Last edited 28 Oct 2020


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In terms of physics, a force is an interaction that changes the motion of an object when it is unopposed. This takes the form of a push or a pull on an object. Forces only exist as a result of an interaction which causes an object with mass to change its velocity; when the interaction stops then the object no longer experiences the force. Since a force has both magnitude and direction, it is considered a vector quantity. Force is measured in the SI unit of Newtons, and represented by the symbol ‘F’.

The two broad categories of forces between objects are contact forces and forces that result from action-at-a-distance.

Contact forces are types of force that occur when the two interacting objects physically connect with one another. These include:

  • Friction: The force that resists the relative motion of solid objects, surfaces, fluid layers and material elements sliding against one another.
  • Tension: The force transmitted through a string, rope, cable or wire when pulled tight by oppositional forces.
  • Normal force: The support force exerted upon an object that is in contact with another stable object. For example, an object on the surface of a table is supported by an upward force being exerted by the table surface.
  • Air resistance: The frictional force air exerts against a moving object. This is also known as ‘drag’.
  • Applied force: A force applied to an object by a person or another object.
  • Spring force: A restoring force exerted by a spring, which acts to restore a spring towards equilibrium.

Action-at-a-distance forces occur when two interacting objects are not in physical contact with one another but still exert a push or pull. These include:

  • Gravitational force: The phenomenon by which all things with mass are brought toward one another.
  • Electrical force: The attractive or repulsive interaction between any two charged objects.
  • Magnetic force: The attraction or repulsion that arises between electrically charged particles due to their motion.

Structural analysis can be used to calculate the effects of forces acting on any component and on a structure overall.

Three properties of forces that should be considered are:

  • Magnitude: The size of the force.
  • Direction: The direction in which the force is acting.
  • Position: The position on which the force acts.

Isaac Newton developed three laws of motion:

  • First law: An object will remain at rest or in uniform motion unless compelled to do otherwise by some external force acting on it.
  • Second law: A force is caused by an acceleration acting on an object.
  • Third law: Action and reaction are equal and opposite.

A shear force is a force applied perpendicular to a surface, in opposition to an offset force acting in the opposite direction. This results in a shear strain. For more information, see Shear force.

An uplift force is any upward pressure applied to a structure that has the potential to raise it relative to its surroundings. Uplift forces can be a consequence of pressure from the ground below, wind, surface water, and so on. For more information, see Uplift force.

Stiffness is the extent to which an element is able to resist deformation or deflection under the action of an applied force. In contrast, flexibility or pliability is a measure of how flexible a component is, i.e. the less stiff it is, the more flexible it is.

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