Newton's First Law of Motion

The Newton's First Law of Motion is a Newtown's law of motion that defines the concept inertia.

AKA: Law of Inertia.
Context:
- It is often stated as: "An object remains at rest or in uniform motion unless acted upon by an unbalanced force".
- It can be expressed as: [math]\displaystyle{ \sum \mathbf{F} = 0 \Leftrightarrow \frac{\mathrm{d}^2 \mathbf{x} }{\mathrm{d}t^2} = 0 \Leftrightarrow \frac{\mathrm{d} \mathbf{v} }{\mathrm{d}t} =0 }[/math] where F is the unbalanced force, x object's position vector and v is the object's velocity.
- It is a restatement of Gaileo's Principle of Inertia.
Example(s)
- Galileo's Principle of Inertia.
- Zero velocity.
- Constant velocity.
- Uniform Motion.
Counter-Example(s)
See: Newton's Laws of Motion, Motion, Velocity, Acceleration, Mass, Length, Time, Force, Momentum, Energy, Inertia.

References

2016

(Wikipedia, 2016) ⇒ https://www.wikiwand.com/en/Newton%27s_laws_of_motion
- First law: When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.

2015

(NASA Website, 2016) ⇒ http://www.grc.nasa.gov/www/k-12/airplane/newton.html
- Newton's first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This is normally taken as the definition of inertia. The key point here is that if there is no net force acting on an object (if all the external forces cancel each other out) then the object will maintain a constant velocity. If that velocity is zero, then the object remains at rest. If an external force is applied, the velocity will change because of the force.

2005

(Hyperphysics Encyclopedia, 2005) ⇒ http://hyperphysics.phy-astr.gsu.edu/hbase/newt.html#ntcon
- Newton's First Law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. It may be seen as a statement about inertia, that objects will remain in their state of motion unless a force acts to change the motion. Any change in motion involves an acceleration, and then Newton's Second Law applies; in fact, the First Law is just a special case of the Second Law for which the net external force is zero.

1996

(The Physics Classroom,1996) ⇒ http://www.physicsclassroom.com/class/newtlaws/Lesson-1/Newton-s-First-Law
- Newton's first law of motion is often stated as

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

There are two clauses or parts to this statement - one that predicts the behavior of stationary objects and the other that predicts the behavior of moving objects. The two parts are summarized in the following diagram.

The behavior of all objects can be described by saying that objects tend to "keep on doing what they're doing" (unless acted upon by an unbalanced force). If at rest, they will continue in this same state of rest. If in motion with an eastward velocity of 5 m/s, they will continue in this same state of motion (5 m/s, East). If in motion with a leftward velocity of 2 m/s, they will continue in this same state of motion (2 m/s, left). The state of motion of an object is maintained as long as the object is not acted upon by an unbalanced force. All objects resist changes in their state of motion - they tend to "keep on doing what they're doing."

There is an important condition that must be met in order for the first law to be applicable to any given motion. The condition is described by the phrase "... unless acted upon by an unbalanced force." As the long as the forces are not unbalanced - that is, as long as the forces are balanced - the first law of motion applies.

1963

(Feynman et al., 1963) ⇒ Richard P. Feynman, Robert B. Leighton and Matthew Sands (1963, 1977, 2006, 2010, 2013) "The Feynman Lectures on Physics": New Millennium Edition is now available online by the California Institute of Technology, Michael A. Gottlieb, and Rudolf Pfeiffer ⇒ http://www.feynmanlectures.caltech.edu/
- QUOTE: Chapter 7: Galileo discovered a very remarkable fact about motion, which was essential for understanding these laws. That is the principle of inertia — if something is moving, with nothing touching it and completely undisturbed, it will go on forever, coasting at a unifor speed in a straight line. (Why does it keep on coasting? We do not know, but that is the way it is.)

Newton modified this idea, saying that the only way to change the motion of a body is to use force. If the body speeds up, a force has been applied in the direction of motion. On the other hand, if its motion is changed to a new direction, a force has been applied sideways. Newton thus added the idea that a force is needed to change the speed or the direction of motion of a body.