This is a fairly simple problem which i believe i know the answer to but the answer doesn't agree with the book i got it from. Theres a 750 gram cart traveling 4 m/s, 2 meters later its traveling at 3 m/s. Calculate the average retarding force assuming only air resistance. Here what I did: KE= (1/2)*m*v^2 so deltaKE=(.5)*(.750)*(4^2-3^2)= 2.625 The Work=f*d. Since the Change in Kinetic Energy equals the work done on the object i did: 2.625=f*2. Hence f=(2.625/2)=1.3125 N. I think thats right but is there something i might be possibly missing?