Amplitude is the maximum displacement of an oscillating object or wave from its equilibrium position. In AP Physics 1 it determines the total mechanical energy of a simple harmonic oscillator (E = ½kA²) but has no effect on the period of a mass-spring system or pendulum, or on wave speed.
Amplitude is the farthest an oscillating system gets from its equilibrium position. For a block on a spring, it's the maximum stretch or compression. For a pendulum, it's the maximum swing from the lowest point. For a wave, it's the height of a crest (or depth of a trough) measured from the undisturbed middle line.
The big idea behind amplitude is energy. When you pull a spring-block system out to amplitude A and release it, all the energy sits in the spring as elastic potential energy, ½kA². As the block passes through equilibrium, all of that energy has converted to kinetic energy. So amplitude is essentially a snapshot of how much total mechanical energy the oscillator has. Double the amplitude and you quadruple the energy, because energy scales with A². What amplitude does NOT do is just as important. It does not affect the period of a simple harmonic oscillator, and it does not affect how fast a wave travels through a medium.
Amplitude shows up in two places in the course. In the simple harmonic motion topics (Topic 6.1, Period of Simple Harmonic Oscillators, and Topic 6.2, Energy of a Simple Harmonic Oscillator), amplitude sets the energy budget of the oscillator and marks the turning points where speed is zero and acceleration is maximum. In the wave topics (Topic 10.1, Properties of Waves, and Topic 10.2, Periodic Waves), amplitude describes the size of the disturbance and connects to how much energy the wave carries. The exam loves amplitude because it's the perfect trap variable. The classic test of whether you actually understand SHM is knowing that changing amplitude changes energy and maximum speed but leaves the period completely alone.
Keep studying AP Physics 1 Unit 6
Period (T) (Units 6 & 10)
This is the relationship the exam tests most. For a mass-spring system, T = 2π√(m/k), and amplitude appears nowhere in that equation. A bigger swing covers more distance but also moves faster, and the two effects exactly cancel. Same period, every time.
Elastic Potential Energy (Unit 6)
At maximum displacement, the oscillator's entire energy is stored as elastic potential energy, U = ½kA². That makes amplitude a direct readout of total mechanical energy, which is why energy bar charts for SHM always anchor to the amplitude position.
Equilibrium Position (Units 6 & 10)
Amplitude is meaningless without a reference point, and that reference is equilibrium. You measure amplitude from the equilibrium position, not from one extreme to the other. The full extreme-to-extreme distance is 2A, a detail that costs points when reading graphs.
Crest and Trough (Unit 10)
On a wave, amplitude is the vertical distance from the equilibrium line up to a crest or down to a trough. Crest-to-trough is 2A. Wave amplitude tells you about the wave's energy, while wavelength and frequency together set its speed.
Amplitude appears in both MCQ and FRQ settings, usually as a 'what changes and what doesn't' question. The 2018 short-answer Q5 gave a block of mass m oscillating with period T_P and amplitude A_P on a spring, then asked you to reason about how changing the setup affects the motion. The 2022 short FRQ Q5 had a spring hanging from a ceiling with a motion detector tracking the oscillation, where amplitude is what you read directly off the position-time graph. Expect to (1) extract amplitude from a graph of position versus time, measuring from equilibrium to a peak, not peak to peak; (2) use E = ½kA² to connect amplitude to total energy or maximum speed via ½mv²max = ½kA²; and (3) justify in writing why doubling amplitude quadruples energy but leaves the period unchanged. That last justification is a favorite for paragraph-length response questions.
Displacement is where the object is right now relative to equilibrium, and it changes constantly throughout the motion, from +A to -A and back. Amplitude is the maximum value that displacement ever reaches, and it stays constant (assuming no energy loss). On a position-time graph, displacement is the curve itself; amplitude is the height of the peaks. If a question asks for x(t), it wants displacement. If it asks how far the oscillation extends, it wants amplitude.
Amplitude is the maximum displacement from equilibrium, measured from the center of the motion to one extreme, not from one extreme to the other.
The total mechanical energy of a simple harmonic oscillator is E = ½kA², so doubling the amplitude quadruples the energy.
Amplitude has no effect on the period of a mass-spring system or a simple pendulum, since T depends only on m and k (or L and g).
At maximum displacement (x = ±A), speed is zero and the magnitudes of acceleration and restoring force are at their maximum.
For waves, amplitude is the distance from the equilibrium line to a crest or trough, and it relates to the wave's energy, not its speed.
Wave speed is set by the medium, so increasing a wave's amplitude does not make it travel faster.
Amplitude is the maximum displacement of an oscillating object or wave from its equilibrium position. For a spring-block oscillator it's the maximum stretch, and it determines the system's total energy through E = ½kA².
No. For a mass-spring system, T = 2π√(m/k), so period depends only on mass and spring constant. A larger amplitude means the object travels farther each cycle but also moves faster, and those effects cancel exactly. Explaining this is a classic FRQ justification.
Displacement is the object's current position relative to equilibrium and changes throughout the motion. Amplitude is the maximum displacement and stays constant in ideal SHM. On a position-time graph, amplitude is the height of the peaks.
Energy quadruples. Since E = ½kA², energy scales with the square of the amplitude, so doubling A multiplies the total mechanical energy (and the kinetic energy at equilibrium) by 4. Maximum speed only doubles, since KE depends on v².
No. Wave speed is determined by the properties of the medium, like tension and mass per length for a string. A bigger amplitude means the wave carries more energy, but it moves through the medium at the same speed.