I've tried doing this question in many different ways: A person pushes a 13.8-kg shopping cart at a constant velocity for a distance of 31.7 m on a flat horizontal surface. She pushes in a direction 31.8 ° below the horizontal. A 30.6-N frictional force opposes the motion of the cart. (a) What is the magnitude of the force that the shopper exerts? Determine the work done by (b) the pushing force, (c) the frictional force, and (d) the gravitational force. I know that I can do parts b, c, and d. I can't remember how to do part a, though. I tried saying that F=ma, but this doesn't work. F=ma(sin31.8) won't work for me either. I'm assuming that a=9.8, even though it is on a horizontal plane. I can't think of what else could be the horizontal acceleration, if there could be anything else.