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An eraser is tied to a string and held by a physics teacher. The eraser is whirled in a circle at constant speed. A 'God's eye' view of the circle is shown in the diagrams below. If the teacher lets go of the string when the eraser is at the indicated position, then which one of the paths best represents the motion of the eraser?
Newton's First Law of Motion: An object at rest will tend to stay at rest; an object in motion will tend to stay in motion with the same speed and direction, unless acted upon by an unbalanced force.
If you were in a car and traveling in a circle at constant speed, then you (and the car) would be accelerating towards the center of the circle. This acceleration is caused by an unbalanced or net force. At all points along the circular path, you would be heading in a direction which is tangent to the path. You would feel a strong inertial tendency to continue in motion in a direction tangent to the circle. The presence of an unbalanced force on your body pushes you towards the center of the circle so that you can make the turn rather than travel straight ahead. This force is supplied by the outside door, the seat belt and harness or friction between you and the seats. If this force were somehow removed, there would no longer be an unbalanced force on your body and you would fly out of the car and tangent to the circle. An object in motion would continue in motion in the same direction when not acted upon by an unbalanced force. That which is true of your motion in this car would be equally true of the motion of the eraser tied to the string (and not tied to the string).
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 which 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. If the string force ceased, the eraser would stop accelerating and begin travel in a straight line in the direction it has at that given moment.
