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a bob of mass m is tied by a massless string whose other end portion is wound on a fly wheel (disc) of radius r and mass m. When released from rest the bob starts falling vertically. When it has covered a distance of h, the angular speed of the wheel will be :

Question

a bob of mass m is tied by a massless string whose other end portion is wound on a fly wheel (disc) of radius r and mass m. When released from rest the bob starts falling vertically. When it has covered a distance of h, the angular speed of the wheel will be :

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Solution

The problem involves the conservation of mechanical energy. Initially, the system has potential energy due to the height of the bob, and finally, when the bob has fallen a distance h, the system has kinetic energy due to the rotation of the flywheel and the falling of the bob.

  1. Initial Energy (Potential Energy): The initial energy of the system is the potential energy of the bob which is given by mgh (mass * gravity * height).

  2. Final Energy (Kinetic Energy): The final energy of the system is the kinetic energy of the bob and the flywheel. The kinetic energy of the bob is (1/2)mv² and the kinetic energy of the flywheel is (1/2)Iω², where I is the moment of inertia of the flywheel and ω is the angular speed. The moment of inertia of a disc (flywheel) is (1/2)mr².

  3. Conservation of Energy: By the conservation of energy, the initial energy equals the final energy. So, mgh = (1/2)mv² + (1/2)(1/2)mr²ω².

  4. Solve for ω: We can solve this equation for ω (the angular speed of the wheel). The velocity v of the bob is related to the angular speed ω by v = rω. Substituting this into the equation gives mgh = (1/2)m(rω)² + (1/2)(1/2)mr²ω². Simplifying this equation gives ω = sqrt((4gh)/(3r)).

So, the angular speed of the wheel when the bob has fallen a distance h is sqrt((4gh)/(3r)).

This problem has been solved

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