Question

An object with a mass of `4` `kg` is hanging from a spring with a constant of `2` `kgs^-2`. If the spring is stretched by `20` `m`, what is the net force on the object?

Answer 1

The force due to the spring will be `F=kx=2*20=40` `N` upward. The weight force of the mass will be `F=mg=4*9.8=39.2` `N` downward. The net force will be `0.8` `N` upward.

Explanation:

There are two forces acting on the object - it's weight force acting downward and a restoring force from the spring acting upward. The magnitudes of these two forces was calculated above, as was the resultant force.

(One quick side note: spring constants tend to be quoted in `kgs^-2`, but the unit `Nm^-1` is equivalent and seems clearer: if the spring constant is `k`, the force it exerts is `k` `N` per metre the spring is stretched.)