# Velocity

**Velocity** is a vector measurement of the rate and direction of motion. The scalar absolute value (magnitude) of velocity is speed. Velocity can also be defined as rate of change of displacement.

In mechanics the average speed *v* of an object moving a distance *d* during a time interval *t* is described by the simple formula:

*v* = *d/t*.

The instantaneous velocity vector

**v** of an object whose position at time

*t* is given by

**x**(

*t*) can be computed as the

derivative

**v** = d**x**/d*t*.

Acceleration is the change of an object's velocity over time. The average acceleration of

*a* of an object whose speed changes from

*v*_{i} to

*v*_{f} during a time interval

*t* is given by:

*a* = (*v*_{f} - *v*_{i})/*t*.

The instantaneous acceleration vector

**a** of an object whose position at time

*t* is given by

**x**(

*t*) is

**a** = d^{2}**x**/(d*t*)^{2}

The final velocity

*v*_{f} of an object which starts with velocity

*v*_{i} and then accelerates at constant acceleration

*a* for a period of time

*t* is:

*v*_{f} = *v*_{i} + *at*

The average velocity of an object undergoing constant acceleration is (

*v*_{f} +

*v*_{i})/2. To find the displacement

*d* of such an accelerating object during a time interval

*t*, substitute this expression into the first formula to get:

*d* = *t*(*v*_{f} + *v*_{i})/2

When only the object's initial velocity is known, the expression

*d* = *v*_{i}*t* + (*a**t*^{2})/2

can be used. These basic equations for final velocity and displacement can be combined to form an equation that is independent of time:

*v*_{f}^{2} = *v*_{i}^{2} + 2*ad*

The above equations are valid for both

classical mechanics and

special relativity. Where

classical mechanics and

special relativity differ is in how different observers would describe the
same situation. In particular, in

classical mechanics, all observers
agree on the value of 't' and the transformation rules for position
create a situation in which all non-accelerating observers would describe
the acceleration of an object with the same values. Neither is true
for

special relativity.

The kinetic energy (movement energy) of a moving object is linear with both its mass and the square of its velocity:

The kinetic energy is a

scalar quantity.