What Causes Motion?

January 18, 2026

Ailon Offer

Williamsville East High School

11th Grade

Why do things move? What causes motion? These questions, being highly fundamental, have concerned many thinkers throughout history. A notable philosopher who attempted to explain motion was Aristotle. He suggested that every movement requires an agent: “Everything that is in motion must be moved by something.”  This approach does seem intuitive even today without an understanding of Newton’s laws; for example, for a car to move on a flat road, there needs to be a running engine that is the agent, and once the engine fails, the car will gradually stop moving. 

Because of its intuitiveness, Aristotle’s motion theory had remained the accepted explanation for 2000 years. In the 17th century, the theory was finally challenged when Galileo Galilei discovered from his experiments that bodies tend to maintain motion when resisting forces are minimized; this led to the belief that objects have a natural tendency to remain in their current state of motion—movement or rest.

 Galileo’s findings inspired Isaac Newton’s First Law of Motion, the principle of inertia, which also integrated mathematics and used more empirical evidence. Newton’s law was universally applied to celestial and terrestrial objects alike, from the planets to objects on Earth, and stated that “Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by the forces impressed.” 

Newton’s First Law means that motion, just like rest, is a natural state that would be preserved unless a force is acted upon. The principle of inertia therefore contrasts Aristotle’s claim that motion must have an agent (like a force) but declares that a change in motion requires a force. Granted, Newton’s First Law changes the question from “What causes motion?” to “What causes a change in motion?” And the answer to that question is force, or more accurately an unbalanced force (when forces of the same magnitude act in opposite directions, they cancel each other out, and the force is balanced and doesn’t affect motion).

The relationship between force and the change in motion is described in Newton’s Second Law of Motion: “A change in motion is proportional to the motive force impressed and takes place along the straight line in which that force is impressed.” Newton simply states that force is proportional to a change in motion. The quantity of motion Newton had intended is widely interpreted as momentum (p), which is mass times velocity (p = mv). This means that force (F) = the change in momentum (derivative) or the change in mass times velocity with respect to time:

(Image credit: The University of Texas at Austin)

In many scenarios the mass of the object will remain constant; for instance, when a chair is moved, its mass does not change, and thus the mass could be pulled out of the derivative. This changes the formula to force equals mass times the change in velocity with respect to time, which is acceleration, creating the formula most associated with Newton’s second law: F = ma 

(Image credit: The University of Texas at Austin) 

Although F = ma is associated with Newton’s Second Law, it never appeared in any of Newton’s writings, as he proved his laws geometrically. It was Jacob Hermann and Leonhard Euler who described it using calculus in the following century. 

F = ma explains the cause of a change in velocity: the greater the net force becomes, the higher the acceleration of the object will be. When the net force is against the direction of the object’s movement, the object will decelerate; going back to an earlier example, when the engine of a moving car stops running, no force would act in the direction of the car’s movement, and the force of friction between the tires and the road (and air resistance) would act against the car’s movement, causing the car to decelerate and come to a gradual stop as shown in the following images.

(Image credit: pathwayz.org) 

In the image towards the left, the traction is similar to or higher than the resistance (zero/positive net force), meaning the car is going at a constant speed or accelerating. Meanwhile, in the image towards the right, there is only resistance against the car’s motion (negative net force), meaning the car is decelerating and losing speed.

As mentioned earlier, F = ma is not relevant in cases where the mass of the object is not constant; for example, the net force of a flying rocket could not be calculated using its measured mass on the ground times its acceleration since its mass is constantly changing as it loses fuel.