# Classical mechanics Diagram of orbital motion of a satellite around the Earth, showing perpendicular velocity and acceleration (force) vectors, represented through a classical interpretation.

Similarly, the first object sees the velocity of the second object as:

When both objects are moving in the same direction, this equation can be simplified to:

Or, by ignoring direction, the difference can be given in terms of speed only:

A force in physics is any action that causes an object's velocity to change; that is, to accelerate. A force originates from within a field, such as an electro-static field (caused by static electrical charges), electro-magnetic field (caused by moving charges), or gravitational field (caused by mass), among others.

As an example, assume that friction is the only force acting on the particle, and that it may be modeled as a function of the velocity of the particle, for example:

where λ is a positive constant, the negative sign states that the force is opposite the sense of the velocity. Then the equation of motion is

For extended objects composed of many particles, the kinetic energy of the composite body is the sum of the kinetic energies of the particles.

The decrease in the potential energy is equal to the increase in the kinetic energy

is constant in time. It is often useful, because many commonly encountered forces are conservative.

The ray approximation of classical mechanics breaks down when the de Broglie wavelength is not much smaller than other dimensions of the system. For non-relativistic particles, this wavelength is

Classical mechanics was traditionally divided into three main branches: