Скачать или смотреть PHY101 Mechanics | Lecture 9: Newton’s Second Law & Universal Gravitation

This lecture serves as the foundation for the remainder of the course, moving from the conceptual "license" of the First Law to the quantitative precision of the Second Law. The instructor explores how we measure the physical world and why gravity behaves the way it does across different scales.

Key Topics Covered:
Newton’s Second Law (F⃗ = m a⃗) [02:40]: Understanding force as a measure of interaction and mass as the resistance to motion (inertia).

Operational Definition of Mass [08:27]: A step-by-step explanation of how to quantify an unknown mass by comparing acceleration ratios ($m_u = m_s * (a_s / a_u)$) using a standard 1 kg block [14:56].

Force as a Vector [21:46]: Experimental proof that interactions add like vectors and the declaration of the Newton (N) as the standard unit of force [27:10].

Newton’s Third Law Clarified [34:03]: Debunking common misconceptions about "action and reaction." The instructor emphasizes that these forces must act on different objects and therefore never cancel each other out [35:07].

Law of Universal Gravitation [37:34]: How Newton hypothesized that the force pulling an apple is the same force keeping the Moon in orbit.

The Moon Test [48:39]: A mathematical derivation comparing the acceleration of a falling object on Earth to the centripetal acceleration of the Moon to prove the 1/r² relationship [55:41].

Normal Force vs. Weight [01:03:00]: Why the normal force is not the "reaction" to gravity and how its magnitude depends on the specific constraints of the surface [01:04:10].