Question

A model airplane with mass 0.5 kg hangs from a rubber band with spring constant 45 N/m. How much is the rubber band stretched when the model airplane hangs motionless?

Answer 1

This is a straightforward application of the equation
Force = Spring constant X extension
With the force equal to the weight (not mass) of the model airplane.

Answer 2

To expand on Farcher's explanation, mathematically we have:\[\huge \text{F}_\text{spring}=k \Delta x\]Here, \(\text{F}_\text{spring}\) is the force that acts parallel to the displacement of the spring. In this case, it is the weight of the object that pulls the spring down.