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Basic Physics - Spinning Head (mine)

  1. Oct 26, 2004 #1
    Hello again all. This is my 6th posting requesting some assistance for some new physics problem. It is getting a bit easier to understand but I would like some review and comments on the following problems. My answers are marked with an astericks. You do not have to necessarily provide me the answers. Please tell me which ones are correct and for those that are not some direction as to where to focus. Thanks for your time and comments.

    1. What is the rotational speed of the minute hand on a clock?
    a. 0.000278 rev/s*
    b. 0.00175 rev/s
    c. 0.0167 rev/s
    d. 0.105 rev/s

    2. A child with a mass of 25 kg is riding on a merry-go-round. If the child has a speed of 3 m/s and is located 2 m from the center of the merry-go-round, what is the child's angular momentum?
    a. 50 kg·m2/s
    b. 75 kg·m2/s*
    c. 150 kg·m2/s
    d. 300 kg·m2/s

    3. Newton's first law for rotational motion states that an object will maintain its state of rotational motion unless acted on by an unbalanced (or net):
    a. force
    b. velocity
    c. inertia*
    d. torque

    4. A child with a mass of 20 kg sits at a distance of 2.0 m from the pivot point of a seesaw. Where should a 16-kg child sit to balance the seesaw?
    a. 1.6 m*
    b. 2.0 m
    c. 2.5 m
    d. 3.2 m

    5. A solid sphere and a solid cylinder are made of the same material. If they have the same mass and radius, which one has the larger rotational inertia about its center?
    a. the sphere
    b. the cylinder
    c. both the same
    d. There is not enough information to say*

    6. A fully-loaded trailer truck is less stable than a race car because the truck:
    a. is more massive
    b. has a higher center of mass*
    c. has bigger wheels
    d. weighs more



    7. If you face a wall with your toes against the baseboard, you find that you cannot stand up on your toes because:
    a. you cannot generate the necessary torque
    b. your rotational inertia is too large
    c. your center of mass is too high
    d. your center of mass cannot move forward over your toes*

    8. An astronaut "floating" in Skylab has an initial rotational motion but no initial translational motion relative to Skylab. She continues to rotate because she:
    a. experiences no net force*
    b. experiences a net force that acts through her center of mass
    c. is weightless
    d. experiences a torque due to the force of gravity

    9. A spinning gyroscope points directly at the North Star when it is located at the North Pole. If it is transported to the South Pole without exerting any torques on it, which way will it point?
    a. up
    b. down*
    c. horizontally

    10. Assume that Gerry sits on a freely rotating stool holding a bicycle wheel with its axle vertical so that it rotates in a clockwise direction when viewed from above. If Gerry turns the wheel over, he will:
    a. not rotate because the system of wheel and Gerry is closed or isolated
    b. not rotate because the two torques cancel
    c. rotate clockwise because angular momentum is conserved*
    d. rotate counterclockwise because angular momentum is conserved
     
  2. jcsd
  3. Oct 26, 2004 #2

    krab

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    Your grade is 6 out of 10.
     
  4. Oct 26, 2004 #3

    arildno

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    PhysicsNovice:
    If I remember correctly, you educate yourself with an on-line physics program.
    That's great!
    But what would really be educational for you, is if you post your own thoughts on why you think your answer is correct.

    Learning physics is first and foremost about developing good ways of thinking physics;
    not primarily to apply rigid formulas to solve artificial problems correctly.

    If you develop your own thoughts in your posts, that's what you'll get responses to, and you'll learn a lot more.
     
  5. Oct 27, 2004 #4
    Thanks Krab for the review of my problems. Good you provide some insight please. Which ones are correct and those that are not some guidance (i.e. a formula, physics law or theory, etc.) Thanks again for reviewing.
     
  6. Oct 27, 2004 #5
    Thanks Arildo. That is a good idea. Your memory is correct about the on-line course I am taking on basic physics. In my previous threads I have posted the questions and my answer first. Then viewers would comment on the problems telling me the ones that are correct so I know that my thinking was obviously correct. The incorrect ones I would get suggestions on how to solve (especially when Doc Al reviewed). From there I would express my thinking on how I derived the first incorrect answer followed by new answer. This way just saved a lot of detailed explanations at the beginning. Again, your suggestion is right on target. I do get the most out of this site when I write out my thought process to solving problems. Thanks again.
     
  7. Oct 27, 2004 #6

    arildno

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    Doc Al is very nice! :smile:

    Well, could you write just a few comments on:
    3,4,5,6 for starters?
     
  8. Oct 27, 2004 #7
    Hey Arildo. Here we go. #3 - I do want to change my answer to (a) force. The reason is that in all of the readings and examples given on-line objects in motion tend to stay that way unless acted upon by a force. Forces seem to be the initiator of maintaining velocity, changes in velocity (acceleration), changes in direction, etc. #4 Of all the problems this is the one I thought I was most sure was correct. Logic made me think that if the 20 kg child was sitting 3 m away from the fulcrum then a 16 kg child would have to sit closer than 3 m. The only answer then would be (a) 1.6 m And if I drew the see-saw with x being the unknow distance I stated that 20 kg is to 3 m as 16 kg is to x. Therefore, 20x = 32 or x = 1.6 #5 This question deals with rotational inertia. I thought the factors affecting RI are the velocity of rotation and the mass of the rotating object. If both the cylinder and sphere have the same radius length then we know that the sphere has a certain length (2r) but the sphere could be any length...oops I just re-read the question and it states that the sphere and the cylinder have the same mass so the length of the cylinder doesn't matter. I change my answer to (c) both the same. #6 I read the question again and I am not sure. When I first answered I was thinking about the car and truck going around a turn and how the force acting on the truck with a higher center of gravity would be unstable. I do not think this is what is meant. I am not sure. Well, if I get the first three correct then you are not only right about this approach I think that I should take my time and do this before posting the problems. How did I do?
     
  9. Oct 27, 2004 #8

    arildno

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    "#3 - I do want to change my answer to (a) force. The reason is that in all of the readings and examples given on-line objects in motion tend to stay that way unless acted upon by a force. Forces seem to be the initiator of maintaining velocity, changes in velocity (acceleration), changes in direction, etc. "
    Your last sentence is a very important observation about force!
    However, answering (a) is untrue.
    It is the net TORQUE an object experiences which causes rotational acceleration; in the absence of a net TORQUE, an object maintains its state of rotational motion.

    I'll look into the others in a while..
     
  10. Oct 27, 2004 #9

    krab

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    Think again, and think of the limitting case. If the child has very very small mass, he should sit very near the fulcrum? I think not!
     
  11. Oct 28, 2004 #10
    Krav, thanks. I can not even use logic correctly. I understand completely that a child of less mass would have to sit further away from the pivot point on the see-saw. Again, using the data furnished in #4...a child of mass 20kg sits 2m away from a pivot point of a see-saw. A child of 16kg would have to sit x meters away...20kg x 2m = 16kg x ?...(20)(2) = (16)x...40 = 16x...40/16 = 16x/16...2.5 = x...Therefore, (c) is correct. Thanks.
     
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