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Homework Help: AP Physics B Test Packet Assistance

  1. Apr 28, 2008 #1
    In preparations for the AP Physics B test, my physics teacher has assigned our class a previous years AP multiple choice section test. I've managed to work through 80% of it with little difficulty, I've just gotten to the questions where I have no clue to how to answer. If I am allowed, I am going to post the questions which I haven't been able to answer, verbatim. I'd appreciate it if anyone who knew the answer could please tell me the correct answer and how to arrive at it.Some of the questions are into to physics material, but most of them lay in the advanced physics. I would appreciate any help in this. :)

    9.An empty sled of mass M moves without friction across a frozen pond at speed Vo. Two objects are dropped vertically into the sled one at a time: First an object of mass m and then an object of mass 2m. Afterward the sled moves with speed Vf. What would the final speed of the sled be if the objects were dropped into it in reverse order?
    A)Vf/3 B)Vf/2 C)Vf D)2Vf E)3Vf

    18.The operating efficiency of a .5A, 120V electric motor that lifts a 9kg mass against gravity at an average velocity of .5m/s is most nearly
    A)7% B)13% C)25% D)53% E)75%

    39.A spring scale calibrated in kilograms is used to determine the density of a rock specimen. The reading on the spring scale is 0.45 kg when the specimen is suspended in air and 0.36kg when the specimen is fully submerged in water. If the density of water is 1000 kg/m^3, the density of the rock specimen is
    A)200 B)800 C)1250 D)4000 E)5000

    43.A simple pendulum and a mass hanging on a spring both have a period of 1s when set into small oscillatory motion on Earth. They are taken to planet X, which has the same diameter as Earth but twice the mass. Which of the following statements is true about the periods of the two objects on planet X compared to their periods on Earth?
    A)Both are shorter B)Both are the same C)Both are longer D)The period of the mass on the spring is shorter,; that of the pendulum is the same E)The period of the pendulum is shorter; that of the mass on the spring is the same

    Two large, flat, parallel, conducting plates are .04m apart, as shown above. The lower play is at a potential difference of 2V with respects to the ground, the upper play is at a potential difference of 10V with respects to the round. Point P is located .01 m above the lower plate.
    45.The electric potential at point P is
    A)10V B)8v C)6V D)4V E)2V

    46.The magnitude of the electric field at point P is
    A)800 B)600 C)400 D)200 E)100

    58.When B^10 is bombarded by neutrons, a neutron can be absorbed and an alpha partical(He^4) emitted. If the B^10 target is stationary, the kinetic energy of the reaction products is equal to the
    A)kinetic energy of the incident neutron B)Total energy of the incident neutron C)Energy equivalent of the mass decrease in the reaction D)Energy equivalent of the mass decrease in the reaction, minus the kinetic energy of the incident neutron E)Energy equivalent of the mass decrease in the reaction, plus the kinetic energy of the incident neutron

    60.A 50,000W radio station transmits waves of wavelength 4m. Which of the following is the best estimate of the number of photons it emitts per second?
    A)10^8 B)10^22 C)10^30 D)^40 E)10^56

    *Image of a circular loop of wire with a horizontal axis through its diameter, with an upward(in regards to the page) electric field*
    64.A wire loop is roatated in a uniform magnetic field about an axis perpendicular to the field. How many times is the induced current in the loop reversed if the loop makes 3 complete revolutions from the position shown.
    A-1 B-2 C-3 D-6 E-12

    67.A satellite of mass m and speed v moves in a stable, circular orbit around a planet of mass M. What is the radius of the satellite's orbit?
    A)GM/mv B)Gv/mM C)GM/v^2 D) GmM/v E)GmM/v^2

    70.A hollow metal sphere 1.0 m in diameter carries a charge of 4.0 μC. The electric field at a distance of 2.0m from the center is most nearly
    A)9000N/C B)18000 N/C C)24000N/C D)36000 N/C E)140000N/C

    65.A particle of charge Q and mass m is accelerated from rest through a potential difference V, attaining a kinetic energy K. What is the kinetic energy of a particle of charge 2Q and mass m/2 that is accelerated from rest through the same potential difference?
    A)K/4 B)K/2 C)K D)2K E)4K

    63.Two blocks of masses M and 2M initially travel at the same speed v but in opposite directions. They collide and stick together. How much mechanical energy is lost to other forms of energy during the collision?
    A)Zero B)1/2Mv^2 C)3/4Mv^2 D)4/3Mv^2 E)3/2Mv^2

    69.A positively charged particle moves to the right without deflection through a pair of charged plates. Between the plates are a uniform electric field E of magnitude 6.0 N/C directed downward and a uniform magnetic field B of magnitude 2.0T directed downward through the page. The speed of the particle is most nearly
    A).33 m/s B).66 m/s C)3.0 m/s D)12 m/s E)18 m/s
  2. jcsd
  3. Apr 28, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Welcome to the forums. Please note that for homework/coursework type questions you are required to show some work before we can help you. So, which question would you like to start on? Have you had an attempt at any of them?

    Also, note I've moved this to intro physics.
  4. Apr 28, 2008 #3
    I've already gotten most of the packet(90 questions) completed, with either correct or what I believe to be correct answers. My only problem with these are I just don't know where to start, or I just don't have the correct formulas. We weren't given any material for this, and many of the questions were just basic overviews of the subjects they were pertaining to. But, for the sake of the forum's guidelines, I'll post whatever work I can find on these questions.

    9.My first guess to this is C since the sled moves on a frictionless track. My reasoning is because the total end mass is the same, which should yield the same speed. My only problem with this is that I've seen this problem before, and that reasoning was proven wrong, so I'm not entirely conclusive about this.

    18.I'm familiar with electric currents and mechanical engines, but I don't have any formulas that connect the two for this problem.

    39.Another concept I'm familiar with(i.e. I have a page of notes on) however I can't recall a single class that compares spring weights to density, even though if I spent a few hours I could manage something.

    43.My guess would be A since the only difference is the value of g, but like the sled, this problem has appeared before to be a trick question, leading me to second guess myself.

    45.No idea, whatsoever.

    46.I could answer this question quite easily, the only problem is that it depends on the answer to 45, which I'm at a loss for.

    58.....*Curls up in a ball*

    60.Our class hasn't even mentioned photons, so I have no clue here.


    67.I have all the gravitational formulas for this, but the only problem is that I'm missing a certain variable in order to use algebra to find r. if anyone knows the correct formula, please fill me in.



    63.I think it is A, but my trick question alarm is ringing in the back of my head.

    69.See 58
  5. Apr 28, 2008 #4


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    Staff Emeritus
    Science Advisor

    Find a definition for efficiency, which can be expressed as the ratio of (work output)/(energy input), or it could be (rate of work output)/(rate of energy input), where rate of energy/work is power.

    What is the rate of electrical power?

    What is the rate of work in lifting "9kg mass against gravity at an average velocity of .5m/s".
    Hint: force * distance = work/energy, and force * (distance/time) = power.

    I'd suggest starting at the top and work down.
  6. Apr 29, 2008 #5
    funny story.

    I got the same assignment. our teacher didn't really teach this year. she didn't even cover electricity/protons/charges/etc, which is about 1/3 of the test

    I could only answer 26 of the 70 questions. And I have always considered myself a good student, especially in the physics area before this year.

    Does anyone have the formulas for any of the problems mentioned above? I know how to figure them if I have a formula.
  7. Apr 29, 2008 #6


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    Staff Emeritus
    Science Advisor

    I don't think this is the best way for you to go about these problems. Why not start your own thread and go through the questions, one at a time, listing what you know about each one, and what formulae you have encountered in class?

    Also, please read the PF guidelines, especially regarding the homework forums.
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