Tag: Newton

In the previous post of this thread, I explored the nature of forces. In this post, I will go into more detail on how forces act on physical objects. The central law is Newton’s third law.

Action-Reaction Pairs (Newton’s Third Law)

Newton’s third law provides a profound insight into how forces actually arise: every force represents an interaction between two objects. The law is famously stated as:

“For every action, there is an equal and opposite reaction.”

This means forces always occur in pairs—an action force exerted by one object onto another, and a corresponding reaction force of equal magnitude but opposite direction exerted by the second object onto the first.

Crucially, these two forces never act on the same object. Instead, each force acts on a different object involved in the interaction, preventing them from simply “canceling out.”

Everyday Examples:

• Walking:
  When you walk, your foot pushes backward against the ground (action), and the ground pushes your foot forward (reaction). It’s the reaction force from the ground that propels you forward.
• Rocket Propulsion:
  Rockets move by expelling hot exhaust gases backward (action), and these gases push the rocket forward (reaction). The expelled gases experience a backward force, while the rocket experiences the equal and opposite forward force.
• Collisions:
  When two cars collide, each exerts a force on the other. Although the forces are equal and opposite, each car experiences its own acceleration depending on its mass, leading to different outcomes for each vehicle.

Newton’s third law emphasizes the interconnected nature of forces: forces never exist in isolation but always represent mutual interactions. This insight is crucial for correctly analyzing motion and solving practical engineering problems.

Forces in Different Reference Frames

In an earlier post, we discussed inertial and non-inertial reference frames. Understanding reference frames is especially important when analyzing forces because the appearance of motion and forces can vary greatly depending on your perspective.

• Inertial frames (frames moving at constant velocity, with no acceleration) allow straightforward application of Newton’s laws. Forces observed in inertial frames reflect real physical interactions—gravitational, electromagnetic, or nuclear.
• Non-inertial frames (accelerating or rotating frames), however, introduce additional inertial forces (often called fictitious forces). These forces arise purely because the frame itself accelerates relative to inertial frames.

Real vs. Inertial Forces:

• Real forces originate from physical interactions between objects. Examples include gravitational attraction between planets or electromagnetic forces in charged particles.
• Inertial forces, in contrast, do not represent direct interactions but result solely from observing motion from an accelerating reference frame. Examples include centrifugal and Coriolis forces experienced on a rotating planet.
  We often learn that inertial forces, such as the centrifugal force do not exist. I will passionately argue against this misconception. Inertial forces clearly exist but they depend on our choice of reference frame.

Example—Experiencing Inertial Forces:

When you’re in a car accelerating quickly, you feel pushed back into your seat. From the car’s reference frame (non-inertial), it seems a backward force is acting on you. Viewed from a resting reference frame, no backward force physically pushes you. Instead, this inertial force emerges because your body tries to maintain its inertia (its original state of motion), while the frame itself accelerates forward.

This distinction is crucial: only real forces originate from interactions, whereas inertial forces emerge from accelerating perspectives. Clearly differentiating between these two types helps us avoid confusion when analyzing complex scenarios, such as weather patterns, planetary motion, or engineering problems involving rotating machinery.

Deeper Insights from Studying Forces

While understanding forces is essential for solving practical problems, studying forces deeply can reveal powerful insights into the fundamental workings of nature. Beyond simply describing motion, forces connect physics to deeper philosophical and theoretical concepts.

Symmetry and Conservation Laws:

One profound insight comes from the relationship between forces and symmetries. In physics, symmetry refers to invariance under transformations—such as translations in space, rotations, or shifts in time. These symmetries correspond directly to conserved quantities, a connection formalized by Noether’s theorem (which we will explore more deeply in future posts).

For instance:

• Spatial translation symmetry (physics looks the same everywhere in space) corresponds to the conservation of momentum.
• Temporal symmetry (laws of physics don’t change with time) corresponds to the conservation of energy.

The Unification of Forces:

Historically, physicists discovered that seemingly separate forces could often be unified into deeper, more fundamental interactions. The electromagnetic force, for example, unified electricity and magnetism into a single framework in the 19th century. Modern physics continues this pursuit, seeking a unified understanding of gravity, electromagnetic, and nuclear forces—something referred to as the quest for a “Theory of Everything.”

Forces and Fields:

Another profound concept is the notion of fields. Rather than viewing forces as mysterious actions-at-a-distance, physicists introduced fields—physical entities that permeate space, mediating forces between objects. Electromagnetic and gravitational forces, for example, are now understood as interactions mediated by electric, magnetic, and gravitational fields, respectively. This field-based perspective becomes particularly essential in advanced topics such as electromagnetism, relativity, and quantum field theory.

Conclusion and Looking Forward

In these two posts on forces, we’ve explored not just how forces operate practically, but also their deep theoretical significance. Understanding action-reaction pairs clarified the inherent symmetry of interactions, while analyzing forces in different reference frames underscored the subtleties of motion. Most importantly, recognizing that studying forces leads us to deeper insights—such as symmetry principles, conservation laws, and unified theories—highlights classical mechanics’ role as the gateway to deeper physical theories.

In our next topic, we’ll examine these ideas in greater detail through Conservation Laws in Newtonian Mechanics, bridging from forces and interactions to deeper principles like energy, momentum, and angular momentum.

In our previous discussions, we explored Newton’s laws of motion and saw how reference frames shape our description of motion. At the heart of Newtonian mechanics is the concept of a force—a physical influence capable of changing an object’s state of motion, causing acceleration.

Forces provide the fundamental way objects interact with one another. Whenever you push or pull an object, or when planets attract each other across vast distances, you witness forces in action. However, not all forces are the same. Some forces act through direct physical contact, while others operate over vast distances, seemingly without any direct interaction.

In this post, we’ll examine these forces more closely, distinguishing between fundamental forces and those that emerge from more basic interactions at the microscopic level.

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Fundamental Types of Forces

Nature, at its most fundamental level, is governed by four basic types of forces: gravitational, electromagnetic, strong nuclear, and weak nuclear. While strong and weak nuclear forces primarily influence subatomic particles, gravitational and electromagnetic forces shape nearly all macroscopic phenomena we experience daily.

Gravitational Force

Gravity is perhaps the most familiar and universal force—an attractive force acting between any two masses. It governs the motion of planets around the sun, keeps the moon in orbit around Earth, and is the reason objects fall toward the ground when released.

A defining feature of gravity is that it always attracts and never repels, with its strength decreasing rapidly with distance according to an inverse-square law. Despite its profound effects, gravitational force is surprisingly weak compared to other fundamental forces—but it dominates at large scales because it accumulates and never cancels out.

Electromagnetic Force

The electromagnetic force encompasses both electric and magnetic interactions. It’s responsible for nearly all the forces we experience directly, apart from gravity. From friction to tension, from the rigidity of solids to chemical bonds, electromagnetic forces shape everyday life at a fundamental level.

Unlike gravity, electromagnetic forces can both attract and repel, allowing complex structures like atoms and molecules to form. At the microscopic level, electromagnetic interactions arise between charged particles such as electrons and protons. At macroscopic scales, these interactions manifest as contact forces—forces that occur when objects physically touch or interact at short distances.

Strong and Weak Nuclear Forces

Although beyond the scope of everyday experiences, these two fundamental forces shape the subatomic world. The strong nuclear force holds atomic nuclei together against electromagnetic repulsion, while the weak nuclear force is involved in certain forms of radioactive decay. We’ll explore these forces further in later parts of our course.

Contact Forces as Emergent Interactions

While gravitational and electromagnetic forces are fundamental and operate at a distance, many of the forces we encounter daily—such as friction, tension, and normal force—are contact forces. These forces aren’t fundamental on their own; instead, they’re emergent phenomena arising from electromagnetic interactions at microscopic scales.

Normal Force

Consider placing a book on a table. The gravitational force pulls the book downward, yet the book remains stationary. Why doesn’t the book fall through the table? The answer lies in the normal force, an emergent electromagnetic interaction.

When the book presses down, atoms in the table are compressed slightly, causing electrons around these atoms to repel electrons in the book. This microscopic electromagnetic repulsion creates a measurable upward force, balancing gravity and preventing the book from moving downward.

Friction

Friction is another familiar contact force, essential for activities such as walking, driving, and holding objects. At a microscopic level, friction arises from the roughness of surfaces and electromagnetic attraction and repulsion between atoms at points of contact.

There are two common forms of friction:

• Static friction, which prevents objects from starting to move.
• Kinetic friction, which opposes motion once objects are sliding against each other.

Both types of friction originate from electromagnetic interactions between atoms on contacting surfaces. Even seemingly smooth surfaces have microscopic irregularities, causing resistance as they slide past each other.

Tension and Elastic Forces

When you pull on a rope or stretch a spring, you encounter tension or elastic forces. These forces come from the electromagnetic interactions holding atoms and molecules together in solid objects.

For example, stretching a spring disturbs its equilibrium configuration at the atomic scale, prompting atoms to pull each other back toward their original positions. This restoring force, governed by Hooke’s law, is fundamentally electromagnetic—atoms resist being displaced from their stable arrangements.

Summary and What’s Next

We’ve now explored how forces connect objects and shape their motion, distinguishing between fundamental forces acting at a distance and contact forces that emerge from underlying microscopic interactions. In the next post, I will deepen our exploration by examining Newton’s third law and how forces always come in action-reaction pairs, completing our conceptual picture of forces in classical mechanics.