Linear acceleration: principle
This activity allows students to discover how an accelerometer works by directly exploring its responses to different movements. It develops an intuitive understanding of acceleration as a vector quantity.
Activity Summary
The student uses FizziQ's Linear Acceleration X and observes the variations of the graph as he moves his smartphone in different directions. By making movements to the right then to the left, fast or slow, it analyzes the sign and amplitude of the acceleration during the different phases of the movement. From his observations, the student establishes general rules on the relationship between the direction of movement, the force applied and the sign of acceleration.
Introduction
The accelerometer is arguably the most important sensor in your smartphone, yet most people have no idea it exists. It detects the phone's orientation for screen rotation, measures your steps for fitness tracking, triggers airbags in cars, stabilizes camera images, and enables motion-controlled games. At its heart, a smartphone accelerometer is a tiny mechanical device: a microscopic mass suspended by silicon springs, all etched onto a chip smaller than a fingernail using MEMS (Micro-Electro-Mechanical Systems) technology. When the phone accelerates, the suspended mass lags behind due to inertia, and the resulting displacement is measured electronically. This experiment introduces students to the accelerometer by having them directly explore how it responds to different movements: fast and slow, left and right, acceleration and deceleration. Through hands-on experimentation, students discover that acceleration is a vector quantity with both magnitude and direction, and that slowing down is just as much an acceleration as speeding up.
Learning Objectives
- Discover that acceleration is positive during speed-up and negative during slow-down along the same direction
- Understand that acceleration is a vector quantity with magnitude and direction
- Identify the three phases of a typical movement: acceleration, constant velocity, deceleration
- Learn to interpret an acceleration-time graph and relate it to physical motion
Scientific Concepts
Instruments and sensors
Scientific instruments
- Linear acceleration (X-axis)
- Absolute acceleration
Sensors
- Accelerometer
Required Materials
- - Smartphone with the FizziQ application - Clear space to carry out movements - Note: the protocol remains adaptable to any comparable acceleration measurement tool.
Experimental Protocol
Open FizziQ and select the **Linear Acceleration X** sensor. This measures acceleration along the phone's horizontal axis, subtracting gravity.
Hold the phone **flat in your hand** with the screen facing up. Note that the X-axis points along the phone's long direction.
Start recording. While holding the phone still, observe the graph: it should show values near **zero** (no acceleration).
Now **move the phone to the right** with a quick push, then let it come to rest. Observe the graph: you should see a **positive spike** (acceleration) followed by a **negative spike** (deceleration).
Move the phone **to the left** with a quick push. Now the pattern is reversed: **negative then positive**.
Try a **slow, gentle** movement to the right. Compare the peak acceleration with the fast movement. The slow movement should show smaller peaks.
Try to move the phone at **constant speed** (no acceleration). The graph should return to zero during the constant-speed phase.
Perform a complete movement cycle: accelerate right, cruise at constant speed, decelerate to a stop. Identify all three phases on the graph.
Record the **peak acceleration** for movements of different speeds. Is the peak acceleration larger for faster movements?
Now tilt the phone so the X-axis points upward. Observe that the reading changes even without movement, due to **gravity affecting the measurement**.
Switch to **Absolute Acceleration** and compare with Linear Acceleration. Note that absolute acceleration includes gravity while linear acceleration subtracts it.
Summarize your findings: write three rules about how acceleration relates to the direction and speed of movement.
Expected Results
Students should observe clear patterns: moving right produces positive then negative acceleration; moving left produces negative then positive. Faster movements produce larger peak accelerations (typically 1-5 m/s² for hand movements, up to 10-20 m/s² for sharp jerks). During constant-speed motion, the linear acceleration returns to approximately zero. Tilting the phone causes the absolute acceleration to change even without movement, demonstrating the gravitational contribution. The three-phase pattern (accelerate, cruise, decelerate) should be clearly visible for deliberate, controlled movements.
Scientific Questions
- What is the difference between speed and acceleration?
- Why does the phone show non-zero acceleration even when tilted but stationary?
- What is the difference between linear acceleration and absolute acceleration?
- How does a MEMS accelerometer work? What is the role of the tiny suspended mass?
- Can you think of three applications of accelerometers in everyday technology?
Scientific Background
The accelerometer of a smartphone is a MEMS (Micro Electro Mechanical Systems) sensor that measures acceleration on three orthogonal axes (X, Y, Z). Its operating principle is based on a small mass suspended by microscopic springs.
During acceleration, the inertia of this mass causes its relative movement relative to the sensor frame, a deformation measured electronically. The acceleration measured on the X axis corresponds to the change in speed along this axis.
When moving the smartphone to the right, we typically observe three phases: 1) positive acceleration during the initial phase where the speed increases, 2) zero acceleration during the movement at constant speed, 3) negative acceleration when slowing down. This profile reverses for a movement to the left.
These observations illustrate Newton's second law (F = ma): acceleration is proportional to the applied force and in the same direction. The maximum absolute value of acceleration generally appears at moments of sudden change: rapid start or sudden braking.
Extensions
- Attach the phone to a toy car and analyze the acceleration during push and release
- Record the acceleration during an elevator ride and identify the floor-to-floor acceleration pattern
- Investigate the accelerometer response during a car ride: identify braking, accelerating, and turning
- Compare the sensitivity of the accelerometer on different smartphones
Frequently Asked Questions
The acceleration reading is never exactly zero even when the phone is still. Why?
Sensor noise and small vibrations produce fluctuations of about ±0.02-0.10 m/s² around zero. This is normal for all MEMS sensors.
Why does the acceleration spike when I start moving but return to zero during constant speed?
Acceleration measures the rate of change of velocity, not the velocity itself. At constant speed, the velocity is not changing, so the acceleration is zero.
The patterns look reversed from what I expected. Is the axis orientation wrong?
The X-axis direction depends on how you hold the phone. The positive X direction may point left or right depending on phone orientation. Experiment to determine which direction corresponds to positive X on your device.
What is MEMS technology?
MEMS stands for Micro-Electro-Mechanical Systems. These are tiny mechanical structures (springs, masses, levers) etched into silicon chips using techniques borrowed from computer chip manufacturing. The accelerometer's sensing element is typically less than 1 mm across.
Detailed Description
The student uses FizziQ's Linear Acceleration X and observes the variations of the graph as he moves his smartphone in different directions. By making movements to the right then to the left, fast or slow, it analyzes the sign and amplitude of the acceleration during the different phases of the movement. From his observations, the student establishes general rules on the relationship between the direction of movement, the force applied and the sign of acceleration. FizziQ turns the smartphone's built-in accelerometer into a real-time acceleration-time graph, letting students switch instantly between the Linear Acceleration X and Absolute Acceleration instruments to compare readings with and without gravity.
Ready to start?
Download FizziQ and try this activity with your students.