1.

A frame of reference that is accelerated with respect to an inertial frame of reference is called a non-inertial frame of reference. A coordinate system fixed on a circular disc rotating about a fixed axis with a constant angular velocity `omega` is an example of non=inertial frame of reference. The relationship between the force `vecF_(rot)` experienced by a particle of mass m moving on the rotating disc and the force `vecF_(in)` experienced by the particle in an inertial frame of reference is `vecF_(rot)=vecF_(i n)+2m(vecv_(rot)xxvec omega)+m(vec omegaxx vec r)xxvec omega`. where `vecv_(rot)` is the velocity of the particle in the rotating frame of reference and `vecr` is the position vector of the particle with respect to the centre of the disc. Now consider a smooth slot along a diameter fo a disc of radius R rotating counter-clockwise with a constant angular speed `omega` about its vertical axis through its center. We assign a coordinate system with the origin at the center of the disc, the x-axis along the slot, the y-axis perpendicular to the slot and the z-axis along the rotation axis `(vecomega=omegahatk)`. A small block of mass m is gently placed in the slot at `vecr(R//2)hati` at `t=0` and is constrained to move only along the slot. The net reaction of the disc on the block isA. (a) `1/2momega^2R(e^(2omegat)-e^(-2omegat))hatj+mghatk`B. (b) `1/2momega^2R(e^(omegat)-e^(-omegat))hatj+mghatk`C. (c) `-momega^2Rcosomegathatj-mghatk`D. (d) `momega^2Rsinomegahatj-mghatk`

Answer» Correct Answer - B
`vecF_(rot)=vecF_(i n)+2m(V_(rot)hati)xxwhatk+m(whatkxxrhati)xxwhatk`
`:.` `mromega^2hati=vecF_(i n)+2mV_( rot) omega(-hatj)+momega^2rhati`
`vecF_(i n)=mrV_(rot)omegahatj` -(i)
But `r=(R)/(4)[e^(wt)+e^(-wt)]`
`:.` `(dr)/(dt)=V_r=(R)/(4)[omegae^(wt)-omegae^(-wt)]` -(ii)
From (i) and (ii)
`vecF_(i n)=2m(Romega)/(4)(e^(wt)-e^(-wt))omegahatj`
`:.` `vecF_(i n)=(mRomega^2)/(2)(e^(wt)-e^(-wt))hatj`
`:.` `vec F_(reaction)=(mRomega^2)/(2)(e^(wt)-e^(-wt))hatj+mghatK`


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