Everything about Force Physics totally explained
In
physics, a
force is a push or pull that can cause an object with
mass to
accelerate. Force has both
magnitude and
direction, making it a
vector quantity. According to
Newton's Second Law, an object will
accelerate in proportion to the
net force acting upon it and in inverse proportion to the object's
mass. An equivalent formulation is that the net force on an object is equal to the
rate of change of
momentum it experiences. Forces acting on three-dimensional objects may also cause them to
rotate or
deform, or result in a change in
pressure. The tendency of a force to cause rotation about an axis is termed
torque. Deformation and pressure are the result of
stress forces within an object.
Since antiquity, scientists have used the concept of force in the study of
stationary and
moving objects. These studies culminated with the descriptions made by the third century BC philosopher
Archimedes of how
simple machines functioned. The rules Archimedes determined for how forces interact in simple machines are still a part of modern physics. Earlier descriptions of forces by
Aristotle incorporated fundamental misunderstandings, which wouldn't be resolved until the seventeenth century when
Isaac Newton correctly described how forces behaved. This theory, based on the everyday experience of how objects move, such as the constant application of a force needed to keep a cart moving, had conceptual trouble accounting for the behavior of
projectiles, such as the flight of arrows. The place where forces were applied to projectiles was only at the start of the flight, and while the projectile sailed through the air, no discernible force acts on it. Aristotle was aware of this problem and proposed that the air displaced through the projectile's path provided the needed force to continue the projectile moving. This explanation demands that air is needed for projectiles and that, for example, in a
vacuum, no projectile would move after the initial push. Additional problems with the explanation include the fact that
air resists the motion of the projectiles.
These shortcomings wouldn't be fully explained and corrected until the seventeenth century work of
Galileo Galilei, who was influenced by the late medieval idea that objects in forced motion carried an innate force of
impetus. Galileo constructed an experiment in which stones and cannonballs were both rolled down an incline to disprove the
Aristotelian theory of motion early in the seventeenth century. He showed that the bodies were accelerated by gravity to an extent which was independent of their
mass and argued that objects retain their
velocity unless acted on by a force, for example
friction.
Newtonian mechanics
Isaac Newton is the first person known to explicitly state the first, and the only, mathematical definition of force—as the time-derivative of momentum:
. In 1687, Newton went on to publish his
Philosophiae Naturalis Principia Mathematica, which used concepts of
inertia, force, and
conservation to describe the motion of all objects.
Newton's second law
A modern statement of Newton's second law is a
vector differential equation:
»
where
is the
spring constant. The corresponding
CGS unit is the
dyne, the force required to accelerate a one gram mass by one centimeter per second squared, or g•cm•s
−2. 1 newton is thus equal to 100,000 dyne.
The
foot-pound-second Imperial unit of force is the
pound-force (lbf), defined as the force exerted by gravity on a
pound-mass in the
standard gravitational field of 9.80665 m•s
−2.
[ The pound-force provides an alternate unit of mass: one slug is the mass that will accelerate by one foot per second squared when acted on by one pound-force.][ An alternate unit of force in the same system is the poundal, defined as the force required to accelerate a one pound mass at a rate of one foot per second squared.][ The units of slug and poundal are designed to avoid a constant of proportionality in Newton's Second Law.]
The pound-force has a metric counterpart, less commonly used than the newton: the kilogram-force (kgf) (sometimes kilopond), is the force exerted by standard gravity on one kilogram of mass.[ The kilogram-force leads to an alternate, but rarely used unit of mass: the metric slug (sometimes mug or hyl) is that mass which accelerates at 1 m•s−2 when subjected to a force of 1 kgf. The kilogram-force isn't a part of the modern SI system, and is generally deprecated; however it still sees use for some purposes as expressing jet thrust, bicycle spoke tension, torque wrench settings and engine output torque. Other arcane units of force include the sthène which is equivalent to 1000 N and the kip which is equivalent to 1000 lbf.
]
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