Final answer:
The statement is false. External forces such as gravity and air resistance can and do affect the motion of a body, either slowing it down or changing its direction. This is consistent with the principles of Newton's laws of motion.
Step-by-step explanation:
External forces such as gravity and air resistance indeed have the ability to slow down or accelerate a body in motion. For instance, gravity continuously pulls objects towards Earth, influencing their motion. As an object falls, it accumulates speed due to gravity. However, as it speeds up, air resistance increases and can significantly slow down the object. This resistance force acts in the direction opposite to the motion of the object. Moreover, considering Newton's Third Law of Motion, every action has an equal and opposite reaction. When a person walks, they exert force on the ground (F₁) and the ground exerts an equal and opposite force (F₂) on them, allowing the person to move forward. The net force is not zero due to the directional aspects of these forces. This is a clear example of how forces work to move bodies and change their state of motion. Furthermore, when an object experiences two unequal forces, it will move in the direction of the stronger force, not the weaker one. Also, the normal reaction is a force that counters the force of gravity but acts perpendicular to the surface, not in the direction of gravitational force. Finally, a rocket launched into space that escapes Earth's gravitational pull will indeed move in a straight line until another external force acts upon it, which is in accordance with Newton's First Law of Motion.