# What is velocity?

Jul 28, 2016

Velocity is the speed and direction of motion of an object.

#### Explanation:

Velocity is a vector quantity of motion in that it has magnitude, or speed, and direction.

To change an object's direction of motion requires a change in velocity direction which requires an external force.

To change an object's speed requires a change in the magnitude of its velocity which requires an external force.

Newtons laws state that in the absence of an external force an object moves in a straight line at constant speed. Newton describes gravity as a force.

Einstein describes gravity as the curvature of spacetime which means that it is not a force. Objects moving in the absence of an external force, which includes motion under gravity, follow a geodesic which is the 4 dimensional equivalent of a straight line.

Another way of looking at velocity is to consider a moving object's momentum. Momentum is the object's mass multiplied by its velocity $p = m v$. Momentum is conserved in the absence of an external force.