Summary
Highlights
If an object is placed on a table and left undisturbed, it will remain at rest indefinitely unless an external force acts upon it. This part of the law aligns with our common observation.
The second part of the law, that a moving object will continue in motion at the same speed indefinitely without an external force, often seems counterintuitive. For example, a kicked football eventually stops due to forces like friction and air resistance, seemingly contradicting Newton's law.
The reason why a moving object on Earth, like a football, slows down and stops is not that Newton's law is wrong, but because external forces such as friction from the ground and air resistance are acting upon it, preventing it from maintaining its constant velocity. If these forces were absent (like in space), the object would indeed continue moving indefinitely.
The term 'external force' in Newton's law primarily refers to the 'net force' or 'resultant force.' Even an object at rest, like a book on a table, experiences forces (gravity pulling it down), but it remains stationary because other forces (like the normal force from the table pushing it up) balance each other out, resulting in a zero net force.
Sir Isaac Newton's First Law of Motion, established in the 1600s, states that an object at rest will stay at rest, and an object in motion will stay in motion with the same speed and in the same direction unless acted upon by an external force. This principle was published in his famous book.
Newton's Third Law states that for every action, there is an equal and opposite reaction. This is crucial for understanding net force. When an object exerts a force on a surface (e.g., a book pushing down on a table due to its weight), the surface exerts an equal and opposite force back on the object. These opposing forces can cancel each other out, leading to a zero net force.
When multiple forces act on an object in opposite directions, the net force is determined by subtracting the smaller force from the larger one. If the forces are equal and opposite, the net force is zero. This zero net force is the condition for an object at rest to remain at rest, or an object in motion to continue in motion at a constant velocity, fulfilling Newton's First Law.