An object of mass 3 kg has a kinetic energy of 96 J. What is its speed?

Kinetic energy $KE = \tfrac{1}{2}mv^2$: twice the speed gives four times the energy. The work done on an object equals the change in its kinetic energy; a brake force $F$ stops it in $d$ when $F \times d = \tfrac{1}{2}mv^2$.