Tru Physics

Newton’s First Law

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An object at rest will remain at rest, and an object in motion will continue in motion with a constant velocity, unless acted upon by a net external force.

In simpler terms, this means that objects tend to maintain their state of motion or rest until something acts on them. This law is one of the three laws of motion that were first described by Sir Isaac Newton in 1687 in his book “Philosophiæ Naturalis Principia Mathematica.” Technically, when translated exactly from its original Latin, his first law reads:

“Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.”

To better understand this law, let’s break it down:

  • An object at rest will remain at rest: This means that if an object is not moving, it will stay still until something makes it move.
  • An object in motion will continue in motion with a constant velocity: This means that if an object is already moving, it will keep moving in the same direction and at the same speed unless something stops it or changes its direction or speed.
  • Unless acted upon by a net external force: This means that if there is a net force acting on the object, the object’s state of motion can be changed.

Mathematically, this law can be expressed as:

\text{If } \Sigma{ \vec{F}} = \vec{0} \text{ then } v=\text{constant}.

Here, \Sigma \vec{F} is the net external force acting on the object.

Bowling is a good example of Newton's first law. Technically, there is a net force acting on the bowling ball due to friction. However, this effect is so small that the ball effectively rolls with constant velocity.
Bowling is a good example of Newton’s first law. Technically, there is a net force acting on the bowling ball due to friction. However, this effect is so small that the ball effectively rolls with constant velocity.

This law has many practical applications. For example, it explains why objects in space continue to move in the same direction and at the same speed unless acted upon by a force, such as the gravitational pull of another object. It also explains why a car continues to move forward even after the driver takes their foot off the gas pedal until friction and air resistance eventually slow it down. In this case, friction and air resistance play the role of the “net external force.”

It’s important to note that this law applies to both stationary and moving objects and that the term “velocity” includes both speed and direction. Additionally, it’s important to understand that the concept of inertia is closely related to this law. Inertia refers to an object’s resistance to changes in its state of motion, and it is what causes objects to maintain their state of motion or rest.

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Comments

2 responses to “Newton’s First Law”

  1. Newton’s Laws of Motion – Tru Physics

    […] The First Law is often referred to as the law of inertia. Inertia decribes an object’s resistance toward a change in its motion. Thus, the main concept of the first law is that objects tend to retain their motion unless a net force causes that motion to change. If the net force on an object is zero, its motion will not change. So, an object at rest will remain at rest. An object in motion will continue in motion with constant velocity. In other words, there is no acceleration when the net force is zero. […]

    Reply
  2. Inertial Frame – Tru Physics

    […] inertial frame of reference, in physics, is a frame of reference in which Newton’s first law of motion — the law of inertia — holds true. That is, in an inertial frame, an object not acted upon by a […]

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