Q1- If M= mass of the earth,R = radius of the earth,then wwhat is the gravitational potential at the centre of the sphere?Aslo define what is gravitational potential?

Q2- two sarellites are in the parking orbits around the earth.Mass of one of is 1 times other.What is the ratio of their time periods of revolutions?

Q3- The coefficient of friction of an inclines plane is 1/root3. if it is inclined at 30 angle with the horizontal, then what will be the downward acceleration of the block placed on the inclined plane?