1. The problem statement, all variables and given/known data A 5.00 kg sack of flour is lifted vertically at a constant speed of 3.50 m/s through a height of 15.0 m. 2. Relevant equations a) How great a force is required? b) How much work is done on the sack by the lifting force? c) What becomes of this work? 3. The attempt at a solution Originally i assumed that acceleration at constant speed is the same as the speed, but then i realized that acceleration at constant speed is zero. F=ma F=5.00kg * 3.50 m/s F = 17.5 kg * m/s so when you include zero in the equation you get that there is no force? But there has to be some force in order to calculate work W=F*S*Cos(Θ) W=F*(15.0m)*Cos(180) ; it would be 180 since its being lifted vertically correct?