Forces and stresses

Forces

Force is an interaction that, when unopposed, changes the motion of an object. A force can cause an object to accelerate, slow down, remain in place, or change shape.
Every force involves the interaction of two or more bodies, either through contact forces (tension, friction, normal, air resistance, applied), or forces acting at a distance (gravitational, electrical, magnetic).
Forces are measured in the unit of Newtons (N) and are depicted as vectors, which means they have both magnitude and direction.
Balanced and unbalanced forces: Balanced forces are equal in size but opposite in direction. If an object is subject to balanced forces it stays still or continues to move at the same speed. Unbalanced forces lead to a change in movement.

Stress in the context of materials and structures is defined as the internal forces that neighbouring particles of a material exert on each other.
It’s often measured in Pascal (Pa) or N/m². Stress = force/area.
Types of stress include tensile stress, compressive stress, and shear stress.
- Tensile stress (or tension) is the stress state leading to expansion; that is, the length of a material tends to increase in the direction of applied force.
- Compressive stress results in a material in the state of compression, shrinking or volume reduction.
- Shear stress is the stress state leading to the deformation of materials in such a manner that parallel internal surfaces slide past one another.
Elastic deformation occurs when the material returns to its original shape after the load is removed. Plastic deformation occurs when the material does not return to its original shape after the load is removed.
The point at which a material changes from elastic deformation to plastic deformation is known as the yield point.
Strain is a measure of deformation representing the displacement between particles in the material body. Strain = change in length/original length.
Modulus of elasticity is a measure of a material’s ability to withstand changes in length when under lengthwise tension or compression. It’s defined as the stress/strain in the region of proportional limit.

Hooke’s Law states that the force needed to extend or compress a spring by some distance is proportional to that distance, demonstrating elasticity.
Young’s Modulus is a mechanical property that measures the stiffness of a solid material. It’s the ratio of stress (on an object) to strain (in the material of the object).