Why are some elements nuclei unstable?

Adding more pulleys to a system of pulleys needed to lift an object. a. increases the force b. decreases the force c. increases the amount of work d. decreases the amount of work

Which simple machine is not part of this compound machine? a. lever b. pulley c. inclined plane d. wheel and axle

If the work required to stretch a spring 3 ft beyond its natural length is 15 ft-lb, how much work is needed to stretch it 27 in. beyond its natural length?