22 Introduction to Dynamics: Newton’s Laws of Motion

Motion captures our attention—not only because it’s everywhere in nature but because it can be spectacular. Think of a dolphin leaping out of the water, a pole vaulter soaring over a bar, a bird in flight, or even a satellite orbiting Earth. Studying motion is the science of kinematics, which describes how objects move: their speeds, directions, and accelerations.

However, kinematics tells us what happens, not why it happens. To understand the causes behind motion, we turn to dynamics, the study of the forces acting on objects and how these forces influence motion.

At the heart of dynamics are Newton’s laws of motion, foundational principles that govern everything from the tiniest biological movements (like muscle contractions) to the paths of celestial bodies.

Isaac Newton: The Founder of Classical Dynamics

Isaac Newton (1642–1727) developed these laws over 300 years ago, and they remain fundamental to physics—and by extension, to many biological and health science phenomena, such as blood flow dynamics, biomechanics, and cellular movement.

Newton’s laws mark a shift from earlier ideas, rooted in ancient philosophers like Aristotle, who based understanding mostly on logic and intuition rather than observation and experiment. Newton’s work, building on that of Galileo and others, ushered in the modern scientific approach based on observation, measurement, and mathematical description.

Galileo’s Role: The Power of Observation

Galileo Galilei (1564–1642), often called the father of experimental science, championed observation as the ultimate test of scientific truth. His telescopic discoveries—such as moons orbiting Jupiter—challenged long-held beliefs and emphasized that understanding nature requires evidence, not just argument.

Galileo’s insights helped pave the way for Newton’s laws by establishing the importance of measuring motion carefully and understanding its principles.

Newton’s First Law: The Principle of Inertia

One of Galileo’s key contributions was the idea that an object in motion tends to stay in motion unless acted upon by an external force—what Newton formalized as the first law of motion or the law of inertia. This is a cornerstone concept in both physics and biology, underlying everything from blood cells flowing through vessels to the motion of animals.

Limits of Newtonian Physics

Newton’s laws describe most everyday motions extremely well. However, they have limits:

• They work best for objects moving much slower than the speed of light.

• They apply to objects much larger than molecules (larger than about 1 nanometer).

For situations involving very small particles (quantum mechanics) or very fast speeds (Special Relativity), Newton’s laws need refinement.