this is from ch 4 of cutnell & johnson physics when you add forces together such as the ones in the problem below, how do you know which ones are positive and which ones are negative? does it have to do with their relation on the xy axis? or does it have to do with the direction the object is trying to travel? hw problem: A boat has a mass of 6800 kg. Its engines generate a drive force of 3300 N, due west, while the wind exerts a force of 940 N, due east, and the water exerts a resistive force of 1200 N due east. What is the magnitude and direction of the boat's acceleration? this is my attempt at the hw problem: sum of the forces = -1610 then i divide that by the mass of the ship giving me -.237m/s^2 W as the a and direction of the ship.