Mechanics · Forces & Newton's laws
Chapter 1 · 4
The idea
Force diagrams, resultant force & equilibrium
What each force on a free-body diagram is and which way it points (weight always vertically down, normal reaction perpendicular to the surface, tension pulling along a string), why forces add as vectors so a resultant is found component-by-component, and what "in equilibrium" means — the resultant is zero, so the components balance.
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Mechanics · Forces & Newton's laws
Force diagrams, resultant force & equilibrium
What each force on a free-body diagram is and which way it points (weight always vertically down, normal reaction perpendicular to the surface, tension pulling along a string), why forces add as vectors so a resultant is found component-by-component, and what "in equilibrium" means — the resultant is zero, so the components balance.
Why it works
The picture comes first
Every mechanics problem starts the same way: draw the free-body diagram — the object on its own, with an arrow for every force acting on it. Get this picture right and the equations write themselves; get it wrong and no amount of algebra saves you. So the first skill is knowing the forces and which way each one points.The cast of forces
- Weight . The pull of gravity, acting at the object's centre and always vertically downwards — never along a slope, never tilted. Its size is mass , with .
- Normal reaction (or ). The push of a surface on the object, always perpendicular to that surface. On flat ground it points straight up; it is not automatically equal to the weight (see below).
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