Free fall in the context of "Galileo Galilei"

General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1916 and is the accepted description of gravitation in modern physics. General relativity generalizes special relativity and refines Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or four-dimensional spacetime. In particular, the curvature of spacetime is directly related to the energy, momentum and stress of whatever is present, including matter and radiation. The relation is specified by the Einstein field equations, a system of second-order partial differential equations.

Newton's law of universal gravitation, which describes gravity in classical mechanics, can be seen as a prediction of general relativity for the almost flat spacetime geometry around stationary mass distributions. Some predictions of general relativity, however, are beyond Newton's law of universal gravitation in classical physics. These predictions concern the passage of time, the geometry of space, the motion of bodies in free fall, and the propagation of light, and include gravitational time dilation, gravitational lensing, the gravitational redshift of light, the Shapiro time delay and singularities/black holes. So far, all tests of general relativity have been in agreement with the theory. The time-dependent solutions of general relativity enable us to extrapolate the history of the universe into the past and future, and have provided the modern framework for cosmology, thus leading to the discovery of the Big Bang and cosmic microwave background radiation. Despite the introduction of a number of alternative theories, general relativity continues to be the simplest theory consistent with experimental data.

The gravity of Earth, denoted by g, is the net acceleration that is imparted to objects due to the combined effect of gravitation (from mass distribution within Earth) and the centrifugal force (from the Earth's rotation).It is a vector quantity, whose direction coincides with a plumb bob and strength or magnitude is given by the norm ${\displaystyle g=\|{\mathit {\mathbf {g} }}\|}$.

In SI units, this acceleration is expressed in metres per second squared (in symbols, m/s or m·s) or equivalently in newtons per kilogram (N/kg or N·kg). Near Earth's surface, the acceleration due to gravity, accurate to 2 significant figures, is 9.8 m/s (32 ft/s). This means that, ignoring the effects of air resistance, the vertical component of velocity of an object falling freely will increase in the downwards direction by about 9.8 metres per second (32 ft/s) every second.

In science and engineering, the weight of an object is a quantity associated with the gravitational force exerted on the object by other objects in its environment, although there is some variation and debate as to the exact definition.

Some standard textbooks define weight as a vector quantity, the gravitational force acting on the object. Others define weight as a scalar quantity, the magnitude of the gravitational force. Yet others define it as the magnitude of the reaction force exerted on a body by mechanisms that counteract the effects of gravity: the weight is the quantity that is measured by, for example, a spring scale. Thus, in a state of free fall, the weight would be zero. In this sense of weight, terrestrial objects can be weightless: so if one ignores air resistance, one could say the legendary apple falling from the tree, on its way to meet the ground near Isaac Newton, was weightless.

The g-force or gravitational force equivalent is a mass-specific force (force per unit mass), expressed in units of standard gravity (symbol g or g₀, not to be confused with "g", the symbol for grams).It is used for sustained accelerations that cause a perception of weight. For example, an object at rest on Earth's surface is subject to 1 g, equaling the conventional value of gravitational acceleration on Earth, about 9.8 m/s.More transient acceleration, accompanied with significant jerk, is called shock.

When the g-force is produced by the surface of one object being pushed by the surface of another object, the reaction force to this push produces an equal and opposite force for every unit of each object's mass. The types of forces involved are transmitted through objects by interior mechanical stresses. Gravitational acceleration is one cause of an object's acceleration in relation to free fall.

Gravitational energy or gravitational potential energy is the potential energy an object with mass has due to the gravitational potential of its position in a gravitational field. Mathematically, it is the minimum mechanical work that has to be done against the gravitational force to bring a mass from a chosen reference point (often an "infinite distance" from the mass generating the field) to some other point in the field, which is equal to the change in the kinetic energies of the objects as they fall towards each other. Gravitational potential energy increases when two objects are brought further apart and is converted to kinetic energy as they are allowed to fall towards each other.

An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration (the rate of change of velocity) of the object relative to an observer who is in free fall (that is, relative to an inertial frame of reference). Proper acceleration is different from coordinate acceleration, which is acceleration with respect to a given coordinate system, which may or may not be accelerating. For example, an accelerometer at rest on the surface of the Earth will measure an acceleration due to Earth's gravity straight upwards of about g ≈ 9.81 m/s. By contrast, an accelerometer that is in free fall will measure zero acceleration.

Highly sensitive accelerometers are used in inertial navigation systems for aircraft and missiles. In unmanned aerial vehicles, accelerometers help to stabilize flight. Micromachined micro-electromechanical systems (MEMS) accelerometers are used in handheld electronic devices such as smartphones, cameras and video-game controllers to detect movement and orientation of these devices. Vibration in industrial machinery is monitored by accelerometers. Seismometers are sensitive accelerometers for monitoring ground movement such as earthquakes.