Mechanics

Force A push or pull that can change an object’s state of motion or shape. Forces are vector quantities (magnitude and direction). The SI unit is the newton (N). Contact forces (e.g., friction, tension) require physical contact; non-contact forces (e.g., gravity, electrostatic) act at a distance. Example: Pushing a stalled car requires a force; the car accelerates only if the net force is non-zero. Newton’s First Law (Law of Inertia) An object at rest stays at rest, and an object in motion stays in uniform motion in a straight line, unless acted upon by a net external force. Inertia is the resistance to change in motion, directly proportional to mass. A body moving with constant velocity has zero net force. Example: A passenger lurches forward when a bus suddenly brakes—their body tends to keep moving forward due to inertia.