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The distance (d) a spring will stretch varies directly as the force (f) applied to the spring. If a force of 12lb is required to stretch a spring 3inchs, what force is required to stretch the spring 5 inches?
If d varies directly with f the math way of saying this is
d=kf where k is a constant of proportionality.
In these problems you are usually given enough information
to find k, as follows:
So for your spring the proper equation relating distance and force
Now you are asked what force is required to stretch the spring 5 inches.
Multiply both sides by 121 to get:
Divide both sides by 3 to get:
That is the required force.