Kinematics

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• Mechanics starts with SI units: displacement in $\text{m}$, time in $\text{s}$, mass in $\text{kg}$, velocity in $\text{m s}^{-1}$, acceleration in $\text{m s}^{-2}$, force in $\text{N}$ and moment in $\text{N m}$. Convert before substituting.
• Distance and speed are scalars; displacement, velocity and acceleration have direction. Choose a positive direction once, draw it, and let the algebraic signs carry the direction.
• On a displacement-time graph, gradient is velocity. On a velocity-time graph, gradient is acceleration and signed area is displacement; total distance needs the magnitudes of areas below and above the axis.
• The constant-acceleration formulae apply only while acceleration is constant. List $s,u,v,a,t$, include signs and choose the equation containing the known quantities and the one unknown you need.
• For variable acceleration, $v=\dfrac{ds}{dt}$ and $a=\dfrac{dv}{dt}$; conversely, $s=\int v\,dt$ and $v=\int a\,dt$. Constants of integration come from the stated initial conditions.
• For a projectile, resolve the initial velocity. Horizontal motion has acceleration $0$ and vertical motion has acceleration $-g$ after choosing upwards as positive; both components use the same time.
• A projectile model normally treats the object as a particle, uses constant $g$ and neglects air resistance. State a limitation in context rather than writing only 'the model is unrealistic'.

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Questions are written in the style of past Edexcel papers (source shown on each) — never copied from them.

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