The force that pulls an oscillating object back toward equilibrium is the:

1. Newton's first law implies that an object oscillating back and forth is experiencing forces. 2. Without force, the object would move in a straight line at a constant speed rather than oscillate. 3. Once released, the restoring force causes the ruler to move back toward its stable equilibrium position, where the net force on it is zero. 4. By the time it gets there it has gained momentum and overshoots, which is what keeps the cycle going. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 16 'Oscillatory Motion and Waves', sections 16.1-16.11_