Forces can be described as a push or pull on an object. They can be due to phenomena such as gravity, magnetism, or anything that might cause a mass to accelerate.
Aristotle famously described a force as anything that causes an object to undergo "unnatural motion"
Though Sir Isaac Newton's most famous equation is, he actually wrote down a different form for his second law of motion that did not use differential calculus
Free body diagrams of a block on a flat surface and an inclined plane. Forces are resolved and added together to determine their magnitudes and the net force.
Galileo Galilei was the first to point out the inherent contradictions contained in Aristotle's description of forces.
Feynman diagram for the decay of a neutron into a proton. The W boson is between two vertices indicating a repulsion.
Images of a freely falling basketball taken with a stroboscope at 20 flashes per second. The distance units on the right are multiples of about 12 millimeters. The basketball starts at rest. At the time of the first flash (distance zero) it is released, after which the number of units fallen is equal to the square of the number of flashes.
Instruments like GRAVITY provide a powerful probe for gravity force detection.
FN represents the normal force exerted on the object.
Fk is the force that responds to the load on the spring
Relationship between force (F), torque (τ), and momentum vectors (p and L) in a rotating system.

Influence that can change the motion of an object.

- Force

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A Pratt & Whitney F100 jet engine being tested. This engine produces a jet of gas to generate thrust. Its purpose is to propel a jet airplane. This particular model turbofan engine powers McDonnell Douglas F-15 and General Dynamics F-16 fighters both.

Thrust is a reaction force described quantitatively by Newton's third law.

Drag (physics)

Drag coefficient Cd for a sphere as a function of Reynolds number Re, as obtained from laboratory experiments. The dark line is for a sphere with a smooth surface, while the lighter line is for the case of a rough surface.
An object falling through viscous medium accelerates quickly towards its terminal speed, approaching gradually as the speed gets nearer to the terminal speed. Whether the object experiences turbulent or laminar drag changes the characteristic shape of the graph with turbulent flow resulting in a constant acceleration for a larger fraction of its accelerating time.
Trajectories of three objects thrown at the same angle (70°). The black object does not experience any form of drag and moves along a parabola. The blue object experiences Stokes' drag, and the green object Newton drag.
The power curve: parasitic drag and lift-induced drag vs. airspeed
Qualitative variation in Cd factor with Mach number for aircraft
thumb|Concorde with 'high' wave drag tail
thumb|Concorde with 'low' wave drag tail
thumb|Hawk aircraft showing base area above circular engine exhaust
Drag coefficient Cd for a sphere as a function of Reynolds number Re, as obtained from laboratory experiments. The dark line is for a sphere with a smooth surface, while the lighter line is for the case of a rough surface.

In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid.


Newton-metre .

Moment arm diagram
The torque caused by the two opposing forces Fg and −Fg causes a change in the angular momentum L in the direction of that torque. This causes the top to precess.
Torque curve of a motorcycle ("BMW K 1200 R 2005"). The horizontal axis shows the speed (in rpm) that the crankshaft is turning, and the vertical axis is the torque (in newton metres) that the engine is capable of providing at that speed.

caption = Relationship between force F, torque τ, linear momentum p, and angular momentum L in a system which has rotation constrained to only one plane (forces and moments due to gravity and friction not considered).


Pressure as exerted by particle collisions inside a closed container
Mercury column
The effects of an external pressure of 700 bar on an aluminum cylinder with 5 mm wall thickness
Water escapes at high speed from a damaged hydrant that contains water at high pressure
Low-pressure chamber in Bundesleistungszentrum Kienbaum, Germany

Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed.

Stress (mechanics)

Residual stresses inside a plastic protractor are revealed by the polarized light.
Roman-era bridge in Switzerland
Inca bridge on the Apurimac River
The stress across a surface element (yellow disk) is the force that the material on one side (top ball) exerts on the material on the other side (bottom ball), divided by the area of the surface.
Glass vase with the craquelé effect. The cracks are the result of brief but intense stress created when the semi-molten piece is briefly dipped in water.
Idealized stress in a straight bar with uniform cross-section.
Shear stress in a horizontal bar loaded by two offset blocks.
Isotropic tensile stress. Top left: Each face of a cube of homogeneous material is pulled by a force with magnitude F, applied evenly over the entire face whose area is A. The force across any section S of the cube must balance the forces applied below the section. In the three sections shown, the forces are F (top right), F, respectively. So the stress across S is F/A in all three cases.
Components of stress in three dimensions
Illustration of typical stresses (arrows) across various surface elements on the boundary of a particle (sphere), in a homogeneous material under uniform (but not isotropic) triaxial stress. The normal stresses on the principal axes are +5, +2, and −3 units.
A tank car made from bent and welded steel plates.
For stress modeling, a fishing pole may be considered one-dimensional.
Toughened glass of car rear window. Variations in glass stress are clearly seen when photographed through a polarizing filter
 (bottom picture).
Simplified model of a truss for stress analysis, assuming unidimensional elements under uniform axial tension or compression.

In continuum mechanics, stress is a physical quantity that expresses the internal forces that neighbouring particles of a continuous material exert on each other, while strain is the measure of the deformation of the material.

Euclidean vector

Geometric object that has magnitude (or length) and direction.

Vector arrow pointing from A to B
Illustration of tangential and normal components of a vector to a surface.
The subtraction of two vectors a and b
Scalar multiplication of a vector by a factor of 3 stretches the vector out.
The scalar multiplications −a and 2a of a vector a
The normalization of a vector a into a unit vector â

Vectors play an important role in physics: the velocity and acceleration of a moving object and the forces acting on it can all be described with vectors.

Dynamics (mechanics)

Diagram of orbital motion of a satellite around the Earth, showing perpendicular velocity and acceleration (force) vectors, represented through a classical interpretation.

Dynamics is the branch of classical mechanics that is concerned with the study of forces and their effects on motion.


Accelerating in the direction of travel.

An oscillating pendulum, with velocity and acceleration marked. It experiences both tangential and centripetal acceleration.
Components of acceleration for a curved motion. The tangential component at is due to the change in speed of traversal, and points along the curve in the direction of the velocity vector (or in the opposite direction). The normal component (also called centripetal component for circular motion) ac is due to the change in direction of the velocity vector and is normal to the trajectory, pointing toward the center of curvature of the path.
Calculation of the speed difference for a uniform acceleration

In classical mechanics, for a body with constant mass, the (vector) acceleration of the body's center of mass is proportional to the net force vector (i.e. sum of all forces) acting on it (Newton’s second law):

Proportionality (mathematics)

Called the coefficient of proportionality or proportionality constant.

The variable y is directly proportional to the variable x with proportionality constant ~0.6.

The force, acting on a small object with small mass by a nearby large extended mass due to gravity, is directly proportional to the object's mass; the constant of proportionality between the force and the mass is known as gravitational acceleration.


Resistance of any physical object to a change in its velocity.

Galileo Galilei
The effect of inertial mass: if pulled slowly, the upper thread breaks (a). If pulled quickly, the lower thread breaks (b)
rotational inertia

An aspect of this property is the tendency of objects to keep moving in a straight line at a constant speed when no forces act upon them.