Final answer:
Newton's first law of motion, also known as the law of inertia, states that a body at rest remains at rest or, if moving, remains in motion at a constant velocity unless acted on by a net external force. Inertia is the tendency of an object to stay at rest or in motion. Forces are interactions between objects that cause changes in motion.
Step-by-step explanation:
The first law of motion, known as Newton's first law, states that a body at rest remains at rest or, if in motion, remains in motion at a constant velocity unless acted on by a net external force. This law is also referred to as the law of inertia. Inertia is the property of an object to resist changes in its state of motion.
When we talk about inertia, we are referring to an object's tendency to stay at rest or in motion. Objects with less mass have less inertia, meaning they are easier to move or stop. On the other hand, objects with more mass have more inertia, making it harder to change their motion.
Forces are interactions between objects that cause changes in motion. When there is no force acting on an object, its state of motion remains unchanged. This means that if the object is at rest, it will stay at rest, and if it is moving, it will continue to move at a constant speed and direction. On the other hand, when the net force on an object is zero, there is no acceleration, and the object remains at rest or moves with constant velocity.