What is the definition of physical activity?

The density of a substance is 4.0 g/cm3. if a sample of the substance has a volume of 25 cm3 then what is the mass

In a hydrogen molecule there are a total of four charges, 2 protons in the two nuclei, and 2 electrons. how many unique charge-pairs are there (without double counting)?

Panel bc in fig. p2.76 is semi-circular, with the 3 meter radius and horizontal straight edge through point b. compute (a) the hydrostatic force of the water on the panel, (b) its center of pressure, and (c) the moment of this force about point b. assume atmospheric pressure on the dry side of the panel