First Law of Motion

Part 1

Physics is the study of objects, how they move, and how they interact. One of the earliest attempts to use mathematical equations to describe physics was the book Mathematical Principles of Natural Philosophy, written by Isaac Newton in the 17th century. There, Newton describes three laws that explain the motion of all objects known to exist at that time. These three laws were incredibly important for the development of physics, and are still mostly true for all everyday objects – from tennis balls bouncing around on a tennis court to planets orbiting the sun.  

The first law states that an object in motion will remain in motion at a constant velocity, unless the object interacts with a force. For example, an object moving on a frictionless surface (think of a really strong air hockey table) will move at a constant speed and in a constant direction until it collides with something. If an object is not moving (that is, has a velocity of zero) then it will remain still until something interacts with it. The ability for objects to stay in motion when in the absence of force is called inertia, which is a property of all matter. Objects will even stay in motion if two opposite forces are interacting with it, as the two forces cancel out to produce net-zero force.  

One consequence of Newton’s first law of motion is that a force is necessary to produce any change in velocity. A change in velocity is called an acceleration if velocity increases, or a deceleration if velocity decreases.  

Part 2

Newton’s first law states that an object will maintain a constant velocity if net-zero force is interacting with the object. In the real world, it is difficult to find an example of an object that doesn’t interact with any force. When an object slides along the ground, it is always met with the force of friction, which causes it to slow down over time. When an object moves through the air, it is likewise always met with some degree of air resistance. These forces cause objects to slow down, and eventually come to a complete stop. On a theoretical frictionless surface and in a vacuum where there is no air resistance, an object in motion will maintain its velocity indefinitely.  

The forces of friction and air resistance point in the opposite direction as does the velocity of a moving object. When a car is moving forward, backwards-facing forces of friction and air resistance begin to slow down the car’s velocity. The driver must apply forward-facing force to the car in order to maintain speed. When the driver is applying enough force to maintain a constant velocity, the car once again experiences net-zero force. That is, the forward-facing force the engine is producing is equal to the backward-facing forces of friction and air resistance when the car is maintaining a constant velocity. The engine, air resistance, and friction all contribute forces to the car, forces which are vector quantities measured in Newtons (N).