Forces is one of the heaviest-weighted topics in OCR GCSE Physics. This bilingual guide breaks down every key concept with worked examples and exam tips. 力学是 OCR GCSE 物理权重最高的模块之一。本文中英双语拆解每个关键考点，助你冲刺高分。
1. Scalar vs Vector 标量与矢量
Scalars have magnitude only: speed, distance, mass, energy, time. Vectors have magnitude and direction: velocity, displacement, force, acceleration, momentum. When adding vectors, direction matters — you cannot simply add magnitudes. 标量只有大小，矢量有大小和方向。矢量相加时方向至关重要。
考点提醒： OCR frequently asks "Is X scalar or vector?" Velocity is a vector, speed is a scalar. Displacement is a vector, distance is a scalar. Force is ALWAYS a vector. 速度是矢量，速率是标量；位移是矢量，路程是标量；力永远是矢量。
2. Contact vs Non-Contact Forces 接触力与非接触力
Contact 接触力： friction, tension, normal reaction, air resistance, upthrust — require physical contact. Non-contact 非接触力： gravitational, electrostatic, magnetic — act at a distance.
关键点： Weight = mg (g ≈ 9.8 N/kg on Earth). Weight changes with gravitational field strength; mass is invariant. W = mg，重量随引力场变化，质量不变。
3. Newton's Three Laws 牛顿三大定律
First Law 第一定律： An object remains at rest or in uniform motion unless acted upon by a resultant force. Resultant force = 0 ⇒ constant velocity. 合外力为零，物体保持静止或匀速直线运动。
Second Law 第二定律： F = ma. Acceleration ∝ resultant force and ∝ 1/mass. The most-used equation in Forces. 加速度与合外力成正比，与质量成反比。
Third Law 第三定律： Action-reaction pairs are equal and opposite but act on DIFFERENT objects — they do not cancel. Balanced forces act on the SAME object. 作用力与反作用力作用在不同物体上，不会抵消。这是考试反复挖坑的地方。
4. Free Body Diagrams & Resultant Force 受力分析与合力
Draw all forces as arrows touching the object: weight (from centre of mass downwards), normal reaction (perpendicular to surface), applied forces, friction. Arrows must be labelled with correct relative lengths. 受力分析图标出所有力：重力从质心向下，法向力垂直于接触面，箭头标注清楚、长度比例正确。
Resultant Force 合力： The single force equivalent to all forces combined. Zero resultant ⇒ equilibrium (stationary or constant velocity). Non-zero ⇒ acceleration per F = ma. 合力为零则平衡，不为零则产生加速度。
例题： A 5 kg box pushed right with 30 N, friction 10 N left. Resultant = 20 N right, a = 20/5 = 4 m/s². 5kg箱子受向右30N推力、向左10N摩擦力，合力20N，加速度4m/s²。
5. Motion, Momentum & Stopping Distance 运动、动量与刹车
Terminal Velocity 终极速度： Falling object: drag increases with speed until drag = weight, resultant = 0, constant terminal velocity reached. 下落物体阻力随速度增大，直到阻力等于重力，达到终极速度。
Stopping Distance 停车距离： = thinking distance + braking distance. Thinking distance ∝ speed and reaction time. Braking distance ∝ speed², affected by road/tyre/brake conditions. 停车距离 = 反应距离 + 刹车距离，后者与速度平方成正比。
Momentum (Higher Tier) 动量： p = mv. Conservation: total momentum before collision = total momentum after, in a closed system. 碰撞前后系统总动量守恒。
6. Moments & Levers 力矩与杠杆
Moment 力矩： M = F × d (d = perpendicular distance from pivot). Unit: Nm. Principle of Moments: sum of clockwise = sum of anticlockwise for rotational equilibrium. 力矩 = 力 × 力臂；平衡时顺时针力矩之和等于逆时针力矩之和。
Levers & Gears 杠杆与齿轮： Longer effort arm ⇒ smaller force needed. A small gear turning a larger gear multiplies torque. 杠杆和齿轮利用力矩原理放大力的效果。跷跷板、横梁、起重机是高频考题场景。
7. Elasticity & Hooke's Law 弹性与胡克定律
Hooke's Law 胡克定律： F = kx. Extension ∝ force (within elastic limit). k = spring constant (N/m). Beyond the elastic limit, plastic (permanent) deformation occurs. 弹簧伸长量与拉力成正比（弹性限度内），超过限度发生塑性形变。
Elastic potential energy: E = ½kx² = area under force-extension graph. 弹性势能等于力-伸长量图下方的面积。
8. Pressure 压强
Solids 固体： P = F/A. Smaller area ⇒ larger pressure. 相同力下，面积越小压强越大。
Fluids 流体： P = hρg. Pressure increases with depth and density. Atmospheric pressure ≈ 101 kPa at sea level, decreases with altitude. 液体压强随深度和密度增大；海平面大气压约101kPa。
9. Exam Strategy 应考策略
MUST-KNOW equations 必背公式： F=ma, W=mg, p=mv, M=Fd, F=kx, P=F/A, P=hρg, E=½kx². Write them down at the exam start. 考试开始时先写公式。
6-Mark Questions 六分大题： State the principle → describe setup → explain what and WHY → link to science. Common topics: terminal velocity, stopping distance, moments on a beam, Hooke's Law experiment. 六分题答题结构：原理→设置→解释→回归科学原理。
Graph Skills 图像分析： Distance-time: gradient = speed. Velocity-time: gradient = acceleration, area = displacement. Force-extension: gradient = k, area = elastic energy. 距离-时间图斜率=速率；速度-时间图斜率=加速度、面积=位移；力-伸长量图斜率=k、面积=弹性势能。
Master these concepts and practise past papers for exam confidence. For GCSE/A-Level tutoring, reach out below. 掌握核心概念，刷透历年真题。如需辅导，欢迎联系。
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