*1-12. The specific weight (wt./vol.) of brass is 520 1b/ft. Determine its density (mass/vol.) in SI units. Use an appropriate prefix. 1-13. Convert each of the following to three significant figures: (a) 20 lb . ft to N . m. (b) 450 1b/ft to kN/m , and (c) 15 ft/h to mm/s. 1-14. The density (mass/volume) of aluminum is 5.26 slug/ft . Determine its density in SI units. Use an appropriate prefix. 1-15. Water has a density of 1.94 slug/ft . What is the density expressed in SI units? Express the answer to three significant figures. #1-16. Two particles have a mass of 8 kg and 12 kg. respectively. If they are 800 mm apart, determine the force of gravity acting between them. Compare this result with the weight of each particle. 1-17. Determine the mass in kilograms of an object that has a weight of (a) 20 mN. (b) 150 kN, and (c) 60 MN. Express the answer to three significant figures. 1-18. Evaluate each of the following to three significant figures and express each answer in SI units using an appropriate prefix: (a) (200 kN) . (b) (0.005 mm), and (c) (400 m) . 1-19. Using the base units of the SI system, show that n Eq. 1-2 is a dimensionally homogeneous equation which gives Fin newtons. Determine to three significant figures the gravitational force acting between two spheres that are touching each other. The mass of each sphere is 200 kg and the radius is 300 mm. *1-20. Evaluate each of the following to three significant figures and express each answer in SI units using an appropriate prefix: (a) (0.631 Mm)/(8.60 kg) , and (b) (35 mm) (48 kg). 1-21. Evaluate (204 mm)(0.00457 kg)/(34.6 N) to three significant figures and express the answer in SI units using an appropriate prefix