13.2 Why Things Oscillate 1

Chapter 13 Objectives Identify characteristics of harmonic motion, such as cycles, frequency, and amplitude. Determine period, frequency, and amplitude from a graph of harmonic motion. Use the concept of phase to compare the motion of two oscillators. Describe the characteristics of a system that lead to harmonic motion. Describe the meaning of natural frequency. Identify ways to change the natural frequency of a system. Explain harmonic motion in terms of potential and kinetic energy. Describe the meaning of periodic force. Explain the concept of resonance and give examples of resonance. 2

Chapter 13 Vocabulary Terms amplitude damping frequency harmonic motion hertz (Hz) natural frequency oscillator period periodic force periodic motion phase phase difference piezoelectric effect resonance stable equilibrium unstable equilibrium

Inv 13.2 Why Things Oscillate Investigation Key Question: What kinds of systems oscillate? 4

13.2 Why Things Oscillate Systems that have harmonic motion move back and forth around a central or equilibrium position. Equilibrium is maintained by restoring forces. A restoring force is any force that always acts to pull the system back toward equilibrium.

13.2 Inertia Newton’s first law explains why harmonic motion happens for moving objects. According to the first law, an object in motion stays in motion unless acted upon by a force.

13.2 Stable and unstable systems Not all systems in equilibrium show harmonic motion when disturbed. In unstable systems there are forces that act to pull the system away from equilibrium when disturbed. Unstable systems do not usually result in harmonic motion (don't have restoring forces).

13.2 The natural frequency The natural frequency is the frequency at which systems tend to oscillate when disturbed. Everything that can oscillate has a natural frequency, and most systems have more than one. Adding a steel nut greatly increases the inertia of a stretched rubber band, so the natural frequency decreases.

13.2 Changing the natural frequency The natural frequency is proportional to the acceleration of a system. Newton’s second law can be applied to see the relationship between acceleration and natural frequency.