Elasticity, weight and equilibrium
Forces can deform an object, change its motion, or turn it. This lesson connects these effects through measurable laws.
1. Deformation and elasticity
Tension stretches, compression shortens, bending curves, and shear slides neighbouring layers. An elastic object returns to its original form after the force is removed. A plastic change remains. For a spring in its linear region, the restoring force magnitude obeys F = kΔx, where k is stiffness in N/m and Δx is extension or compression in metres. On a force-extension graph, k is the gradient. Beyond the limit of proportionality the graph is no longer straight; beyond the elastic limit permanent deformation can occur.
2. Weight and centre of mass
Mass measures inertia and is expressed in kilograms. Weight is the gravitational force W = mg in newtons. In a uniform gravitational field the distributed force can be represented by one downward force acting at the centre of mass. The normal force, tension, friction and support reactions must be drawn separately on a free-body diagram.