📚 How to Tackle the Jun-18 Experimental Investigation | 如何应对2018年6月实验探究题
The June 2018 A-Level Physics paper included an experimental scenario that tested students' ability to design, carry out and analyse an investigation. This article breaks down the key markscheme points from that paper, using a typical dynamics experiment as a model to illustrate the essential skills required for high marks.
2018年6月的A-Level物理试卷包含了一个实验情境，考查学生设计、实施和分析探究的能力。本文基于该试卷的评分要点，以典型的动力学实验为例，梳理获得高分所需的关键技能。
1. Understanding the Question: A Glider on an Air Track | 审题：气垫导轨上的滑块实验
In the Jun-18 question, students were presented with a scenario where a glider on an air track is pulled by a falling mass over a pulley. The aim was to investigate the relationship between the resultant force on the glider and its acceleration.
在6月18日的试题中，学生面对的是这样一个场景：气垫导轨上的滑块通过细绳和滑轮被下落的砝码拉动，目的是研究作用在滑块上的合力与其加速度之间的关系。
2. Identifying Variables Clearly | 明确识别变量
Independent variable is the force applied (varied by changing the hanging mass). Dependent variable is the acceleration of the glider. Control variables include total mass of the system (glider plus added masses).
自变量是施加的力（通过改变悬挂砝码的质量来变化），因变量是滑块的加速度，控制变量包括系统的总质量（滑块加上所载质量）。
3. Minimising Systematic Error: Compensating for Friction | 减少系统误差：补偿摩擦
The air track must be tilted slightly so that the component of gravity along the track cancels friction. This is checked by giving the glider a small push and confirming it moves with constant velocity.
必须将气垫导轨略微倾斜，使重力沿导轨的分力抵消摩擦。可以通过轻推滑块检查它是否做匀速运动来验证。
4. Random Errors and Use of Repeats | 随机误差与重复测量
For each force value, the time taken by the glider to pass through a light gate (or between two light gates) should be measured at least three times. Averaging reduces random uncertainties and the range can estimate the spread.
对于每一个力值，滑块通过光电门（或两个光电门之间）的时间应至少测量三次，取平均可减小随机不确定度，极差可估计散布程度。
5. Choosing Suitable Measuring Instruments | 选择合适的测量仪器
A digital light gate with a millisecond timer provides precise time measurements. A ruler or metre rule is used to measure the distance between light gates. The hanging mass is measured with a digital balance reading to 0.01 g.
使用带毫秒计时器的数字光电门可得到精确时间，用米尺测量光电门之间的距离，用可读到0.01 g的数字天平测量悬挂砝码的质量。
6. Designing an Efficient Data Table | 设计有效的数据表格
The table should include columns for hanging mass, force (mg), distance, time, calculated acceleration, and repeats. All quantities must have appropriate units in the header.
表格应包括悬挂质量、力（mg）、距离、时间、计算出的加速度以及重复测量等列，所有物理量的单位都应写在表头中。
7. Correct Calculation of Acceleration | 正确计算加速度
s and card length, or if using a single gate to measure time for a card of known length, the formula a = (v² - u²) / (2s) or from time and distance. The exact method must be specified.
加速度 a 可利用运动学公式确定。如果滑块通过两个光电门，由遮光板长度和时间得出速度 u 和 v，则 a = (v² - u²) / (2s)；若用单个门测量遮光时间，也可用时间–距离关系。具体方法必须明确说明。
a = (v² - u²) / (2s)
8. Plotting the Graph and Choosing Axes | 绘制图形并选择坐标轴
Plot acceleration a on the y-axis against force F on the x-axis. Both axes should be labelled ‘a / m s⁻²’ and ‘F / N’, with linear scales chosen to fill more than half the graph paper.
将加速度 a 画在 y 轴，力 F 画在 x 轴，坐标轴标注为‘a / m s⁻²’和‘F / N’，选择线性标度使数据点占据图面一半以上。
9. Drawing Best-Fit and Worst Acceptable Lines | 绘制最佳拟合线与最差可接受线
Draw a single straight line of best fit through the data points. Then draw the worst acceptable line (either steepest or shallowest) that still passes through all error bars, to determine the maximum gradient range.
通过数据点画一条最佳拟合直线，然后画出仍穿过所有误差棒的最差可接受线（最陡或最平缓），以确定斜率的最大范围。
10. Determining Slope and Its Uncertainty | 确定斜率及其不确定度
The slope m is Δa / ΔF. Its absolute uncertainty is Δm = (|slope_best - slope_worst|) / 2. Then percentage uncertainty = (Δm / m) × 100%.
斜率 m = Δa / ΔF，其绝对不确定度为 Δm = (|最佳斜率 - 最差斜率|) / 2，然后百分比不确定度 = (Δm / m) × 100%。
Δm = |mbest - mworst| / 2
11. Linking Slope to Physical Law and System Mass | 将斜率与物理定律和系统质量关联
slope, and the percentage uncertainty in M equals the percentage uncertainty in the slope.
根据牛顿第二定律 F = Ma，有 a = (1/M) F，斜率等于 1/M，其中 M 为系统总质量。因此 M = 1 / 斜率，M 的百分比不确定度等于斜率的百分比不确定度。
a = F / M
12. Evaluation and Suggested Improvements | 评估与改进建议
Compare the experimental M with the directly measured total mass. A significant discrepancy suggests unaccounted systematic errors, such as residual friction, pulley inertia, or string elasticity. Suggest using a pulley with low moment of inertia, lubricating the air track, or using a force sensor.
将实验得出的 M 与直接测量的总质量比较，显著差异表明存在未考虑的系统误差，例如残存摩擦、滑轮转动惯量或细线弹性。建议使用低转动惯量的滑轮、润滑导轨或采用力传感器。
Published by TutorHao | 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