Every part of a structure is being pushed or pulled by its neighbours. Engineers sort those internal forces into a few kinds:

• Compression: the material is squeezed. A column under a roof, a stack of books under a hand.
• Tension: the material is stretched. A rope holding a lamp, a bridge cable, a stretched rubber band.
• Shear: layers of the material try to slide past each other, like cutting paper with scissors.
• Bending: a beam sags, so its top edge is squeezed and its bottom edge is stretched.

A force alone does not tell you whether a part will survive. What matters is the force spread over the area that carries it. That is stress: stress = force ÷ area, measured in pascals (Pa), or megapascals (MPa, millions of pascals). A 1 kN load (about the weight of a 100 kg mass) hanging from a rod with a cross-section of 100 mm² gives 1 000 N ÷ 0.0001 m² = 10 MPa.