Chapter 1: The Collision Dilemma

Imagine you're behind the wheel of a high-speed supercar, racing at an astonishing speed of a million miles per second. As you zip through the landscape, a tiny fly suddenly appears in your path. With no time to dodge it, you collide with the insect, which splatters against your windshield. While the fly is destroyed in the impact, your vehicle remains virtually untouched, save for a smear of red on the glass.

Now, consider this: how much greater is the force the car exerts on the fly compared to the force the fly applies to the car? Would you guess 10%? 20%? Perhaps even 99.99%? The surprising answer is actually 0%. The fly and the car exert equal forces upon each other, yet the consequences of that force are vastly different. This paradox illustrates a fundamental concept in physics that we often overlook: the equal but opposite forces described by Newton's laws.

Section 1.1: Clarifying Newton's 2nd Law

Many people simplify Newton’s 2nd Law as "every action has an equal and opposite reaction." However, this phrasing can be misleading. A more accurate interpretation is that "every force has an equal and opposite force." The confusion often arises from the term “opposite.” In this context, opposite means that the forces act in different directions but possess the same magnitude.

This leads to another question: if forces are equal and opposite, shouldn’t they negate one another, preventing any acceleration? For instance, when an apple rests on a table, the upward normal force from the table balances the weight of the apple, resulting in no movement. Why doesn’t this scenario contradict Newton’s 3rd Law, unlike others?

Subsection 1.1.1: Forces at Play

The explanation lies in understanding that the forces involved are not related to Newton's 3rd Law. The normal force can exceed the weight of the apple if the table accelerates upward, or vice versa if the apple were to crush the table. In essence, the normal force and the apple's weight act on the same object, resulting in no acceleration.

The actual pairs of forces related to Newton’s 3rd Law involve the gravitational force the Earth exerts on the apple and the equal force the apple exerts on the Earth, which act on different objects and therefore do not cancel each other out.

Chapter 2: Momentum and Its Implications

Imagine a gorilla and an ant both delivering a punch with the same effort. The ant’s punch might not hurt much, but the gorilla’s punch would send you flying, demonstrating that the actual force is more about mass and momentum than merely the effort exerted.

In the context of Newton’s 2nd Law, force is defined as the rate of change of momentum:

F = dp/dt

This means that forces relate to changes in momentum rather than the momentum itself. For example, if a gorilla punches you at 20 m/s and then at 21 m/s, the impact remains high, but the change is minimal. Conversely, if an ant punches you at 1 m/s and then at 16 m/s, the change is significant despite the lower absolute impact.

Returning to our collision scenario with the car and the fly, we see that the change in momentum for both is equal, despite the fly experiencing a dramatic effect from the collision.

Misunderstandings of Newton's 3rd Law often stem from conflating the concepts of impact and the change in impact over time. In reality, Newton’s 3rd Law is rooted in the principle of momentum conservation. If momentum is conserved, the total change in momentum within a system is zero, leading to the conclusion that the changes in momentum between two objects must be equal and opposite.

Understanding these dynamics clarifies the seemingly peculiar nature of Newton's laws, revealing the deeper principles that govern motion and interaction in our physical world.

Thank you for joining this exploration of classical mechanics, and stay tuned for more insights in future discussions!