12092006, 08:02 AM  #1 (permalink) 
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Physics Conceptual Questions
I need some help on some concept questions in a calculusbased physics class, its quite a few so I wont post them here. If anyone is willing to help, please email me at threewingedfury@hotmail.com THANKS!

12092006, 08:42 AM  #3 (permalink) 
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quite a few questions
ok cool  I just didnt think people would want to deal with so many questions lol
The displacement of an object oscillating on a spring is given by x(t)=xmcos(wt+p). If the initial displacement is zero and the initial velocity is in the negative x direction, then the phase constant p is: a. 0 rad b. pi/2 rad c. pi rad d. 3pi/2 rad e. 2pi rad I figured this one was d. Two uniform circular disks have the same mass and the same thickness are made from different materials. The disk with the smaller rotational inertia is: a. the one made from the more dense material b. the one made from the less dense material c. neither  both are the same d. the disk with the larger angular velocity e. the disk with the larger torque I thought this one was b. A hoop (I=MR^2) rolls with constant velocity and without sliding along level ground. Its rotational kinetic energy is: a. half its translational kinetic energy b. the same as its translational kinetic energy c. twice its translational kinetic energy d. four times its translational kinetic energy e. one third its translational kinetic energy This one, I have no clue. When a certain rubber band is stretched a distance x, it exerts a restroing force of magnitude F = ax+bx^2 where a and b are constants. The work done in stretching this rubber band from x = 0 to x = L is: a. aL^2 + bLx^3 b. aL + 2bL^2 c. a + 2bL d. bL e. aL^2/2 +bL^3/3 No clue on this one either. Vectors A and B have magnitude L. When drawn with their tails at the same point, the angle between them is 30 degrees. The value of A dot B is a. zero b. L^2 c. square root(3)L^2/2 d. 2L^2 e. none of these Vectors A and B have magnitude L. When drawn with their tails at the same point, the angle between them is 60 degrees. The value of A x B is a. zero b. L^2 c. square root(3)L^2/2 d. 2L^2 e. none of these I really hate this stuff  I just cant get a grasp of it. 
12092006, 09:10 AM  #4 (permalink) 
paranoid
Location: The Netherlands

On this forum we tend not to do anybodies homework outright, however we will try to help you understand the stuff.
Please put some work in explaining how you got the answers you put above, and also at what point the other questions elude you or where your specific troubles lie. Personally I can't help you, these questions go over my head. But if you explain in more detail what concepts you're having trouble with I'm sure other members can be of much more assistance.
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12092006, 09:11 AM  #5 (permalink) 
pigglet pigglet
Location: Locash

Really quick  on 1. the xmcos(wt+p)...define more clearly xm.
For the work question: W=integral(F.dx)L0. It's pretty straight forward. dot product = mag(a)mag(b)cos(theta). cross product is another vector. you can calculate its magintude to be a scalar quantity, but how you interpret the answers you have given is up to you. ie. none of them is a vector  so what does "value" of AxB mean? the other two i'll have to think about  it's been a while since i've looked at some of these formulae.
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12092006, 09:18 AM  #6 (permalink) 
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The problem just has x of m  Im thinking its between 0 and 2pi, maybe 0  but all I have in my book is : The value of the phase constant depends on the displacement and the velocity of the particle at time zero. So I dunno.
So are you just saying for the cross product, just multiply? 
12092006, 09:45 AM  #7 (permalink)  
pigglet pigglet
Location: Locash

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12092006, 10:56 AM  #9 (permalink)  
pigglet pigglet
Location: Locash

Quote:
note, for the cross product  the formula you gave is for the magnitude of the cross product, but that is not the same as the answer for A x B. A x B is a vector quantity.
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12092006, 01:22 PM  #11 (permalink)  
pigglet pigglet
Location: Locash

Quote:
you are given the function for the displacement : x(t) = x_m*cos(wt+p). you have the initial conditions. x(0)=0, dx/dt=k, where k is some unknown negative quantity. you can deduce two values of p (not considering periodicity) from the x(0)=0 condition. To determine which value it is, you consider the derivative of the displacement function, dx/dt. the derivative of the displacment function is the velocity. it is positive at one point, and negative at the other. therefore, you need to determine the sign of the derivative of x(t) in order to determine which value is the correct one for your problem.
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12092006, 04:15 PM  #14 (permalink)  
Tone.

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12092006, 04:34 PM  #15 (permalink)  
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^ but they are made of different materials, so the density would be different in them  does the density not matter?
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Last edited by threewingedfury; 12092006 at 05:50 PM.. Reason: Automerged Doublepost 

12092006, 06:39 PM  #16 (permalink)  
Tone.

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Ahhh I made a mistake. You are correct, it's b. I=1/2 mr^2 (this works for hoops and discs), so the less dense material, which has a larger radius, will have a greater rotational inertia. 

12092006, 08:25 PM  #17 (permalink)  
Junkie
Location: In the land of ice and snow.

Quote:
If rotational kinetic energy is .5 I w^2 (it seems like it should be) translational velocity is v translational kinetic energy is .5 m v^2 (v = wr) w = angular velocity = v/r (r = radius) rotational k.e. is then 0.5 mr^2 * (v/r)^2 = the key to your answer (provided i didn't mess things up) 

12092006, 08:54 PM  #18 (permalink)  
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are you trying to say they are the same?
because I get for the translational  .5m(v/r)r^2 and for the rotational .5mr^2(v/r)^2, which arent the same Quote:
Last edited by threewingedfury; 12092006 at 10:33 PM.. Reason: Automerged Doublepost 

12092006, 10:48 PM  #19 (permalink)  
Junkie
Location: In the land of ice and snow.

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12102006, 06:43 AM  #21 (permalink)  
pigglet pigglet
Location: Locash

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for the displacement, check this: problem i think you'll find something suspiciously similar to your problem there.
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