A body continues to be in its state of rest or of uniform motion along a straight line, unless it is acted upon by some external force to change the state. Two conditions combined: (1) rest body stays at rest; (2) moving body continues at constant velocity — both in absence of net external force.

• Key word: 'net external force.' If multiple forces act but their vector sum = 0, First Law still applies (body stays in its current state).
• Uniform motion = constant velocity (constant speed AND constant direction — straight line). Circular motion at constant speed is NOT uniform motion in the First Law sense because direction changes.
• NEET: 'Which of the following demonstrates Newton's First Law?' → A body moving at constant velocity on a frictionless surface. OR: A book at rest on a table (two forces balanced — weight down, normal up. Net force = 0, body stays at rest.)

Newton's First Law was called the 'law of inertia' by Galileo before Newton. It is so named because it is inertia that makes a body resist changes in its state of motion. The law explains WHY — because of inertia — a body does not change state unless forced.

• Galileo's contribution: Through thought experiments on frictionless inclined planes, Galileo concluded that if there is no friction, a ball rolling on a horizontal surface would continue indefinitely — the first statement of inertia.
• Newton's First Law is the QUANTITATIVE formulation of Galileo's law of inertia — any body (regardless of mass) remains in its state without a net external force.
• NEET assertion-reason type: 'Assertion: Newton's First Law is also called law of inertia. Reason: Inertia is the cause of the property described by the First Law.' Both TRUE, R is correct explanation of A.

Newton's First Law provides a qualitative definition of force: force is any external agent that changes the state of rest or uniform motion of a body. This defines force conceptually ('what force does') — without quantifying it. Second Law (F = ma) quantifies force.

• If no net force acts on a body, then the velocity of the body cannot change — it remains at rest or at constant velocity.
• First Law defines force qualitatively (what force DOES). Second Law defines force quantitatively (how MUCH force). They are not redundant — completly different aspects.
• NEET: 'Newton's First Law of Motion gives the definition of...' → Force (qualitative). Note: Second Law gives quantitative definition (F = ma).

An inertial reference frame is a frame of reference in which Newton's First Law holds — a body at rest remains at rest, and a body in motion continues at constant velocity, if no net force acts. Accelerating frames (buses braking, rotating frames) are non-inertial.

• In a non-inertial frame (e.g., accelerating bus), fictitious forces appear (pseudo-forces). Passengers feel thrown forward when bus brakes — this is a pseudo-force in the non-inertial frame of the bus.
• Earth is approximately inertial for most physics problems. Strictly, Earth rotates (centrifugal effect), but this is negligible at the scale of typical NEET problems.
• Deep space far from massive bodies = truly inertial frame. An inertial frame is defined by the absence of pseudo-forces — any frame moving at constant velocity relative to an established inertial frame is also inertial.