Driving a Car: Should you use the Cruise Control?

From Newton's Second Law, we know that:

F = ma

This means that a force “F” acting on a body of mass “m” produces an acceleration “a”.

This principle can be observed every time we drive a car.

Imagine starting your car from a parking spot and gradually increasing its speed—from 0 km/h to 10 km/h, then 20 km/h, 30 km/h, and so on until it reaches 100 km/h. At that point, you engage Cruise Control.

Scenario 1: Accelerating from 0 km/h to 100 km/h

The velocity of the car is changing, which means the car is accelerating. Since acceleration is present, a net force must be acting on the car.

During this phase, the engine's power is used for two purposes:

1. Increasing the car's kinetic energy (i.e., increasing its speed)

2. Overcoming resistive forces such as air drag, tyre rolling resistance, and drivetrain losses

In other words, part of the engine's output goes into making the car go faster, while the rest is spent fighting forces that oppose motion.

Scenario 2: Cruising at 100 km/h

Once the car settles at a constant speed of 100 km/h, the velocity is no longer changing.

Therefore:

a = 0

and from Newton's Second Law:

F = m × 0 = 0

This does not mean that no forces exist. Rather, it means that the driving force produced by the engine is exactly balanced by the opposing forces, resulting in zero net force and hence zero acceleration.

At this stage, the engine is no longer using power to increase the car's speed. Its power output is devoted entirely to overcoming air resistance, tyre rolling resistance, and drivetrain losses.

The Benefit of Cruise Control

On a long, level road, Cruise Control helps maintain a steady speed without unnecessary acceleration or deceleration. By avoiding frequent changes in speed and supplying only the power needed to balance resistive forces, the system can improve fuel efficiency and make driving more economical.

A smooth, constant speed is often the most fuel-efficient way to travel.