O que é um/uma Newton (N)?
Formal Definition
The newton (symbol: N) is the SI derived unit of force. It is defined as the force required to accelerate a mass of one kilogram at a rate of one meter per second squared. In SI base units: 1 N = 1 kg·m·s⁻². The newton is named after Sir Isaac Newton (1643–1727), whose laws of motion form the foundation of classical mechanics.
The definition follows directly from Newton's second law of motion: F = ma, where F is force in newtons, m is mass in kilograms, and a is acceleration in meters per second squared. This relationship makes the newton the natural force unit in the SI system, connecting mass, length, and time in a coherent way.
Intuitive Scale
One newton is approximately the force of Earth's gravity on a small apple (about 102 grams). This is a fitting coincidence given the famous (though likely apocryphal) story of Newton being inspired by a falling apple. In everyday terms, one newton is a relatively small force — roughly the weight of a stick of butter or a small smartphone. A person weighing 70 kg exerts a gravitational force of about 686 newtons on the floor.
Etymology
Named After Isaac Newton
The unit is named after Sir Isaac Newton (1643–1727), the English mathematician, physicist, and astronomer who formulated the laws of motion and universal gravitation. Newton's Principia Mathematica (1687) laid the mathematical foundation for classical mechanics and remained the dominant framework for physics for over two centuries, until Einstein's theory of relativity.
The name "newton" for the unit of force was adopted at the 9th General Conference on Weights and Measures (CGPM) in 1948, as part of a broader effort to complete the coherent system of SI derived units. Before this, force was often expressed in dynes (CGS system), kilogram-force (technical system), or pounds-force (imperial system).
Pronunciation and Style
When used as a unit name, "newton" is written in lowercase (as are all SI units named after people), while the symbol "N" is capitalized. The plural in English is "newtons" (e.g., "a force of 50 newtons"). In many other languages, the unit name is adapted to local pronunciation but the symbol N remains universal.
Precise Definition
SI Definition
The newton is a derived SI unit defined as: 1 N = 1 kg·m·s⁻². This definition is exact and follows from Newton's second law. Since the kilogram, meter, and second are all defined in terms of fundamental physical constants (the Planck constant, the speed of light, and the cesium-133 hyperfine frequency), the newton is ultimately traceable to these invariant quantities.
Relationship to Weight
The weight of an object is the gravitational force acting on its mass. On Earth's surface, where standard gravitational acceleration is g = 9.80665 m/s² (by convention), the weight in newtons of an object of mass m kilograms is: W = m × g. A 1 kg mass weighs approximately 9.81 N on Earth, approximately 1.62 N on the Moon, and approximately 3.71 N on Mars.
Key Conversions
1 N = 100,000 dynes = 0.224809 pounds-force = 0.101972 kilogram-force = 7.23301 poundals = 0.001 kilonewtons. One kilonewton (kN) = 1000 N, and one meganewton (MN) = 1,000,000 N.
História
Newton's Laws of Motion
The scientific foundation for the newton as a force unit is Isaac Newton's three laws of motion, published in Philosophiae Naturalis Principia Mathematica in 1687. The second law — force equals mass times acceleration — directly defines the relationship that the newton unit captures. Newton did not, however, define a unit of force in modern terms; the Principia used geometric rather than algebraic methods.
The CGS Era
Before the SI system, the centimeter-gram-second (CGS) system was dominant in physics. The CGS unit of force was the dyne, defined as the force needed to accelerate one gram by one centimeter per second squared (1 dyn = 1 g·cm·s⁻² = 10⁻⁵ N). The dyne was impractically small for engineering use, which led to the widespread adoption of non-coherent units like the kilogram-force.
The Kilogram-Force Problem
For centuries, engineers used the kilogram-force (kgf) — the gravitational force on a one-kilogram mass — as a practical unit of force. This created confusion because the kilogram served double duty as both a mass unit and (informally) a force unit. The distinction between mass and weight was blurred in everyday language, and engineers routinely conflated the two. This ambiguity led to errors in calculations, particularly when working at non-standard gravitational conditions.
Adoption of the Newton
The 9th CGPM in 1948 established the newton as the SI unit of force, specifically to resolve the mass-force ambiguity. The name honored Isaac Newton and provided a clear, unambiguous unit of force within the coherent SI framework. Adoption was gradual: scientists adopted the newton quickly, but engineers — particularly in countries using imperial units — were slower to change. Today, the newton is the standard force unit in virtually all scientific and most engineering contexts worldwide.
The Mars Climate Orbiter
The importance of consistent force units was dramatically illustrated in 1999 when NASA's Mars Climate Orbiter was destroyed because one engineering team used pound-force-seconds while another used newton-seconds for impulse calculations. The resulting navigation error caused the spacecraft to enter the Martian atmosphere at too low an altitude and disintegrate. The $327.6 million loss underscored the critical importance of unit consistency.
Uso atual
Science and Education
The newton is the standard force unit in physics education and research worldwide. Force diagrams (free-body diagrams), Newton's laws problems, and mechanics courses all use newtons as the primary force unit. Physics textbooks, scientific papers, and laboratory reports universally express force in newtons or their multiples (kN, MN, GN).
Engineering
In engineering practice outside the United States, the newton is the standard unit for all force-related calculations. Structural loads, material strengths, aerodynamic forces, and hydraulic pressures are all expressed using newton-based units. Even in the US, aerospace engineering (post–Mars Climate Orbiter) has largely adopted SI units. The newton per square meter (pascal) is the SI unit of pressure, and the newton-meter is the SI unit of torque.
Product Specifications
Consumer products marketed internationally increasingly use newtons for force specifications. Climbing equipment (rope strength in kilonewtons), sports equipment (racket string tension in newtons), and industrial hardware (spring forces in newtons) are all rated in SI force units. European regulations require force-related specifications on products to be stated in newtons.
Everyday Use
Weight and Gravity
The most common everyday application of the newton is in expressing weight — the gravitational force on an object. A medium apple weighs about 1 N. A liter of milk weighs about 10 N. A typical adult weighs 600–900 N. A loaded grocery bag weighs about 50–100 N. While most people think of weight in kilograms or pounds, these are technically mass units; the newton is the correct unit for the force of gravity.
Sports and Recreation
Climbing ropes are rated by their maximum impact force in kilonewtons — typically 8–12 kN. Carabiners are rated for minimum breaking strength, usually 20–25 kN along the major axis. Tennis racket string tension is measured in newtons, typically 180–270 N (40–60 lbs). Fishing line breaking strength can be expressed in newtons.
Pushing and Pulling
Everyday forces can be expressed in newtons for perspective: opening a door requires about 5–15 N, typing on a keyboard key requires about 0.5–0.6 N, pressing an elevator button about 2–5 N, and pulling a suitcase about 20–50 N. The force required to crush an aluminum can is about 100–150 N.
Vehicles and Safety
Seat belt pretensioners apply about 2,000–4,000 N of force to restrain an occupant during a crash. Airbag deployment forces range from 1,000 to 5,000 N. Tire grip on dry pavement generates about 8,000–12,000 N of friction force for a typical car. These force levels, expressed in newtons, inform vehicle safety engineering.
In Science & Industry
Classical Mechanics
The newton is fundamental to all calculations in classical mechanics. Newton's second law (F = ma), the universal law of gravitation (F = Gm₁m₂/r²), Hooke's law for springs (F = kx), and Coulomb's law for electrostatic force (F = kq₁q₂/r²) all express force in newtons within the SI framework. The coherence of SI units means that using kilograms, meters, seconds, and newtons consistently produces correct results without conversion factors.
Particle Physics
The fundamental forces of nature are expressed in newton-compatible units. The strong nuclear force between quarks reaches approximately 10⁵ N at typical hadronic distances. The electromagnetic force between an electron and proton in a hydrogen atom is about 8.2 × 10⁻⁸ N. The gravitational force between two protons separated by one femtometer is about 1.87 × 10⁻³⁴ N — illustrating the extraordinary weakness of gravity compared to other fundamental forces.
Nanotechnology
Atomic force microscopes (AFMs) measure forces in the piconewton (10⁻¹² N) to nanonewton (10⁻⁹ N) range. These instruments can detect the force required to break a single chemical bond (about 1–10 nN), the adhesion force of a single bacterium (about 0.1–10 nN), and the elastic properties of individual protein molecules. The sensitivity of AFMs extends to below 1 piconewton in specialized configurations.
Interesting Facts
One newton is approximately the weight of a medium apple (about 102 grams on Earth) — a fitting coincidence given the famous story of Newton and the falling apple that supposedly inspired his theory of gravity.
The Mars Climate Orbiter was destroyed in 1999 because one engineering team used pound-force-seconds and another used newton-seconds. The $327.6 million spacecraft entered the Martian atmosphere too low and disintegrated.
The gravitational force between two people standing one meter apart (each weighing 70 kg) is approximately 0.00000033 N — far too weak to feel, but enough for precision instruments to detect.
An atomic force microscope can measure forces as small as a few piconewtons (10⁻¹² N) — roughly the force exerted by a single myosin motor protein pulling on an actin filament inside a living cell.
The thrust of the Saturn V rocket at liftoff was approximately 34,000,000 N (34 MN). This was enough force to accelerate the 2,800-tonne rocket upward against gravity and send astronauts to the Moon.
The strongest recorded bite force of any animal belongs to the saltwater crocodile, at approximately 16,000 N. A human bite force is about 700 N, and a great white shark's is about 18,000 N.
Earth's gravitational pull on the Moon is approximately 2 × 10²⁰ N — twenty billion billion newtons. This force keeps the Moon in orbit at an average distance of 384,400 km.
A single gecko foot pad generates about 10 N of adhesive force using van der Waals interactions between millions of nanoscale hair-like structures (setae) and the surface.