Laws of Motion — Class 11 Physics

This chapter explains what governs motion through Newton's three laws. Aristotle wrongly believed a constant force is needed to keep a body moving; Galileo's study of inclined planes led instead to the law of inertia. Newton's first law states that a body continues in rest or uniform motion unless acted upon by a net external force, defining inertia and force. The second law states that the rate of change of momentum equals the applied force, , giving the newton as the SI unit of force; impulse equals the change in momentum. The third law states that to every action there is an equal and opposite reaction, the two forces acting on different bodies. From the second and third laws follows the law of conservation of linear momentum for an isolated system, illustrated by recoil of a gun. Equilibrium of a particle requires the net force to be zero. Common forces in mechanics include gravity, normal reaction, tension and friction; static, kinetic and rolling friction are governed by and . For circular motion the centripetal force is provided by friction or by banking of roads, and free-body diagrams are used to solve mechanics problems systematically.