Investigating Free Fall Using a PASCO Light Gate and a Picket Fence

Free fall is one of the most fundamental ideas in physics. Objects accelerate towards the Earth at a constant rate (approximately $9.8 \, \text{m/s}^2$) provided air resistance is small. But how do students measure this acceleration accurately?

A PASCO light gate and picket fence provide one of the cleanest, quickest, and most precise methods for determining acceleration due to gravity.
This experiment turns an abstract equation into real, high-quality data.

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How the Equipment Works

• The light gate shines a narrow infrared beam.

• The picket fence is a clear plastic strip with evenly spaced black bars.

• As the fence falls through the gate, each bar interrupts the beam.

• The PASCO interface records the time at which each bar breaks the beam.

From this, the software calculates:

• instantaneous velocity at each bar

• acceleration as velocity increases

• a velocity–time graph showing a straight line for free fall

It is far more accurate than stopwatch timing or video analysis.

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The Experiment

Equipment:

• PASCO light gate

• Picket fence strip

• PASCO interface or Capstone software

• Clamp stand

• Padding/tray to catch the fence

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Method

1. Secure the light gate to a clamp stand.

2. Hold the picket fence vertically just above the light gate.

3. Release it without pushing so it falls freely through the beam.

4. Let the PASCO software record the time intervals.

5. Use Capstone to generate the velocity–time graph.

6. Determine the acceleration from the gradient of the graph.

This gives students a direct measurement of $g$.

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Typical Results

A sample velocity–time dataset might produce:

Time (s)	Velocity (m/s)
0.02	0.20
0.04	0.39
0.06	0.59
0.08	0.78
0.10	0.98

The velocity–time graph is almost a straight line.

The gradient of that line is approximately:

$$a \approx 9.7 \, \text{m/s}^2$$

which is extremely close to the accepted value of

$$g = 9.81 \, \text{m/s}^2$$

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Why This Method Works So Well

• Minimal reaction time error: the equipment times the fall automatically.

• Multiple data points: several bars generate dozens of readings.

• Instantaneous velocity: avoids approximations from distance–time data.

• Straight-line graph: makes determining $g$ simple and clear.

• Suitable for GCSE and A Level: conceptually straightforward but highly accurate.

Students see that physics doesn’t just describe the world — it measures it with precision.

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Skills Highlight

• Using data-logging equipment

• Producing velocity–time graphs

• Determining gradients and acceleration

• Understanding sources of error (air resistance, alignment, release method)

• Applying the equations of motion to real data

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Why It Works in Teaching

The PASCO light gate offers a near-faultless measurement of free fall. Students gain confidence in interpreting graphs and handling real scientific datasets — crucial skills for exam practicals and A Level progression.

It transforms the idea of constant acceleration from a formula into a beautifully clear line of data points.