Force diagrams & model words

1. Circle the body. Decide exactly which particle, block or connected system you are analysing.
2. Remove the scene. Replace that body by a simple box or point. Do not copy the floor, person or other objects into the free-body diagram.
3. Add external forces. Draw every force acting on the selected body, with an arrow and a clear label. The arrow points in the force direction; its label gives the force name or magnitude.

• Weight $mg$ acts vertically downward. Here $m$ is mass in kg and $g$ is the acceleration due to gravity. In Paper 4 use $g=10\text{ m s}^{-2}$ unless told otherwise.
• Normal reaction $R$ acts perpendicular to a contact surface. “Normal” means perpendicular here.
• Friction $F$ acts along a contact surface and opposes relative motion or the tendency to move.
• Tension $T$ acts along a taut string, pulling away from the body at the point of attachment. A taut string is stretched tight rather than hanging slack.
• A stated pull, thrust or resistance acts in the direction described by the question.

• A smooth surface has no frictional force.
• A rough surface may exert friction. It does not tell you that friction is already at its maximum.
• A light string has negligible mass. Across a small smooth pulley, whose axle friction is ignored, its tension is modelled as the same throughout.
• An inextensible string does not stretch. This matters when connected particles move; it will be used in the Newton's laws chapter.

Every model word removes or links something. Write its mathematical consequence before choosing equations.