Physics: Free-Body Diagrams and Newton's Second Law

Newton's Laws of Motion explain how forces change an object's motion, and free-body diagrams are the key tool for visualising them. Every force acting on a body is drawn as an arrow from a single point, with direction showing the force's direction and length showing its relative magnitude. For a car accelerating along a flat road, four forces act: thrust, friction, weight Fg, and the normal force FN. Because the car stays on the ground, the vertical arrows are equal, so FN = Fg and there is no vertical acceleration. Horizontally, thrust exceeds friction, and the difference in arrow lengths represents the net force. Newton's second law, F_net = ma, then links this net forward force to the car's forward acceleration. At constant speed the horizontal arrows become equal, and the net force falls to zero.