Welcome to Newton's First Law of Motion, also known as the Law of Inertia. This fundamental principle states that an object at rest will remain at rest, and an object in motion will continue moving at constant velocity in a straight line, unless acted upon by an unbalanced external force. This law describes the natural tendency of objects to resist changes in their state of motion.
Inertia is the fundamental property that makes Newton's First Law work. It's the tendency of objects to resist changes in their state of motion. A bowling ball has much more inertia than a tennis ball, which is why it takes more force to start or stop a bowling ball. Mass is the measure of inertia - the more massive an object, the greater its resistance to changes in motion.
牛顿第一定律，也被称为惯性定律，是经典力学的基础定律之一。这个定律描述了物体的运动状态与外力之间的关系，是我们理解物体运动的重要起点。
牛顿第一定律的表述是：任何物体都要保持匀速直线运动或静止状态，直到外力迫使它改变运动状态为止。更正式的表述是：一个物体如果没有受到外力作用，或者所受外力的合力为零，它将保持静止或匀速直线运动状态。
让我们看看牛顿第一定律的第一部分：静止物体的惯性。桌上的书完美地展示了这一点。书保持静止是因为没有不平衡的力作用在它上面。重力将它向下拉，但桌子以相等的力向上推。只有当外力如你的手作用在书上时，它才会改变静止状态。
现在我们来看运动物体的惯性。运动的物体倾向于保持运动状态。想象一块石头在光滑的冰面上滑行，它会一直保持运动，直到摩擦力或其他外力迫使它停下来。在理想情况下，如果没有任何阻力，物体会永远保持匀速直线运动。
惯性是物体保持原有运动状态的性质，它与物体的质量密切相关。质量越大的物体，惯性越大，越难改变其运动状态。我们在日常生活中经常体验到惯性：汽车急刹车时乘客向前倾斜，火车启动时乘客向后倾斜，大卡车比小汽车更难停下来。这些都是惯性的表现。牛顿第一定律告诉我们，这种惯性是物体的基本性质。
Now let's examine the second part of Newton's First Law: objects in motion stay in motion. A hockey puck sliding on ice demonstrates this perfectly. Once set in motion, the puck continues moving at constant velocity in a straight line. It only slows down because of friction with the ice and air resistance. In the absence of these forces, the puck would continue moving forever at the same speed and direction.
Newton's First Law has countless real-world applications that we experience daily. Seat belts in cars protect us because when a car suddenly stops, our bodies want to continue moving forward due to inertia. We lean when buses turn corners because our bodies resist the change in direction. In space, spacecraft can coast for years without using fuel because there's no friction to slow them down. Understanding inertia helps engineers design safer vehicles and explains the physics behind many sports like ice hockey and curling.