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Definition of Inertia:
Inertia is the property of an object that describes its natural tendency to keep on doing whatever it is doing.
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Inertia is simply the tendency of any object to keep on doing whatever it is doing. It is the natural tendency of an object to stubbornly maintain the same speed and the same direction that it has at any given moment. An eraser that is moving in a circle is at all points along its path heading in a direction tangent to the circle. It is the inertial tendency of the eraser to continue moving tangent to the path. If a net or unbalanced force were not acting upon the eraser, it would travel straight ahead tangent to the circle. The presence of an unbalanced force on the eraser causes it to change its direction and follow the curved path of the circle. This unbalanced force is supplied by tension in the string. Without the tension force, there would be no circular motion.
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Many students understand that the eraser will maintain its state of motion if there is no net force. But they are a little confused about what exactly the state of motion refers to. They might think that the state of motion of the eraser is the state of moving in a circle. But don't be fooled! The state of motion refers to the speed and direction that an object has at a specific moment in time. For the eraser moving in a circle, the state of motion is constantly changing. The direction is always tangent to the circle, but the tangent direction is undergoing a constant change because of the string's inward pull on the eraser. Without the string force, the eraser would travel in the direction it has at that given moment.
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