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# Load, restoring force and extension

The load is the force that causes an elastic material to deform.

The force with which an archer pulls back the string of a bow is the load.

The restoring force is the force that causes an elastic material to return to its original shape.

The restoring force is equal in magnitude but opposite in direction to the load.

The force that causes an archer's bow to snap back is the restoring force.

When analysing springs, the displacement $\vecphy{x}$ of the spring's end from its equilibrium position as a result of the load being applied is called the extension.

The direction of the restoring force $\vecphy{F_{\text{s}}}$ depends on whether the spring is compressed (B) or stretched (C).