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1. Newton's First Law and Inertial Reference Frames:

Galilei's law of inertia and Newton's First Law are valid only in inertial reference frames, those are reference frames which are not accelerating.

2. Newton's Second Law:

The pull, e.g. from an extended spring, acting on an object of a certain mass, attached to the spring, can be quantitatively expressed by the vector relationship F=ma. F is the force, "m" the mass, and "a" the acceleration. This law, like Newton's First, is ONLY valid in an inertial reference frame.

3. Superposition of Forces and Net Force:

The gravitational force acting on a mass m on Earth is mg. A mass m held in your hand doesn't accelerate, therefore the net force on it must be zero, and thus your hand is exerting an upward force equal to mg.

4. Newton's Third Law, Action = —Reaction:

The contact force between two objects can be described as a pair of forces, equal in magnitude but opposite in direction. Every-day examples are described such as snaking of a garden hose, releasing air from a balloon (jet action) and the recoil of a gun. Professor Lewin shows a demonstration with Hero's engine.

5. Consequences of Newton's Third Law:

While an apple is falling towards the Earth, the Earth is falling (moving) towards the apple!

6. Decomposing Forces in x and y Directions:

Hang an object of mass "m" from two strings each with negligible mass. The net force on the object is zero. The sum of the tension vectors in the strings must equal mg pointing vertically up.

7. Bizarre Demo with a 2 kg Block and 2 Strings:

Two identical strings, one suspending a mass, the other string is suspending from the mass. Which one breaks when you pull on the lower string, the upper string or the lower string? Professor Lewin pulls a fast one!