A disk with radius R has uniform surface charge density σ.

Part A

By regarding the disk as a series of thin concentric rings, calculate the electric potential V at a point on the disk's axis a distance x from the center of the disk. Assume that the potential is zero at infinity. (Hint: Use the result that potential at a point on the ring axis at a distance x from the center of the ring is V=14πϵ0Qx2+a2√ where Q is the charge of the ring.)

Express your answer in terms of the given quantities and appropriate constants.

Part B

Calculate −∂V/∂x.

Express your answer in terms of the given quantities and appropriate constants.

air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.