South African NSC Physical Sciences

## Kinds of force A force is a push or a pull, measured in newtons (N). It is a vector, so it has a magnitude and a direction, and two forces on the same object can only be combined by taking direction into account. The forces met most often in mechanics are these. | Force | Symbol | What it is | |---|---|---| | Weight | $w$ or $F_g$ | The gravitational force the Earth exerts on an object on or near its surface, always towards the centre of the Earth | | Normal force | $N$ or $F_N$ | The force, or the component of a force, that a surface exerts on an object in contact with it, perpendicular to the surface | | Frictional force | $f$ | The force that opposes the motion of an object, or its tendency to move, and acts parallel to the surface | | Applied force | $F$ | A push or pull put on the object from outside, for example by a person or a motor | | Tension | $T$ | The force a string, rope or cable exerts on whatever it is attached to, directed along the string and away from the object | Weight is calculated from the mass of the object and the gravitational acceleration at its position: $$w = mg$$ where $m$ is mass in kg and $g = 9.8\ \mathrm{m\cdot s^{-2}}$ near the surface of the Earth. Mass is the amount of matter in an object and does not change with position, whereas weight changes with the gravitational field the object is in. ## Force diagrams and free-body diagrams A force diagram shows the object and every force acting on it, drawn as arrows from the object, with the…