6. The ground-floor of a building serves as parking space. The inhabited (main) part of the building, above the parking area, rests on 9 columns that are fixed in the ground. The mass of the main part is 30,000 kg. The building is hit by an earthquake whose first jolt has an average horizontal acceleration of 0.8g. The structure was not damaged in the earthquake. (a) What total force was exerted by the main part of the building on the tops of the columns during the first jolt? (b) If the cross section of each column is a square of 10cm x 10 cm, what was the shear stress at the top of each column? main