# forces

## 04. Forces

### Types of Forces

• Static friction
• Frictional force between surfaces at rest with respect to each other.
• It increases with increasing applied force up to a maximum value (which is proportional to normal contact force).
• Kinetic friction
• Frictional force acting between surfaces in relative motion.
• Viscous forces
• Resistive force experienced by a solid moving in a fluid.
• Dependent on speed of object v, e.g. $$F_D\propto v$$ at low speeds and $$F_D\propto v^2$$ at high speeds.

## Hanging Forks This simple demonstration can be done anywhere at home using the following items:

1. an empty glass
2. a toothpick
3. two forks
The video below demonstrates how to do it. When the forks are balanced on the mouth of the glass with the toothpick, the centre of gravity of the forks-and-toothpick system will adjust itself so that it lies vertically below the pivoting point. This is possible because the forks form a V-shape within which the centre of gravity can exist.