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Blazing goIITian

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15 Apr 2009 14:04:49 IST

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Rotation queries ...

Kinematics , NewtonsLawsCirMotion , Work Power Energy collisions , Rotation , Gravitation , Simple Harmonic Motion , Fluid Mechanics , Materials Wave Sound , Super Position , units dimensions , com momentum , shm , wave sound

Q1 . Three thin rods, mass=m , length = l .....lie along axes x,y,z . . . . . . . .wwith one end each at the origin , the MoI about the z-axis for the system ??

Q2. A ball is rolling on a rough horizontal surface , the force of fricn tries to ?? . . .

1) increase/decrease/do nothing to the linear velocity

2) increase/decrease/do nothing to the angular velocity

3) increase/decrease/do nothing to the linear momentum

4) increase/decrease/do nothing to the angular momentum . . .

can some1 please provide explanation for these 2 ques ...

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Comments (6)

VARUN RAJ

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15 Apr 2009 18:23:35 IST

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q2) answer is d as angular momentum is constnat abt the bottommost pt

pritam majumdar

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15 Apr 2009 18:50:14 IST

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for z axis,acording to theorem of perpendicular axis the MOI will be sum of MOI of first rod and MOI of second rod through the axis passing through one of its ends.that willbe ML²/3 + ML²/3= 2ML²/3.

for the next question ,in arough surface the friction will provide the necesassary torque and torque =I 9angular accn.).so it will increase the angular accn.and hence the angular velocity

Siddhartha Gupta

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16 Apr 2009 11:13:22 IST

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varun , i don't think u read the ques properly . . . its not an MCQ . . .i want an answer for all 4 choices with explanations ............ @pritam , the answer for the first one is correct :) but what abt the third rod lying abt the z-axis?also , the ans for the 2nd ques is ....fricn decreases the linear velocity ...but i have no idea wat it does to ang vel , ang acc & ang mom .......................

Prakhar Banga

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17 Apr 2009 11:19:19 IST

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There will not be any Moment of Inertia of the rod lying along the z axis about the z axis, because it is thin and the distance of every element of the rod from the axis is 0.

As for the second question, the ball is rolling on a rough horizontal surface. The velocity of the point of contact is zero and no friction acts because there is no relative motion anywhere. So the friction doesn't try to change anything. However, the case of a ball rolling on an inclined plane would have been very different.

NugoRama

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17 Apr 2009 11:22:21 IST

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1st ques::

3 rods ... zero for the one lying along z axis ..and ml^2/3 for the other two...

tht makes moi for system equal to ml^2/3 ..

NugoRama

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17 Apr 2009 11:25:04 IST

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for second question::

it does nothing to the angular momentum ...it remains constant..

it may inc/dec/remain constant both the lineaar speed as well as angular speed ..there fore as well as the linear momentum.

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