📚 A-Level Physics Unit 4: Jan 2020 Exam Report – Experimental Investigation | A-Level物理 Unit 4 2020年1月考试报告：实验探究
The January 2020 A-Level Physics Unit 4 examination tested students' ability to design, analyse and evaluate experimental investigations, with a focus on mechanics and materials. Many candidates lost marks not because of a lack of knowledge, but due to poor experimental thinking and unclear presentation. This article summarises key lessons from the examiner's report to help you avoid common pitfalls.
2020年1月的A-Level物理Unit 4考试重点考查了学生设计、分析和评估实验探究的能力，尤其是力学和材料学实验。许多考生丢分并非因为知识不足，而是由于实验思维不清晰、表达不规范。本文总结考官报告中的关键教训，帮助你避开常见失分点。
1. Overview of the Experimental Question | 实验题目概述
In the Unit 4 exam, the experimental section typically carries 12–15 marks. It requires students to outline a practical procedure, describe measurements, record and process data, and evaluate sources of error. The January 2020 paper presented a scenario where candidates had to determine the acceleration of free fall, g, using a falling ball that triggers a switch at the start and hits a trapdoor at the bottom.
在Unit 4考试中，实验题通常占12-15分，要求学生写出实验步骤、描述测量方法、记录和处理数据并评估误差来源。2020年1月的试卷出了这样一个题目：使用小球下落撞击起始开关并在底部撞开活门，以此测定重力加速度g。
Many candidates underestimated the precision required in writing the plan. The examiner noted that vague language such as 'drop the ball and measure the time' was insufficient. Instead, a sequence of steps specifying how to release the ball, how to read the timer, and how to vary the independent variable was expected. Marks were awarded for practical details that reduce random and systematic errors.
很多考生低估了书写实验计划时对精确性的要求。考官指出，诸如“放下球并测量时间”这样模糊的描述是不足以得分的。他们期望看到一系列步骤，具体说明如何释放小球、如何读取计时器以及如何改变自变量。能减少随机误差和系统误差的实用细节都能赚取分数。
2. Effective Experimental Design | 有效的实验设计
A strong experimental design starts with a clear aim and a short list of equipment. In the trapdoor experiment, the independent variable was the drop height, h, and the dependent variable was the time of fall, t. The plan should include: 1) measuring h from the bottom of the ball to the trapdoor using a metre ruler; 2) releasing the ball from rest without imparting any initial velocity; 3) using a digital timer with millisecond resolution; 4) repeating each height at least three times to calculate a mean t.
一个优秀的实验设计需要从明确的实验目的和简短的器材清单开始。在活门实验中，自变量是下落高度h，因变量是下落时间t。实验计划应包括：1）用米尺测量从球底到活门的高度h；2）让小球从静止释放，不施加任何初速度；3）使用分辨率为毫秒的数字计时器；4）每个高度至少重复三次，以计算时间平均值。
Common design flaws in the Jan 2020 scripts included forgetting to measure the height before each trial or assuming the timer starts exactly when the ball is released. Using a mechanical switch at the point of release was a commendable detail that many top‑scoring candidates included. Another important aspect was keeping the ball's mass and shape constant to avoid introducing uncontrolled variables.
2020年1月试卷中常见的设计缺陷包括：忘记在每次实验前测量高度，或者假设计时器与小球释放同时开始。在释放点使用一个机械开关是一项值得称赞的细节，很多高分考生都写上了这一点。另一个重要方面是保持小球的质量和形状不变，以避免引入不受控制的变量。
3. Identifying and Controlling Variables | 识别与控制变量
In any physics investigation, candidates must clearly identify independent, dependent and control variables. For the g experiment, the drop height was changed deliberately, the fall time was measured, and the ball's mass, diameter and release mechanism were kept constant. The examiner observed that many students did not list control variables explicitly, losing straightforward marks.
在任何物理探究中，考生必须清楚地识别自变量、因变量和控制变量。就测量g的实验而言，下落高度是有意改变的，下落时间是被测量的，而小球的质量、直径和释放机制则保持不变。考官发现许多学生没有明确列出控制变量，白白失去了容易得到的分数。
| Variable Type | Example |
|---|---|
| Independent (change) | Height h / m |
| Dependent (measure) | Fall time t / s |
| Control (keep same) | Ball mass, ball diameter, release method, air currents |
Failing to mention how to keep control variables constant was a recurring issue. For instance, saying 'use the same ball' is not enough; you should state 'use the same steel ball of mass 20.0 g' and explain that this keeps air resistance characteristics identical. When air currents are a concern, performing the experiment in still air or using a protective shield can be mentioned.
没有说明如何保持控制变量不变是一个反复出现的问题。例如，只说“使用同一个球”是不够的；你应该说“使用同一个质量为20.0 g的钢球”，并解释这样可使空气阻力特性保持一致。当气流成为问题时，可以提到在静止空气中进行实验或使用防护屏。
4. Choosing Appropriate Apparatus | 选择合适的仪器
ch to measure the fall time. However, a stopwatch (±0.1 s) introduces a huge percentage uncertainty for short drops. A digital timer triggered by switches, capable of reading to 0.001 s, was far more suitable. The examiner emphasized that the instrument must match the scale of the quantity being measured.
选择精度足够的仪器通常会被赋予相应分数。在2020年1月的考试中，很多考生建议用秒表测量下落时间。但是，秒表（±0.1 s）对于短下落距离带来的百分比不确定度很大。由开关触发的数字计时器，能读到0.001 s，则合适得多。考官强调，测量仪器必须与被测物理量的标度相匹配。
s acceptable, but candidates had to describe how to avoid parallax error, for example by viewing the scale perpendicularly and using the bottom of the ball as the reference point. A set square or plumb line could be used to ensure the ruler is vertical.
测量高度时，带有毫米刻度的米尺（±0.5 mm）是可以接受的，但考生必须说明如何避免视差错误，例如垂直观察刻度并以球的底部作为参考点。可以使用三角尺或铅垂线确保直尺垂直。
5. Making Precise Measurements | 进行精密测量
Repetition is the simplest way to reduce random errors. The examiner noted that explaining 'repeat and average' earned an instant mark, yet a surprising number of candidates omitted it. For each height, the ball should be dropped at least three times and the mean time calculated, discarding any anomalous results that are clearly out of trend.
重复是减少随机误差最简单的方法。考官指出，只要说明“重复并取平均值”就能立刻得分，但仍有相当多的考生漏掉了这一点。对于每个高度，小球至少应下落三次并计算平均时间，剔除明显偏离趋势的异常结果。
Another precision technique is to measure the height before and after each drop to check if the setup has moved. Recording the starting height with a set square reduces alignment errors. When reading the timer, taking the reading at eye level and waiting for the digital display to stabilise prevents systematic misreads.
另一种提高精度的技巧是在每次下落前后都测量高度，以检查装置是否移动。用三角尺标记起始高度可以减少对准误差。读取计时器时，视线与显示齐平并等待数字显示稳定，可避免系统性的读数错误。
6. Recording Data in a Table | 在表格中记录数据
A well‑constructed data table is essential. The Jan 2020 report revealed that many tables lacked units in column headings, used inconsistent significant figures, or failed to include a column for mean time. The correct format is to list the independent variable first, then the dependent variable readings, and finally the mean. Each column heading must contain the quantity name, symbol and unit in brackets, e.g. Time t / s.
构建良好的数据表格至关重要。2020年1月的报告显示，许多表格在表头中缺少单位，使用了不一致的有效数字，或者没有包含平均时间的一列。正确的格式是先列出自变量，然后列出每次测量的因变量读数，最后是平均值。每个表头必须包含物理量名称、符号和带有括号的单位，例如 Time t / s。
| Height h / m | t₁ / s | t₂ / s | t₃ / s | Mean t / s |
|---|---|---|---|---|
| 0.200 | 0.202 | 0.198 | 0.203 | 0.201 |
| 0.400 | 0.286 | 0.290 | 0.287 | 0.288 |
Candidates also lost marks by recording raw data to too many decimal places. Since the timer read to 0.001 s, recorded times should be given to three decimal places. The mean should then be rounded to the same precision. Inconsistent rounding suggests a lack of understanding of instrument precision.
考生还因为原始数据的小数位数过多而丢分。由于计时器可以读到0.001 s，记录的时间应保留三位小数。平均值应四舍五入到相同的精度。不一致的四舍五入说明对仪器精度的理解不够。
7. Plotting a Graph and Drawing a Line of Best Fit | 绘图与画最佳拟合线
Graphical analysis was a significant part of the experimental question. Most candidates were expected to plot a graph of t² against h to determine g. The equation of motion from rest is h = ½ gt², which rearranges to t² = (2/g)h. Therefore, the graph of t² (y‑axis) against h (x‑axis) should yield a straight line through the origin, with gradient m = 2/g.
图形分析是这道实验题的重要部分。多数考生被要求绘制t²对h的图像来确定g。从静止下落的运动方程是h = ½ gt²，整理后得到t² = (2/g)h。因此，以t² (y轴) 对h (x轴) 作图，应得到一条过原点的直线，斜率m = 2/g。
h = ½ gt² → t² = (2/g)h
When drawing the line of best fit, candidates had to balance the points, ensuring roughly equal numbers lie above and below the line. Many simply joined the first and last data points, which ignores the scattering in between. A thin, sharp pencil and a transparent ruler are essential. The examiner penalised lines that were obviously biased or forced through the origin without justification.
画最佳拟合线时，考生必须使数据点均衡分布，线上线下的点数大约相等。许多人只是连接了第一个和最后一个数据点，这忽略了中间的离散情况。需要用到削尖的铅笔和透明直尺。考官对明显有偏差或未经论证就强行通过原点的直线均给予扣分。
8. Determining Uncertainty from the Graph | 从图形中确定不确定度
To estimate the uncertainty in the gradient, candidates should have drawn the steepest and shallowest 'worst‑fit' lines that still pass through the error bars. The difference between the maximum gradient mmax and the minimum gradient mmin gives the absolute uncertainty Δm. The percentage uncertainty in g would be the same as in the gradient, because g = 2/m.
为了估计斜率的不确定度，考生应画出最陡和最缓的“最差拟合线”，且这两条线仍要通过误差棒。最大斜率mmax与最小斜率mmin之间的差值即为绝对不确定度Δm。由于g = 2/m，g的百分比不确定度与斜率相同。
t many students failed to draw error bars on the graph, making it impossible to draw worst‑ epresenting ± the absolute uncertainty in each quantity, such as ±0.005 s for time and ±0.001 m for height.
2020年1月的报告指出，许多学生没有在图上画误差棒，导致无法正确画出最差拟合线。另一些人则在没有展示计算过程的情况下随便猜了一个不确定度。一定要用竖向和横向的短线标出误差棒，它们代表每个量的±绝对不确定度，例如时间的±0.005 s和高度±0.001 m。
9. Calculating Values and Error Propagation | 计算数值与误差传递
Once the best gradient is obtained, g is calculated from g = 2/m. If the gradient was, say, 0.204 s² m⁻¹, then g = 9.80 m s⁻². The absolute uncertainty in g can be found using Δg = (Δm/m) × g. Candidates had to express the result in the form g to matching decimal places and keeping the uncertainty to one or two significant figures.
一旦获得最佳斜率，g便可通过 Published by TutorHao | A-Level Physics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图