Physics · Laws Of Motion · NEET
No. This is the biggest confusion. Newton's First Law says a body moving at constant speed in a straight line will keep moving forever if the net force is zero. On Earth a moving ball stops only because friction and air push against it. Remove those forces (like in deep space) and it never stops. Force is needed to CHANGE motion, not to continue it.
Every body continues in its state of rest or of uniform motion in a straight line unless it is compelled by a net external force to change that state. 'Uniform motion in a straight line' means constant speed AND constant direction. The word 'net' matters: many forces may act, but if they cancel out, the body's state does not change.
Because it defines inertia - the natural property of a body to resist any change in its state of rest or motion. The First Law simply describes what a body does when left alone (no net force): it shows inertia. That is why 'First Law of Motion' and 'Law of Inertia' are two names for the same idea.
No. Net force zero means zero acceleration, not zero velocity. A body can be at rest (velocity = 0) or moving at constant velocity - both are allowed when net force is zero. NEET traps you here: 'in equilibrium' does not mean 'stationary', it means no net force and so constant velocity (which may be zero).
The First Law is qualitative: it tells you WHEN motion changes (only when a net force acts) and defines force and inertia. The Second Law is quantitative: it tells you HOW MUCH the motion changes, through F = ma. The First Law is actually a special case of the Second Law: when F = 0, a = 0, so velocity stays constant.
A person standing on the floor of an elevator drops a coin. The coin reaches the floor in time t1 if the elevator is at rest, and in time t2 if the elevator is moving uniformly (constant velocity). Then:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Galileo. Through his experiments he showed that a body in motion does not need force to keep moving; it stops only due to friction. Newton built on Galileo's idea and stated it formally as the First Law of Motion.
No, not directly. The First Law holds only in inertial frames (frames at rest or moving at constant velocity). An accelerating bus is a non-inertial frame, where you must add a pseudo-force to use Newton's laws.
It defines two things: force (that which changes a body's state of rest or uniform motion) and inertia (the tendency of a body to resist that change). That is why it is called the Law of Inertia.
No. Uniform motion in the First Law means constant speed in a straight line. If direction changes (like a car turning), velocity changes, so a net force must be acting even if the speed is constant.
