While riding a bike one must use forces to keep the bike moving. By applying a force on the pedals the wheels will begin to rotate. The harder you push the faster the wheels move then the faster you will go. There are also external forces such as gravity. Gravitys goal is to pull any mass towards the center of the earth. Therefore you must keep balence or if not you will fall and gravity will pull you down. Lastly, friction is involved. The wheels on the ground undergo friction.