To understand why the **tangential force** is the one that causes the variation in the vehicle's speed in curves, it is important to consider how a vehicle behaves when taking a curve.

- **Curves and forces:** When taking a curve, the vehicle experiences several forces. One of them is the **tangential force**, which acts along the direction of the vehicle's movement. This force is responsible for changes in the vehicle's speed, as it can increase or decrease the speed depending on whether the driver accelerates or brakes.

- **Importance of speed:** Before entering a curve, it is crucial to **reduce speed** or brake if necessary. This is because excessive speed can cause the vehicle to skid or lose control, as mentioned in the steps for taking a curve properly.

- **Control in the curve:** Within the curve, it is important **not to accelerate or brake abruptly**. This helps maintain control of the vehicle and prevents the tires from losing grip. Acceleration should be gradual, which implies a change in speed due to the action of the tangential force.

- **Exiting the curve:** When exiting the curve, you should **gradually increase speed**. This gradual increase in speed is again an effect of the tangential force, which allows the vehicle to return to an appropriate speed to continue its journey safely.

In summary, the **tangential force** is key to controlling the vehicle's speed in curves, ensuring that grip and control of the vehicle are maintained at all times.