430: "Two 1 kg lumps of clay sit in some mud. A boy picks up one lump of clay and rops it onto the second lump from a height of 1 m. The lumps stick together when they collide. If the second lump is driven into the mud a distance of 1cm, what is the average force exerted on the mud?" Answer: B: 520N. My answer: A: 500N Okay so the stupid little **** drops the 1kg thing of clay, so vf^2 = 2*10*1 = 20, or vf ~ 4.5 Then, conserving momentum, m1v1 = (m1+m2)v2 == 4.5*1 = 2*v2, v2 = 2.25. Then since it goes .01m into the mud, vi^2 = 2*a*.01 = 2.25^2 ~ 5, so solving, a ~ 250. F=ma, so 2*250 = 500. What am I doing wrong? Thanks.