Ch. 10.4External and Internal Forces

There are two types of forces that designers have to consider:

External Forces are forces that act on a structure from the outside.

Forces that act between two different parts of a structure are called internal forces.

External Forces Gravity is a non contact force that acts downward on an object.

Applied/ contact forces act from the outside. These include wind, earthquakes, weight of people on the floors, weight of the building itself.

You can apply an external force (such as push, pull, lift) on an object.

Point of application is the location on an object where an external force is applied.

Plane of application is the imaginary flat surface through which the applied for passes.

Diagram:

Magnitude and direction of the applied force can be the same, but the point of application and plane of application may be different.

If I pushed high on an object it will likely tip over. If I push low, it will probably just move in that direction.

Another important external force is the force in the direction opposite gravity.

If you are sitting in your chair, why aren’t you being pulled completely to the center of the earth?

A chair will apply a force on whoever is sitting. The magnitude of the downward force (gravity) equals the magnitude of the upward force.

Internal Forces Four types: Tension, compression, torsion and shear.

Internal forces act between different parts of the same structure.

Tension Tension: an internal force pulling the particles of an object apart.

If an object is pulled too far, it will eventually break.

i.e.,: An elastic band, electrical power lines, trampoline, guitar strings, suspension bridge

Compression Compression: An internal force that presses or squeezes the particles of an

object together.

Object generally returns to its original shape afterwards.

i.e.,: spring in a mattress, soccer ball

Torsion Torsion: Internal twisting force created in an object as a result of a twisting

motion being applied to the object.

i.e., twisting a wash cloth, doorknob is turned

Shear Shear: forces acting in an object as a result of pushes and/or pulls in

opposite directions; usually results in rips or tears in an object

i.e.,: scissors to cut paper