Principle 3.3.1. The Law of Inertia.
An object at rest stays at rest and an object in motion stays in motion at a constant speed in a straight line unless acted upon by a net external force.
Section 3.3 The Law of Inertia (Newton’s First Law)
The term inertia—more commonly referred to as mass—is the capacity of an object to resist any change to its motion. The more mass something has, the more net external force it is going to take to change the object’s motion.
Definition 3.3.2. Mass.
The mass of an object, typically represented by \(m\text{,}\) is the capacity of that object to resist changes to its motion . The SI unit for mass is the kilogram (kg).
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as you will see in the next section, mass can be operationally defined as the amount of net force per acceleration required to speed up or slow down the object
Historical Note 3.3.3. Aristotelian Motion.
Before Newton’s Laws of Motion, a prevailing theory of motion was put forth by Aristotle: that the natural state of an object was to be at rest as close as possible to the center of the universe (which was, of course, the center of the Earth), and that if an object was disturbed, it would move and then return to rest.
In contrast, the Law of Inertia states something fundamental about the behavior of objects: that an object has two possible natural states: at rest or moving in a straight line at constant speed.
Exercises Activity: Explanation Practice
For each of the following contexts, predict whether or not you expect the system’s velocity to change. Explain your reasoning using the Explanation Task Steps. Clearly state any assumptions you make about each context!
1.
You are riding your bicycle down a flat street (not pedaling). System: you and the bicycle.
2.
You are driving down the highway in your car, obeying the speed limit. System: you and your car.
3.
A heavy book sits on a table in your house. System: the book.
4.
A fully loaded delivery truck is parked on an icy hill when its emergency brake fails. System: the delivery truck and everything in it.
5.
A spaceship with its engines off is traveling from Mars back home to Earth. System: the spaceship.
