# Physics of cycling: Forces. Basic notions

## Forces on the cyclist and the bicycle. Basic notions

When we ride a bicycle, various forces act on us. If we focus on forward motion, the most significant ones for reaching and maintaining a certain speed are the following.

- Air resistance or aerodynamic drag
- Rolling resistance (friction of the wheels on the ground)
- Gravitational pull force (due to the weight of the bicycle-cyclist combination)
- Acceleration force (only when there is a change of speed, it does not exist at constant speed).
- Marginal forces: Friction of the chain, gears and bearings, bending losses of the frame, etc.

These are the five main forces that must be overcome to reach and maintain a certain speed. The marginal forces are not considered in the following, but are minimised by correct maintenance and lubrication. The final state of the movement will depend on the difference between the forces in the direction of displacement and the forces in the opposite direction of displacement.

**Cyclist riding on the flat**

**Cyclist ascending**

When pedalling uphill, aerodynamic resistance becomes less important in favour of the force of gravity, which is determined by the weight of the cyclist-bicycle combination.

But not all of our weight will oppose the forward movement. To see this more clearly, we can set up a coordinate system with the x-axis parallel to the ground and the y-axis perpendicular. The total weight will point towards the centre of the earth, but we can break it down into two components according to each of the established coordinate axes. Thus we will obtain a component on the y-axis, "y-weight" (perpendicular to the ground) and another on the x-axis, "x-weight" (parallel to the ground). This last component parallel to the ground is the part of our weight that actually opposes our upward movement.

**Cyclist descending**

In the case of descents, the "x-weight" component will point in the direction of movement and will therefore be added to the weight we exert when pedalling, facilitating forward movement. The steeper the slope of the descent, the greater the value of "x-weight". We will be able to stop pedalling when this force exceeds the value of the sum of the forces opposite to the direction of movement.

In future articles we will expand and deepen all these concepts.

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