Question

A block of mass m is hanging from the ceiling by a spring whose spring constant is k. Now the spring is pulled further down by x units.
Find the spring force acting on the block at that instant?

• A. mg
• B. kx
• C.
• D. None of these

Answer 1

The correct option is C

When you hang the block on the spring, it will be pulled down by block's weight downward and will have extension ${}^{\prime }{x}_0^{\prime }$.
Drawing free body diagram

Now the spring is pulled down further by x
Total extension in the spring = $x_0+x=(\frac{mg}{k}+x)$
So spring force = K$(\frac{mg}{k}+x)$