Atruck runs into a pile of sand, moving 0.80 m as it slows to a stop. the magnitude of the work that the sand does on the truck is 6.0 times 10^5 j. (a) determine the average force that the sand exerts on the truck. (b) did the sand do positive or negative work? (c) how does the average force change if the stopping distance is doubled? indicate any assumptions you made.