Describe an experiment to measure free-fall acceleration
Plan a falling-object timing method, calculate g from distance and time, and use repeated measurements to evaluate the result.
AS Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 2. The falling-object activity is an online description and calculation practice; it does not provide supervised laboratory competence. Full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use SI units and signed values, and recall that velocity includes direction.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
For an object released from rest under constant acceleration, how does displacement depend on time?
What you will learn
- Describe a falling-object experiment to determine g
- Use measured distance and time to calculate acceleration
- Identify timing and distance controls that improve evidence
A timed drop links distance to acceleration
One method uses a small steel ball, a measured vertical drop height, an electromagnetic release and an electronic contact plate. Switching off the electromagnet releases the ball and starts the timer; impact on the plate stops it. Measure the vertical distance from the bottom of the ball at release to the contact plate.
Release the ball from rest and minimise sideways motion. For each height, repeat the timing and use a mean. Change the height over a safe range and record the corresponding times with units. The purpose is to test how distance depends on time squared, not just to collect one reading.
Use the model and evaluate the method
For release from rest with uniform acceleration, s = ½gt². Calculate g = 2s/t² for each pair or plot s against t²; the gradient of a straight-line graph is g/2. A result close to the supplied or accepted value supports the model within the method's uncertainty.
Timing delay, an inaccurate release-to-plate distance, sideways motion and inconsistent release conditions can affect the result. Repeat readings reduce random timing scatter but do not remove a shared calibration or zero error. This is a description and analysis exercise; online content cannot establish practical competence.
Pause and explain
On a graph of fall distance s against t², what is the gradient?
Worked example
A ball falls 0.800 m from rest in 0.404 s. Calculate g from the model s = ½gt².
Show the worked solution
- Rearrange: g = 2s/t².
- Substitute g = 2(0.800)/(0.404²).
- Give the acceleration unit m s⁻².
Answer g ≈ 9.80 m s⁻².
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe how you could use a falling ball to estimate g and obtain repeated data.
Give me a hint
Specify how the fall starts and stops the timer, what distance is measured and how the data are varied.
Compare my reasoning
- Measure a vertical release-to-impact distance and release the ball from rest.
- Use an electronic release and impact detector to time the fall.
- Repeat for several safe heights and calculate a mean time for each height.
Measure the distance from the ball's lower surface at release to the contact plate, release from rest with an electromagnet, time to impact electronically, repeat each height and use several heights for analysis.
Look for these in your work
- Distance reference points identified
- Timing start and stop described
- Repeated measurements and varied heights included
A ball falls 1.25 m from rest in 0.505 s. Estimate g.
Give me a hint
Use g = 2s/t² and keep the squared time in the denominator.
Compare my reasoning
- Calculate t² = 0.505² = 0.255025 s².
- Calculate 2s = 2.50 m.
- Divide 2.50 by 0.255025.
g ≈ 9.80 m s⁻².
Look for these in your work
- Equation rearranged correctly
- Time squared
- Unit m s⁻²
A learner repeats timings at three heights and plots s against t². The points form a straight line through the origin with gradient 4.85 m s⁻². Estimate g and state one limitation of the conclusion.
Give me a hint
Compare the graph with s = ½gt² and distinguish model agreement from practical certainty.
Compare my reasoning
- The gradient corresponds to g/2.
- Calculate g = 2 × 4.85.
- State what the straight-line pattern supports and what it cannot certify.
g ≈ 9.70 m s⁻². The linear pattern is consistent with the constant-acceleration model, but timing, distance and systematic errors still limit the estimate.
Look for these in your work
- Gradient interpreted as g/2
- Estimate doubled correctly
- Evidence limitation stated
Common mistakes
- Measuring the height to the ball's centre in one trial and its bottom in another
- Using a single timing without repeats or an uncertainty check
- Claiming a written method proves supervised practical competence
What can you explain now?
For an object released from rest under constant acceleration, how does displacement depend on time?
Compare with the explanation
s = ½at²
This follows from s = ut + ½at² with u = 0.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How does the s-versus-t² graph reveal g, and which errors can remain even after repeated trials?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.