Physics of Downhill Mountain Biking

By Sierra Mielens


Balanced Forces

Balanced forces are when two or more forces are acting in opposing directions with an equal amount of force (

An example of balanced forces in downhill mountain biking is pedaling to try to get some speed to resist the force of friction on the bike. For a moment, the force the biker applies to the pedals will be the same as the gravity working against them. This means that the force the biker applies to the pedals and the force of gravity will cancel each other out.

Unbalanced Forces

Unbalanced forces are when two or more forces acting on an object are not equal to each other (

An example of unbalanced forces is when the biker brakes too much, causing enough friction on their tire to stop their speed. If there was a sudden stop like this, it would cause the biker to fall over their handle bars because of their momentum. The amount of friction acting on the bike would be overwhelming compared to the amount of speed the biker had.


Gravity is a force that pulls anything with mass or matter to the ground (

An example of gravity in downhill mountain biking is taking a jump and being momentarily in the air, but being pulled back down. The bike and the biker's mass would need to be known to calculate how much gravity is acting on them. Gravity will pull the bike and the biker down to the groud because they have masses.


Friction is the resistance that an object encounters when it is moving across another object or surface (

An example of friction is braking. Braking on a bike is caused by small pads putting pressure against the rim of the tires. This creates friction to slow the bike down, but it is also dependent upon how much pressure the biker applies to the brakes.

Newton's Laws

First Law & Inertia

Newton's 1st Law states that an object has a tendency to stay in motion unless it is affected by an unbalanced force. Inertia is an object's tendency to resist another force.

An example of this in downhill mountain biking is falling off of the bike. When you fall, you carry the momentum that you had before you fell (inertia). Newton's 1st Law explains this. You will continue to move at that rate until another force, in this case friction from the ground, stops or slows you down.

Second Law

Newton's 2nd Law states that force equals mass times acceleration, or F=mXa.

An example of Newton's Second Law is starting to bike from a stop. Your acceleration is determined by how hard you pedal and what gear you are in. It also depends on your mass and the mass of the bike. Calculate how much force you are applying to the pedals, find the mass of yourself and the bike, and then divide mass by force. The formula can be rearranged to adapt to almost any circumstance.

Third Law

Newton's 3rd Law states that for every action force, there is an equal and opposite force reacting on the first force/object.

An example of Newton's Third Law in downhill mountain biking is when you are pushing your bike. When you push on the bike, the bike exerts an equal and opposite force on you.