Suppose a spacecraft is moving in a region of space where the gravitational force acting upon it is negligible. Suggest how can it change its velocity.
The spacecraft can change its velocity by firing its engine. When the engine fires, the exhaust gases exert a force in the direction opposite to the motion of the spacecraft.
As per Newton's third law, the exhaust gases and the spacecraft exert equal and opposite forces on each other. Since these forces act on two different objects, they do not balance each other. This net force acting on the spacecraft changes its velocity. For instance, if the engine fires in the direction of its motion, the spacecraft will slow down.
Explanation
The textbook context explains that if the net force on an object is zero, it cannot change its velocity. To change velocity, a net force must act on it. The context provides the example of a rocket firing its engine, where exhaust gases exert an opposite force on the rocket (Newton's third law) to change its velocity, such as slowing it down for a landing.