How won't the answer be 10m per sec. A 100kg box is sitting on top of a shelf and has 500J of gravitational potential energy. if the box were to fall off the shelf what its velocity be?

At that speed, its kinetic energy would be quite a bit more than its previous potential energy.To solve this, assume that all the potential energy will be converted to kinetic energy. Replace the energy and the mass into the equation for kinetic energy, and solve for speed.