And this is true for physical as well as chemical reaction. So what we mean is let's suppose the following physical reaction. So here we have an object, our marker. And the marker has a certain amount of potential energy as a result of its position with respect to the ground. So remember, the Earth exerts a gravitational force, and that gives it a certain amount of gravitational potential energy. Now, as soon as I let go of my marker, the marker will begin to travel downward.