In the sport of mountain biking you experience both accelerations and decelerations. This is what makes mountain biking so exciting, but it also the reason why you should always wear a helmet! Acceleration is defined as a change in velocity divided by a given amount of time. A rider accelerates as he or she exerts energy and starts to pedal, but can also accelerate as he or she rides down a hill, and this acceleration is due to the force of gravity mentioned earlier. As you accelerate, the weight of the wheels affects the acceleration process three times as much as the weight of the bike and its rider. So one pound in a wheel feels and behaves more like three pounds. This explains the reason why wheels and tires are made as light as possible. Once the rider eventually coasts to a stop or applies the brakes, he or she experiences a deceleration. This deceleration is what causes the bumps and bruises a mountain biker is bound to get as they ride and eventually crash.