Force

Force is anything that can accelerate an object. The more force that is applied to an object of a given mass, the more the object will accelerate.

Force is defined by Newton's second law as the product of mass and acceleration (F = m x a). The unit of force using the International System of Units (SI) or the metric system is the newton (N). ("Newton" is usually written as a lower-case word even though it is named after Sir Isaac Newton.) Mass is measured in kilograms (kg), and acceleration is measured in meters per seconds squared (meters/seconds2), such that 1 N = 1 kg x 1 m/s2.

For instance, if a 5-kg block is sitting at rest and is then pushed horizontally with an initial acceleration of 3 m/s2, the force that sets the block in motion is 15 newtons [5 kg x 3 m/s2].

The British system, which is used in the United States, measures force in pounds. At the surface of the Earth, the mass of a 1-kilogram brick weighs 2.2 pounds, or exerts 2.2 pounds of force due to gravity.

In conditions of no gravity (or very little gravity), an object is virtually weightless and will not be pulled toward the Earth. It still, however, has mass, expressed in kilograms.

Because of gravity, weight is a force in the downward direction. Thrust is force in the direction that an aircraft is moving. Jet engines are rated according to the amount of thrust, expressed in newtons or pounds of force, that they can generate.

In aerodynamics, four forces affect the flight of an aircraft. These are lift, drag, thrust, and weight.