📚 Year 7 OCR Physics: Case Study Practical Drills | Year 7 OCR 物理：案例分析实战演练
Welcome to our case study practical drills for Year 7 OCR Physics. These exercises will help you apply scientific ideas to real-world scenarios, which is a key skill in for your exams and in practical work. Each case presents a situation, followed by the physics reasoning, the calculations and the evaluation you are expected to write.
欢迎来到我们的 Year 7 OCR 物理案例分析实战演练。这些练习帮助你运用物理概念解决真实情境问题，这既是考试中的关键能力，也是实验探究中的核心技能。每个案例都会给出一种场景，随后展示应写的物理解释、计算过程与评估分析。
Work through each case with a pencil: label the forces, write the equation first, then substitute the numbers, and finish with a sentence that explains what the result means. That structure is exactly what earns marks.
请用铅笔逐个完成案例：先标注力，再写出公式，然后代入数值，最后用一句话说明结果的含义。这一结构正是得分的关键所在。
1. Pushing a Box: Balanced and Unbalanced Forces | 推箱子：平衡力与不平衡力
In this first case a student tries to push a heavy box across the floor. She pushes with a small force, but the box stays still. The reason is that the static friction force between the box and the floor acts in the opposite direction and balances her push, so the forces are balanced and there is no change in motion.
在第一个案例中，一名学生试图把一只重箱子推过地面。她用较小的力推，箱子却纹丝不动。原因是箱子与地面之间的静摩擦力方向与推力相反，与之平衡，因此力是平衡的，运动状态不发生改变。
When she increases the pushing force, it eventually overcomes the maximum static friction. The forces become unbalanced, a resultant force appears, and the box accelerates in the direction of the push. Once the box is moving, the friction changes to sliding friction, which is usually slightly smaller than the maximum static friction.
当她加大推力时，推力最终会超过最大静摩擦力。力变得不平衡，出现合力，箱子沿推力方向加速。箱子开始运动之后，摩擦力转变为滑动摩擦力，通常略小于最大静摩擦力。
If the push is then adjusted so that it exactly equals the sliding friction, the resultant force is zero again and the box moves at a steady speed. The key forces to label on a free-body diagram are the applied push, friction, weight and the normal contact force from the floor.
如果随后把推力调整到恰好等于滑动摩擦力，合力再次为零，箱子就以稳定速度运动。在受力示意图上需要标注的主要力有：推力、摩擦力、重力和地面给予的支持力（法向接触力）。
resultant force = applied push − friction
A concise conclusion for this case is: balanced forces keep an object still or moving at a steady speed, while an unbalanced resultant force changes its speed or direction.
这个案例的简练结论是：力平衡时物体保持静止或匀速直线运动；存在不平衡的合力时，物体的速度大小或方向就会改变。
2. Weight and Mass on the Moon | 月球上的重量和质量
An astronaut has a mass of 60 kg on Earth. Mass is the amount of matter in an object, measured in kilograms, and it does not change with location, so her mass on the Moon is still 60 kg.
一名宇航员在地球上的质量为 60 千克。质量是物体所含物质的多少，用千克作单位，不随地点改变，因此她在月球上的质量仍为 60 千克。
about 10 N/kg, so her weight is found using weight = mass × gravitational field strength.
重量是一种力，用牛顿作单位，取决于她所在位置的重力场强度 g。地球上的 g 约为 10 牛/千克，因此她的重量用"重量 = 质量 × 重力场强度"计算。
W = m × g
= 96 N, roughly 100 N. She feels much lighter because the pull of gravity on her is smaller.
在地球上她的重量为 60 千克 × 10 牛/千克 = 600 牛。月球上的重力场强度弱得多，约为 1.6 牛/千克，因此她在月球上的重量为 60 千克 × 1.6 牛/千克 = 96 牛，约等于 100 牛。她感觉轻得多，是因为重力对她的拉拽变小了。
| Quantity / 物理量 | On Earth / 地球 | On the Moon / 月球 |
| Mass 质量 | 60 kg | 60 kg (unchanged 不变) |
| Gravitational field strength 重力场强度 | 10 N/kg | 1.6 N/kg |
| Weight 重量 | 600 N | 96 N |
The exam trap here is mixing up mass and weight. Mass is a scalar property of the object; weight is a force that changes when the gravitational field strength changes.
这里的考试陷阱是把质量和重量混淆。质量是物体自身的属性，用千克表示；重量是一种力，会随重力场强度改变而变化，用牛顿表示。
3. Measuring Speed with a Stopwatch | 用秒表测量速度
A common case study asks you to measure the average speed of a toy car. A student marks a 2.0 m track on the bench and times the car three times with a stopwatch, recording 1.6 s, 1.5 s and 2.4 s.
一个常见的案例研究要求你测量玩具小车的平均速度。一名学生在桌面上标出 2.0 米的轨道，用秒表测了三次，读数分别为 1.6 秒、1.5 秒和 2.4 秒。
s reading is much larger than the others, so it is likely to be an anomalous result caused by a slow reaction time when stopping the watch.
平均速度用"速度 = 路程 ÷ 时间"计算。取一个典型读数：2.0 米 ÷ 1.6 秒 = 1.25 米/秒。2.4 秒这个读数明显大于其他两个，很可能是按停秒表时反应慢造成的异常值。
speed = distance ÷ time v = s ÷ t
The best response is to repeat the measurement several times, ignore the anomaly, and calculate a mean of the reliable readings. You should also quote the answer with the correct unit and to the same precision as the data.
最好的做法是重复测量若干次，剔除异常值，再对可靠读数求平均值。你还应写出正确的单位，并保留与原始数据相同的有效数字位数。
Sensible improvements to mention are: use a light gate or video timing app to remove human reaction time, check that the bench is level, keep the car rolling in a straight line, and use a longer track so that the timing uncertainly matters less.
可以提出的合理改进包括：使用光电门或录像计时软件消除人为反应时间，检查桌面是否水平，让小车沿直线滚动，以及使用更长的轨道使计时误差的影响相对减小。
Finally, distinguish instantaneous speed (the reading on a car speedometer at one moment) from average speed (the total distance divided by the total time).
最后要区分瞬时速度（汽车速度表某一时刻的读数）与平均速度（总路程除以总时间）。
4. Density and the Floating Egg | 密度与浮蛋案例
mass ÷ volume and is usually measured in g/cm³ for small objects or kg/m³ for larger ones.
密度表示单位体积内包含多少质量。它用"密度 = 质量 ÷ 体积"计算，小物体常用克/立方厘米作单位，大物体常用千克/立方米。
Pure water has a density of 1.0 g/cm³, so the block sinks because it is denser than water.
一块密封方块的质量是 500 克，体积是 250 立方厘米，因此密度为 500 克 ÷ 250 立方厘米 = 2.0 克/立方厘米。纯水的密度是 1.0 克/立方厘米，所以方块因密度大于水而下沉。
ρ = m ÷ V 1 g/cm³ = 1000 kg/m³
In the floating egg case, an egg with a density of about 1.05 g/cm³
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