Potential energy is converted to kinetic energy, which is the energy exerted by a moving object. An active pendulum has the most kinetic energy at the lowest point of its swing when the weight is moving fastest.
Viscosity of liquids is essentially the 'thickness' of the liquid. For instance, honey and water have different viscosities. Honey has a higher one and therefore, liquids with high viscosity do not flow as well as liquids with low viscosity (water).
