The period T of an earth satellite is related to the radius R of its orbit by the equation T^2 = A R^3 where A is a constant. The moon may be assumed to move in a circular orbit of radius RM about earth. The period of the moon's orbit is 28 days. The radius of the orbit of a geostationary satellite about earth is Rg. The orbital radii are related by the expression: RM = K Rg Find the value of K. I found K to be 3.04 but am not sure if i have done it correctly. Can anyone help? Also, a constant potential difference is applied beteen two conducting plates creating a uniform electric field. A very small negatively charged sphere is introduced between the plates. It is found that the values of the weight of the sphere, the charge on it and the electric field between the plates are such as to cause it to remain stationary. At time t=0 the upper plate starts to move with uniform velocity towards the lower plate. The potential difference is kept constant, and the plates remain parallel throughout the motion. Sketch a graph to show the variation of the electric field strength E between the plates with time t. As E = voltage/distance, I drew a straight line through the origin. Is this correct? Describe what happens to the charged sphere while the upper plate is moving. I have said that the charged sphere would accelerate towards the positive plate as the electric field strength would be increasing, because the electric force no longer balances with the charge and the weight of the sphere. Don't think this is correct though. Can anyone suggest how I would answer this?