A little block with mass m rest on a frictionless horizontal table, at a distance r from a hole in the center of the table and is tied by an unelastic cord. The cord is tied by the other edge to another block with mass M . A constant circle motion is given to the little block with a radius r and speed v . Wich v is needed for the big block to stay still once you don't hold it?
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Since we have an unelastic cord we know that the centripetal acceleration is equal to gravity acceleration, so a c = g = r v 2 Then we have a force F that is given by the big block mass and g , and that force pulls the little block, and have g M = F a c = r v 2 = M F And finally v = m F r