📚 Unit 6: Further Mechanics and Thermal Physics (pp. 252-278) | 第6单元：进阶力学与热物理（第252-278页）
This unit brings together two major areas of A-level physics: further mechanics, including momentum, circular motion and simple harmonic motion, and thermal physics, including internal energy, specific heat capacity and ideal gas behaviour. The material is highly synoptic and appears regularly in both calculation and explanation questions.
本单元整合了 A-level 物理的两大核心领域：进阶力学（包括动量、圆周运动和简谐运动）与热物理（包括内能、比热容和理想气体行为）。这部分内容综合性很强，经常以计算题和解释题的形式出现在考试中。
1. Momentum and Impulse | 动量与冲量
Momentum is defined as the product of an object' s velocity: p = mv. Momentum is a vector quantity, so direction must always be considered.
动量定义为物体质量与速度的乘积：p = mv。动量是矢量，因此必须始终考虑方向。
p = mv
is equivalent to N s. Impulse is the change in momentum produced by a force: J = FΔt = Δp.
动量的国际单位是 kg m s⁻¹，等价于 N s。冲量是力产生的动量变化：J = FΔt = Δp。
J = FΔt = Δp
On a force-time graph, impulse is the area under the curve, even when the force is not constant. This is particularly useful when the force varies during a collision or impact.
在力-时间图像中，冲量是曲线下的面积，即使力不是恒定的也成立。这在碰撞或冲击过程中力发生变化时特别有用。
2. Conservation of Momentum | 动量守恒
The principle of conservation of momentum states that if no external resultant force acts on a system, the total momentum remains constant. This applies to collisions and explosions.
动量守恒定律指出：如果系统不受合外力作用，总动量保持不变。这适用于碰撞和爆炸。
m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
When applying this equation, assign one direction as positive and give velocities in the opposite direction negative signs. For explosions, the total momentum after the explosion is zero if the original object was stationary.
应用此方程时，需要先规定正方向，并将反方向的速度设为负值。对于爆炸，如果原物体最初静止，则爆炸后总动量为零。
In recoil problems, such as a gun firing a bullet, the gun and bullet move in opposite directions with equal and opposite momentum. The heavier gun moves backwards much more slowly than the bullet moves forwards.
在反冲问题中，例如枪支发射子弹，枪和子弹以大小相等、方向相反的动量运动。较重的枪向后移动的速度远小于子弹向前移动的速度。
3. Elastic and Inelastic Collisions | 弹性与非弹性碰撞
Collisions are either elastic or inelastic. In an elastic collision, both momentum and kinetic energy are conserved. In an inelastic collision, momentum is conserved but kinetic energy is not.
碰撞分为弹性碰撞和非弹性碰撞。在弹性碰撞中，动量和动能都守恒。在非弹性碰撞中，动量守恒但动能不守恒。
A perfectly inelastic collision occurs when the colliding bodies stick together and move with a common velocity. The maximum kinetic energy loss occurs in this case, usually as heat, sound or deformation.
完全非弹性碰撞发生在碰撞物体粘在一起并以共同速度运动时。这种情况下动能损失最大，通常转化为热、声或形变能。
| Property | Elastic | Inelastic |
|---|---|---|
| Momentum | Conserved | Conserved |
| Kinetic energy | Conserved | Not conserved |
| Behaviour | Bounce apart | May stick or deform |
To determine whether a collision is elastic, calculate the
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