Which one of the following astronomers proved that the Earth and other planets revolve around the Sun?

Sand falls vertically on a conveyor belt at a work rate of 0⋅1 kg/s. In order to keep the belt moving at a uniform speed of 2 m/s, the force required to be applied on the belt is :

The power required to lift a mass of 8⋅0 kg up a vertical distance of 4 m in 2 s is (taking acceleration due to gravity as 10 m/s²):

One block of 2⋅0 kg mass is placed on top of another block of 3⋅0 kg mass. The coefficient of static friction between the two blocks is 0⋅2. The bottom block is pulled with a horizontal force F such that both the blocks move together without slipping. Taking acceleration due to gravity as 10 m/s², the maximum value of the frictional force is :