52 kg box is pushed 7.0 meters by a force with a magnitude of 149 N. The coefficient of kinetic friction is 0.25. Determine normal force, force of gravity and frictional force. For the normal force I have 52 * 9.8 = 509.6 N Which is also the the force of gravity. However which is positive and which is negative? I assume normal force is positive since it pushes up against gravity pushing downward? Also, how would I convert Newtons into Joules?