Click on the screenshot above to view the "Forces" animation by Laszlo Lipot

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What is a force?

A force is a pushing, pulling, twisting, shearing or bending effect on an object.

External forces

External forces are forces applied to an object by people, tools, machines or other objects.

Internal forces

Internal forces within materials are the molecular bonds within materials and their reactions to external forces.  
All materials are composed of elements or compounds. Each element has its own distinct chemical and physical properties that make it a solid, liquid or a gas, hard, soft, ductile or malleable, a metal or non-metal etc.  

The materials we use to make products retain their shape because of the strength of their internal molecular bonds. These internal bonds resist external forces to various degrees, depending on the element and the strength of its internal molecular bonds. The shape of the material will not change unless a force greater than the strength of the material's internal molecular bonds is applied to it.  

The strength of a material's internal molecular bonds can be weakened in some cases by: 

  • heating: e.g. heat causes metals to expand and weaken the molecular bonds holding the metal in its original shape. When a metal is red hot, or white hot, it can easily be hammered, bent and twisted into new shapes. If the metal is heated further, its molecular bonds weaken to the point that the metal turns to a liquid and flows.
  • work hardening: work hardening occurs when a metal is hammered or bent repeatedly and stresses are created in the metal. The metal first gets very hard at the point where it was hammered or bent repeatedly, due to the strain imposed on the material's molecular bonds. If hammering or bending continues, the metals molecular bonds will be strained to breaking point.

These internal stresses can be relieved in metals by processes called annealing and normalising. These processes involve heating a metal to a point at which the metal will expand, its molecules move further apart and the stresses between the molecular bonds are relieved. On cooling, the metal contracts, the molecules move closer together and settled in equilibrium, i.e. in an unstressed state. It remains in its hammered, bent or twisted shape but the stresses between the molecules have been relieved.

Measuring Forces

The Newton (N) is the standard unit used for measuring a force. One Newton (N) is the amount of force required to support 100g.

Describing Forces

A force is described by its:

  • magnitude, i.e. the size of the force
  • direction
  • location, i.e. where the force is applied.


What is a load? Click the link to view an animation and description of a static load and a dynamic load acting on a structure.

What is "Factor of safety"?. Click the link to view a description of Factor of Safety

What is damage tolerance? Click the link to view an animation explaining damage tolerance.


Click here to view the PDF version of this resource. MDTE PDF Forces and safety PDF
Copyright Laszlo Lipot.