When water vapor is cooled it three forms into water droplets why does this represent a conservation in mass

What is meant by the statement" the melting poin of glycerine is 13 degree celsius

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Ablock having mass m slides down an inclined plane. the force of friction between the block and the inclined plane is f, the block's weight is m g, and the normal force is n. (a) draw a free – body force diagram showing the forces acting on the block. (b) write down all relevant newton’s equations for a given situation.