Forces & Newton's laws

The topic on one screen

• A force diagram is the start of the solution. Draw only forces acting on the chosen body: weight $mg$, normal reactions, tensions, friction and any applied forces. Motion arrows are not forces.
• Newton's first law gives equilibrium when the resultant force is zero. Newton's second law is the vector equation $\mathbf{F}=m\mathbf{a}$, so resolve it separately in two perpendicular directions.
• Weight is $mg$ and acts vertically downwards. A normal reaction is perpendicular to the contact surface; it is not automatically equal to $mg$ on an inclined plane or when another force has a perpendicular component.
• Newton's third-law forces act on different bodies. For connected particles, draw a separate diagram and $F=ma$ equation for each particle, while a light inextensible string over a smooth pulley gives common tension and common acceleration magnitude.
• On an incline at angle $\alpha$, the weight components are $mg\sin\alpha$ down the plane and $mg\cos\alpha$ perpendicular to it. Resolve relative to the plane unless another choice is clearly simpler.
• Friction opposes actual or impending relative motion and satisfies $F\leq\mu R$. Use $F=\mu R$ only when the body is moving or is stated to be in limiting equilibrium.
• Mechanics models need interrogation: particles have no size, strings are light and inextensible, pulleys are smooth, and constant friction coefficients simplify real contact forces.

Where students actually lose marks

Try it — exam-style

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

Drill it properly