5.3:
Newton’s First Law: Introduction
Before Galileo and Newton, scientists held a 2000-year-old belief that objects have a natural tendency to come to rest if left alone.
Also, a constant force is necessary to keep them moving.
Galileo disproved this idea with a thought experiment using a bronze ball that is rolled over smooth double inclined planes.
A ball rolled down from the first plane runs up the same height on the second plane.
If the second plane has a smaller inclination angle, the ball moves to a longer distance to reach the same height.
If the second plane is horizontal, the ball should keep moving forever to reach the same height.
Galileo concluded that force is not required to keep a body in motion.
Newton summarized these and other observations in three laws of motion that are fundamental to mechanics.
The first law states that every object remains stationary or under a constant velocity unless acted on by a net external force.
5.3:
Newton’s First Law: Introduction
Motion draws our attention. Motion itself can be beautiful, causing us to marvel at the forces needed to create spectacular sights, such as that of a dolphin jumping out of the water, the flight of a bird, or the orbit of a satellite. The study of motion is kinematics, but kinematics only describes the way objects move—their velocity and acceleration. Dynamics considers the forces that affect the motion of moving objects and systems. Newton's laws of motion are the foundation of dynamics. These laws provide examples of the breadth and simplicity of the principles under which nature functions. They are also universal laws, in that they apply to similar situations on Earth and in space. Isaac Newton's (1642–1727 AD) laws of motion were just one aspect of the monumental work that has made him legendary. The development of Newton's laws marked the transition from the Renaissance to the modern era. This transition was characterized by a revolutionary change in the way people thought about the physical universe. For centuries, natural philosophers had debated the nature of the universe based mainly on certain rules of logic, with great weight given to the thoughts of earlier classical philosophers such as Aristotle (384–322 BC). Among the many great thinkers who contributed to this change were Newton and Galileo.
Galileo (1564-1642 AD) contributed to the formation of what is now called Newton's first law of motion. Newton made use of the work of his predecessors, which enabled him to develop laws of motion, discover the law of gravity, invent calculus, and make great contributions to the theories of light and color. Amazingly, many of these developments were made with Newton working alone, without the benefit of the peer interactions that take place among scientists today.
Newton's first law of motion suggests that a body stays in the same state, whether at rest or in motion (moving with constant velocity), until or unless an external force is applied to it. Note the expression “constant velocity”; this means that the body maintains a path along a straight line since neither the magnitude nor the direction of the velocity vector changes. Newton's first law says that there must be a cause for any change in velocity (a change in either magnitude or direction) to occur. We can think of this law as preserving the status quo of motion. It is general, and can be applied to anything from an object sliding on a table, to a satellite in orbit, to blood pumped from the heart. Experiments have verified that any change in velocity (speed or direction) must be caused by an external force.
This text is adapted from Openstax, College Physics, Section 4.0: Introduction to Dynamics: Newton's Laws of Motion and Openstax University Physics, Section 5.2: Newton's First Law.