Gravity affects every aspect of our lives, and it plays a big role in mountain biking, especially when it come to hills. Whenever you pedal up a hill, you have to oppose the force of gravity that is working against you. However, the opposite is true for riding down a hill. In this case gravity is working to your advantage and is pulling you down the hill. So, as you bike down hills, you can accelerate due to your own power as well as accelerate due to the force of gravity.
Lets take a more in-depth look at this force called gravity!
The force of gravity exerts a constant acceleration of 9.81 m/sec^2 towards the center of the earth. While gravity is being exerted downward, a normal force is being exerted on the rider opposing gravity. This normal force acts perpendicular to the earth’s surface, and for our purposes the trail a biker is riding. So for example, if a biker were on a flat surface, both gravity and the normal force would be acting upon the biker, but in opposing directions, thereby canceling each other out and can be ignored. However, this combination of gravity and the normal force is what makes climbing up hills difficult, and riding down hills a breeze.