Sir Isaac Newton introduced three fundamental laws of motion in 1687, forming the foundational concepts of classical mechanics. These laws describe the relationship between a body and the forces acting upon it, explaining how objects move. The Second Law is the most practical for quantifying and understanding the mechanics of movement and change in motion.
The Simple Definition and Formula
Newton’s Second Law states that the acceleration of an object is directly proportional to the net force acting upon it and inversely proportional to its mass. This means that pushing an object harder speeds it up faster, but a heavier object speeds up slower when pushed with the same effort. The law is expressed by the algebraic equation: F = m a, where F is the net force, m is the mass, and a is the acceleration.
The term “net force” refers to the vector sum of all individual forces acting on the object. Since force and acceleration are vector quantities, they always point in the same direction. If the net force is zero, the object will not accelerate, which is consistent with the First Law of Motion.
Understanding the Variables
The formula F = m a relates three distinct physical quantities, each with a specific definition and unit of measurement.
Force (F)
Force is fundamentally a push or a pull exerted on an object. The standard unit for force is the newton (N), defined as the force necessary to accelerate a one-kilogram mass at a rate of one meter per second squared.
Mass (m)
Mass represents the amount of matter contained within an object and quantifies its resistance to a change in motion. It is measured using the standard unit of the kilogram (kg).
Acceleration (a)
Acceleration is the rate at which an object’s velocity changes over time. This change can involve speeding up, slowing down, or changing direction, and its standard unit is meters per second squared (m/s²).
Everyday Applications
The principles of the Second Law are at work in nearly every movement observed in daily life. Consider the difference between pushing an empty shopping cart and one filled with groceries. Applying the same force results in the empty cart accelerating much faster because its mass is significantly lower, demonstrating the inverse relationship between mass and acceleration.
The law also explains the design of vehicle safety features, such as airbags and crumple zones. In a car crash, the force experienced by a passenger is determined by their mass and the rate of deceleration. Engineers design these features to increase the time over which the car slows down, thereby reducing the large force applied to the occupants.
Throwing a ball illustrates the direct relationship between force and acceleration. A professional pitcher applies a much larger force to the ball than a child, resulting in greater acceleration and final speed.