Activation energy is the energy needed for a chemical reaction to start while the rate of reaction is how a reactant turns into the product. Catalysts are used to speeden up the rate of a chemical reaction.
The catalyst do this by lowering the activation energy needed for the reaction to occur, leading to the increase in the rate at which the same reaction occurs. It should be noted that a catalyst is not a reactant, neither does it get used up in the chemical reaction. It just provides an alternative energy pathway with a reduced activation energy, hereby, helping the reaction to proceed faster than it would without the catalyst.
The gravitational potential energy of the ball decreases as it rolls down the hill while the gravitational kinetic energy of the ball increases due to its acceleration in velocity.
