Published: Jun-2026 | Category: Fun With Science
Have you ever wondered how a relatively small force can support a much larger load?
In this practical physics investigation, students explore the fascinating world of pulleys, forces and acceleration using a Compound Atwood’s Machine. By combining a fixed pulley with a movable pulley, the system demonstrates mechanical advantage and allows a smaller mass to support a larger load.
Using a Wireless Light Gate Sensor and EasySense software, students can accurately measure acceleration and investigate how changing the mass distribution affects the motion of the system.
A traditional Atwood’s Machine uses two masses connected by a string passing over a pulley. A Compound Atwood’s Machine takes this a step further by introducing a movable pulley.
This arrangement creates a mechanical advantage of two, meaning that the load is supported by two sections of string. As a result, a smaller pulling mass can support a larger load.
By carefully transferring small masses between the load and the pulling side, students can investigate how the acceleration of the system changes and compare experimental results with theoretical predictions.
This practical allows students to:
To carry out this investigation you will need:
The Wireless Light Gate measures the acceleration of the pulley system directly, providing accurate and repeatable data for analysis.
The load is suspended from a movable pulley while a smaller pulling mass hangs from the free end of the string.
Students begin by measuring the acceleration of the system with a particular mass arrangement. Small masses are then transferred from the load side to the pulling side, altering the force balance while keeping the total mass approximately constant.
For each configuration, acceleration is recorded using the Light Gate.
Students then calculate the mass ratio:
R = 2(2MP − ML) / (4MP + ML)
where:
A graph of acceleration against R can then be plotted.
Theory predicts that:
a = gR
This means the graph should produce a straight line through the origin, with a gradient equal to the acceleration due to gravity.
As the pulling mass increases relative to the load, the acceleration increases.
Students should observe a clear linear relationship between acceleration and the calculated mass ratio. The gradient of the graph can be used to estimate the value of gravitational acceleration, providing an excellent opportunity to compare experimental and theoretical results.
After completing the practical, students can explore questions such as:
Common sources of error include:
Pulley systems are used throughout engineering and everyday life, including:
Understanding mechanical advantage helps engineers design systems capable of moving heavy loads efficiently and safely.
The Compound Atwood’s Machine practical is available as part of our growing collection of free Practical Explorer activities.
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This practical uses the Wireless Light Gate Sensor to accurately measure acceleration and timing events.
Learn more about the Wireless Light Gate Sensor
The Practical Explorer collection provides ready-to-use investigations designed to support practical science teaching in secondary schools and colleges.
From mechanics and motion to energy, electricity and waves, each activity includes teacher guidance, student worksheets and EasySense data logging support.
Happy experimenting!
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