectile is fired over level ground with a velocity of 200m/s at an angle
    of 25◦ above the horizontal. Just before it hits the ground its speed is 150m/s.
    Over the entire trip the change in the internal energy of the projectile and air is:
    A. +19, 000 J
    B. −19, 000 J
    C. +44, 000 J
    D. −44, 000 J
    E. 0

    I calculated the answer and i got 44000, but i dont know whether its (+) or (-).
    the answer says its +44000 (C).

    A 0.75-kg block slides on a rough horizontal table top. Just before it hits a horizontal ideal spring its speed is 3.5m/s. It compresses the spring 5.7 cm before
    coming to rest. If the spring constant is 1200N/m, the internal energy of the block
    and the table top must have:
    A. not changed
    B. decreased by 1.9J
    C. decreased by 2.6J
    D. increased by 1.9J
    E. increased by 2.6J

    I got the answer of 2.6J, but again, i dunno whether its (+) or (-).

    the answer says its -2.6J (B)

    33. A block of mass m is initially moving to the right on a horizontal frictionless surface at a speed v. It then compresses a spring of spring constant k. At the instant when the kinetic energy of the block is equal to the potential energy of the spring, the spring is compressed a distance of:
    A. v√(m/2k)
    B. (1/2)mv2
    C. (1/4)mv2
    D. (mv2)/(4k)
    E. (1/4)√(mv/k)

    my answer (not listed in choice?):
    x = v√(m/k)

    61. A uniform disk, a thin hoop, and a uniform sphere, all with the same mass and same outer
    radius, are each free to rotate about a fixed axis through its center. Assume the hoop is
    connected to the rotation axis by light spokes. With the objects starting from rest, identical
    forces are simultaneously applied to the rims, as shown. Rank the objects according to their
    angular accelerations, least to greatest.

    A. disk, hoop, sphere
    B. hoop, disk, sphere
    C. hoop, sphere, disk
    D. hoop, disk, sphere
    E. sphere, disk, hoop

    answer is D, but why is that so? i know the hoop has the greatest rotational inertia, but how is I inversely proportional to angular acceleration?

    is it because of Torque = I x ∞?

    A disk starts from rest and rotates around a fixed axis, subject to a constant net torque. The work done by the torque during the second 5 s is ______ as the work done during the first 5 s.
    A. the same
    B. twice as much
    C. half as much
    D. four times as much
    E. one-fourth as much

    I dont understand this question at all, but answer is D.
  2. jcsd
  3. Dec 9, 2012 #2


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    You need to know what "internal energy" means. Do you know?

    How did you get that 44000 J?
    The projectile loses mechanical energy, where does that energy go?
    The book is correct.

    Again, it is the question, what is internal energy.

    Think what happens when you rub your hands together when they are cold? Where does the mechanical energy go when an object is subjected to friction?

    You got x=v2 √(m/k), but you have to give x in terms of the original speed, v. How are v2 and v related? What is the speed if the kinetic energy is half the initial one?

    If ∞ is the angular acceleration, yes. Denote it with alpha. τ(torgue)=I*α.

    How do you get the work in case of rotation? And how do you get the angular displacement in case of constant angular acceleration?

    The book's answer is not correct again, I am afraid.

  4. Dec 9, 2012 #3
    Umm no i dont know what internal energy is, can you please explain?
    I just did 1/2(5)(1502-2002) = -43750J
    but why is the answer positive?

    the energy turns into heat? but does it go into the surroundings?

    and why is this answer negative and not positive?

    ummmm i cant think of it :S

    work = F*r*d?

    θ = 1/2(α)(Δt)2?
  5. Dec 9, 2012 #4


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    In 1), you calculated the difference between the final and initial kinetic energy. It is negative, so mechanical energy is lost. Yes, it turns into heat and it warms up the surroundings, but it warms up the projectile, too.
    Internal energy of an object is the energy of the random motion of its molecules/atoms. The molecules have 1/2 mv^2 translational kinetic energy, but can have also rotational and vibrational energies. The average energy per degrees of freedom is kAT, kA is the Boltzmann constant and T is the absolute temperature.

    When you rub your cold palms together, your work against friction increases the kinetic energy of the molecules of your hand, rises the internal energy, and you feel that your hands warmed up.

    In the problem, the surroundings and the projectile or the block gained energy, equal to the lost mechanical energy. The question was the change of internal energy. It increased. So the sign is ...?
    In problem (2) friction will increase the total internal energy again.

    In problem 3, The new kinetic energy is equal to the potential energy, so how is it related to the initial kinetic energy? Energy is conserved.

    The work in case of rotational motion is Frθ (torque multiplied with angular displacement). The second equation is correct. Calculate the work during the first 5 seconds and then the work during the first 10 seconds.
    What is the work then during the second 5-s interval?

  6. Dec 9, 2012 #5
    so is the answer for #2 wrong? answer says that it decreases internal energy.

    and mu prof just replied to my email, and he said that the ans for both ques is that internal energy decreases in both cases? now im really confused... but your answer makes so much sense.

    the new kinetic energy is half of the initial kinetic energy?

    OHHH i get it now.

    i just use (1/2)(1/2mv2) = 1/2kx2

    thanks so much :smile:
    work during first 5: (25/2)τ*α
    work during last 5: (75/2)τ*α

    its 3 times as much work? but theres no 3 in there...
  7. Dec 9, 2012 #6


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  8. Dec 9, 2012 #7
    Thanks so much! I feel like I know so much more now. Hope I do well on the exam tmr! :)
  9. Dec 9, 2012 #8


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    I hope your prof will think the same.:smile:

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