# Introduction to simple harmonic motion review

Overview of key terms, equations, and skills for simple harmonic motion, including how to analyze the force, displacement, velocity, and acceleration of an oscillator.

## Key terms

Term | Meaning |
---|---|

Oscillatory motion | Repeated back and forth movement over the same path about an equilibrium position, such as a mass on a spring or pendulum. |

Restoring force | A force acting opposite to displacement to bring the system back to equilibrium, which is its rest position. The force magnitude depends only on displacement, such as in Hooke’s law. |

Simple harmonic motion (SHM) | Oscillatory motion where the net force on the system is a restoring force. |

## Equations

Equation | Symbols | Meaning in words |
---|---|---|

$\lvert F_s \rvert = k\lvert x\rvert$ | $F_s$ is spring force, $k$ is the spring constant, and $x$ is displacement | The magnitude of the spring force is directly proportional to the spring constant and the magnitude of displacement |

$x(t) = A\cos(2 \pi f t)$ | $x$ is displacement as a function of time, $A$ is amplitude, $f$ is frequency, and $t$ is time | Displacement as a function of time is proportional to amplitude and the cosine of $2\pi$ times frequency and time |

## Force, displacement, velocity, and acceleration for an oscillator

Simple harmonic motion is governed by a restorative force. For a spring-mass system, such as a block attached to a spring, the spring force is responsible for the oscillation (see Figure 1).

Since the restoring force is proportional to displacement from equilibrium, both the magnitude of the restoring force and the acceleration is the greatest at the maximum points of displacement. The negative sign tells us that the force and acceleration are in the opposite direction from displacement.

The mass's displacement, velocity, and acceleration over time can be visualized in the graphs below (Figure 2-4).

## Analyzing graphs: Period and frequency

We can graph the movement of an oscillating object as a function of time. Frequency $f$ and period $T$ are independent of amplitude $A$. We can find either quantity by taking any two analogous points on the graph and calculating the time between them. It’s often easiest to measure the time between consecutive maximum or minimum points of displacement.

**Finding displacement and velocity**

Distance and displacement can be found from the position vs. time graph for simple harmonic motion. Velocity and speed can be found from the slope of a position vs. time graph for simple harmonic motion.

## Common mistakes and misconceptions

**Sometimes people confuse period and frequency.**These quantities are the inverse of each other. If we can find one, we can also find the other through the relationship:

This means that if the frequency is large, the period is small, and vice versa.

## Learn more

For deeper explanations of simple harmonic motion, see our videos:

To check your understanding and work toward mastering these concepts, check out our exercises: