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Internal forces are the forces that act within a body or object. They are caused by the interactions between different parts of the object and can be either tensile, compressive, or shear forces.
Internal forces can cause objects to deform or change shape. They can also cause objects to break or fail if the forces exceed the object's strength.
Stress is the measure of the internal forces acting within an object per unit area. It is typically represented by the symbol σ and is measured in units of force per unit area (such as N/m² or Pa).
Strain is the measure of how much an object deforms under the influence of stress. It is typically represented by the symbol ε and is dimensionless (no units).
Stress and strain are directly related through a material's Young's modulus, which is a measure of its stiffness. The equation σ = Eε, where E is the Young's modulus, describes this relationship.