7.3 Newton’s First Law of Motion
The Law of Inertia
An object at rest remains at rest or, if in motion, remains in motion at a constant velocity unless acted on by a net external force.
Simply put, this means that an object will maintain its state, whether stationary or in motion, unless acted upon by another force. To understand this law in the context of our own world, imagine a hockey puck on ice. If the puck is stationary, it will not move unless it is pushed by something such as a hockey stick, another puck, or maybe a strong gust of wind. However, to understand the second portion of the law, you have to be a bit imaginative. Although an ice rink is made to be as smooth and slick as possible, so that players can glide around easily, there is still a small amount of friction between the ice and any object in contact with it. However, let’s assume that no frictional force exists. In this case, if a hockey puck were hit down the ice, Newton’s first law says that it will continue going straight indefinitely, assuming the ice also continues indefinitely.
Gravity, Inertia, and Mass
Gravity and inertia are two closely related aspects in biomechanics that are both connected to mass.
Gravity is the force that pulls us toward the ground and on Earth. It is assumed to be constant at 9.81 m/s2. Mass, measured in kilograms (kg), is the amount of matter in something. Mass is also related to inertia, which is the ability of an object to resist changes in its motion—in other words, to resist acceleration. As we know from experience, some objects have more inertia than others. It is more difficult to change the motion of a large boulder than that of a basketball, for example, because the boulder has more mass than the basketball. In other words, the inertia of an object is measured by its mass.
What is the difference between weight and mass?
Often, the term mass is confused and/or used interchangeably with the term weight. However, they do not mean the same thing. Instead, weight is a measure of gravitation, or gravitational pull, on an object, which can be calculated using both gravity (9.81 m/s2) and mass (kg). This is because objects with mass are attracted to one another, and that attraction is proportional to the mass of the objects.
Take you and the earth, for example. You have a specific mass (kg), and the Earth’s gravity is 9.81m/s2. Therefore, if you multiply those two values, the resulting number would be your weight (N).
[latex]\text{Your weight (N)} = \text{Your mass (kg)} \times 9.81 \, \text{m/s}^2[/latex]
For example, how much do you weigh if your mass is 68 kg?
[latex]68 \, \text{kg} \times 9.81 \, \text{m/s}^2 = 667.08 \, \text{N}[/latex]
Therefore, if your mass is 68 kg, your weight is 667.08 N.
“4.2 Newton’s First Law” from Introduction to Biomechanics by Rob Pryce is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
“How much do you weigh?” image: OpenAI. (2025). ChatGPT. [Large language model]. https://chat.openai.com/chat Prompt: Create an illustration of a person who weighs 68kg standing on the Earth. Convert to black and white and remove clouds.
