A metal disk of radius 4.0 cm is mounted on a frictionless axle. Current can flow through the axle out along the disk, to a slid
ing contact on the rim of the disk. A uniform magnetic field B= 1.25 T is parallel to the axle of the disk. When the current is 5.5 A, the disk rotates with constant angular velocity. What's the frictional force at the rim between the stationary electrical contact and t?
This can be solved using momentum balance, since momentum is conserved, the momentum at point 1 is equal to the momentum of point 2. momentum = mass x velocity m1v1 = m2v2 (0.03kg x 900 m/s ) = 320(v2) v2 = 27 / 320 v2 = 0.084 m/s is the speed of the astronaut
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