Objects don't continue moving forever due to various forces and factors that work against their motion. This can primarily be explained through two key principles: friction and air resistance, as well as Newton's first law of motion.
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Friction: When an object moves along a surface, it experiences friction, which is a force that opposes its motion. Friction can arise from the interaction of the object's surface with the surface it's moving on, and it converts kinetic energy into thermal energy, slowing the object down.
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Air Resistance: For objects moving through air (or any fluid), there is drag or air resistance that opposes their motion. This force increases with the speed of the object and can significantly affect the motion of objects, especially at high speeds.
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Newton's First Law of Motion: This law states that an object at rest will remain at rest, and an object in motion will remain in motion at a constant velocity unless acted upon by a net external force. In practical situations, external forces like friction and air resistance are almost always present, causing the object to eventually slow down and stop.
In an idealized frictionless environment, such as in space where there is no air resistance or friction, an object would continue to move indefinitely once set in motion, because no net external force would be acting on it. However, in our everyday experience on Earth, we are surrounded by forces that ultimately bring objects to a stop.