Gravity is the force of attraction between objects and the center of the earth. It is always present and performing work either for or against you. Gravity is the reason it is harder to bike up a hill than to go down one. When traveling up a hill gravity is working against you. It is a force that pulls you and your bike toward the center of the earth, opposite to your preferred direction. As you continue to travel farther up the hill the force of gravity gets stronger. Because Force = Mass x Gravity (Newton’s second law), a greater force must be applied to the pedals to compensate for the opposing force of gravity.

On the way back down the hill gravity is still doing work on you and your bike only now it is to your advantage.