sir, i am very doubtful regarding a question as to how to take sections while deriving the center of mass of various bodies..?

ex: while considering hollow hemisphere we take section mentioning or varying with theta(angle made with either horizontal or vertical) but when it is the case with the solid hemisphere..we take the section to be varying with the height alone...or varying with the verticle distance..plz ...tell me the way how and where to apply these???

Three identical balls are placed on a frictionless horizontal surface touching each other. They stick to each other because of adhesive. Another ball of same radius and mass m is placed over the void created by the three balls. Find the forces applied by the balls kept on the floor to each other if the system remains in equilibrium.

A car loaded with water having total mass 1000 kg moves on a straight horizontal Road starting from rest under the action of a force of 100 Newton. The water spills throw a small hole in the bottom at a rate of 0.1 KG per second the velocity of cart after 300 seconds is nearly.

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