Considering that the book is moving with constant speed, the force applied by Anna must be the same that the friction force:

If we clear the previous equation:
Answer:
When the polythene rod is rubbed with the woolen cloth, static electric charges move from the cloth and into the rod. The rod becomes negatively charged as negative charges move from the cloth and into the rod leaving the cloth positively charged as well.
a) The wind is generated because there are different values of pressure in the amtophera. That is, it is generated due to a pressure difference between two atmospheric points. Generally the movement is performed when the air travels from the highest pressure point, to the lowest pressure point. This is also a direct cause of different types of wind speeds.
b) If the cloud moves from one direction to another, it will indicate that from the starting point the pressure is higher, and the point towards which it is directed, the pressure is lower. If we place this on a Cartesian plane with reference to the cardinal points, we can know the approximate place or area where the pressures are different.
Mass of the displaced material. In water it would be the mass of the water that the volume of the ball displaces.