Velocity


Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector quantity, meaning that both magnitude and direction are needed to define it. The scalar absolute value of velocity is called, a quantity that is measured in metres per second in the SI system. For example, "5 metres per second" is a scalar, whereas "5 metres per second east" is a vector. If there is a change in speed, direction or both, then the object is said to be undergoing an acceleration.

Definition

Average velocity

The average velocity of an object over a period of time is its change in position, divided by the duration of the period, given mathematically as

Instantaneous velocity

The instantaneous 'velocity' of an object is the limit average velocity as the time interval approaches zero. At any particular time, it can be calculated as the derivative of the position with respect to time:
From this derivative equation, in the one-dimensional case it can be seen that the area under a velocity vs. time is the displacement,. In calculus terms, the integral of the velocity function is the displacement function. In the figure, this corresponds to the yellow area under the curve.
Although the concept of an instantaneous velocity might at first seem counter-intuitive, it may be thought of as the velocity that the object would continue to travel at if it stopped accelerating at that moment.

Difference between speed and velocity

While the terms speed and velocity are often colloquially used interchangeably to connote how fast an object is moving, in scientific terms they are different. Speed, the scalar magnitude of a velocity vector, denotes only how fast an object is moving, while velocity indicates both an object's speed and direction.
To have a constant velocity, an object must have a constant speed in a constant direction. Constant direction constrains the object to motion in a straight path thus, a constant velocity means motion in a straight line at a constant speed.
For example, a car moving at a constant 20 kilometres per hour in a circular path has a constant speed, but does not have a constant velocity because its direction changes. Hence, the car is considered to be undergoing an acceleration.

Units

Since the derivative of the position with respect to time gives the change in position divided by the change in time, velocity is measured in metres per second.

Equation of motion

Average velocity

Velocity is defined as the rate of change of position with respect to time, which may also be referred to as the instantaneous velocity to emphasize the distinction from the average velocity. In some applications the average velocity of an object might be needed, that is to say, the constant velocity that would provide the same resultant displacement as a variable velocity in the same time interval,, over some time period. Average velocity can be calculated as:
The average velocity is always less than or equal to the average speed of an object. This can be seen by realizing that while distance is always strictly increasing, displacement can increase or decrease in magnitude as well as change direction.
In terms of a displacement-time graph, the instantaneous velocity can be thought of as the slope of the tangent line to the curve at any point, and the average velocity as the slope of the secant line between two points with coordinates equal to the boundaries of the time period for the average velocity.

Special cases

  • When a particle moves with different uniform speeds v1, v2, v3,..., vn in different time intervals t1, t2, t3,..., tn respectively, then average speed over the total time of journey is given as If, then average speed is given by the arithmetic mean of the speeds
  • When a particle moves different distances s1, s2, s3,..., sn with speeds v1, v2, v3,..., vn respectively, then the average speed of the particle over the total distance is given as If, then average speed is given by the harmonic mean of the speeds

    Relationship to acceleration

Although velocity is defined as the rate of change of position, it is often common to start with an expression for an object's acceleration. As seen by the three green tangent lines in the figure, an object's instantaneous acceleration at a point in time is the slope of the line tangent to the curve of a graph at that point. In other words, instantaneous acceleration is defined as the derivative of velocity with respect to time:
From there, velocity is expressed as the area under an acceleration vs. time graph. As above, this is done using the concept of the integral:

Constant acceleration

In the special case of constant acceleration, velocity can be studied using the suvat equations. By considering a as being equal to some arbitrary constant vector, this shows
with as the velocity at time and as the velocity at time. By combining this equation with the suvat equation, it is possible to relate the displacement and the average velocity by
It is also possible to derive an expression for the velocity independent of time, known as the Torricelli equation, as follows:
where etc.
The above equations are valid for both Newtonian mechanics and special relativity. Where Newtonian mechanics and special relativity differ is in how different observers would describe the same situation. In particular, in Newtonian 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. In other words, only relative velocity can be calculated.

Quantities that are dependent on velocity

Momentum

In classical mechanics, Newton's second law defines momentum, p, as a vector that is the product of an object's mass and velocity, given mathematically aswhere m is the mass of the object.

Kinetic energy

The kinetic energy of a moving object is dependent on its velocity and is given by the equationwhere Ek is the kinetic energy. Kinetic energy is a scalar quantity as it depends on the square of the velocity.

Drag (fluid resistance)

In fluid dynamics, drag is a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid. The drag force,, is dependent on the square of velocity and is given aswhere
is the minimum speed a ballistic object needs to escape from a massive body such as Earth. It represents the kinetic energy that, when added to the object's gravitational potential energy, is equal to zero. The general formula for the escape velocity of an object at a distance r from the center of a planet with mass M iswhere G is the gravitational constant and g is the gravitational acceleration. The escape velocity from Earth's surface is about 11 200 m/s, and is irrespective of the direction of the object. This makes "escape velocity" somewhat of a misnomer, as the more correct term would be "escape speed": any object attaining a velocity of that magnitude, irrespective of atmosphere, will leave the vicinity of the base body as long as it does not intersect with something in its path.

The Lorentz factor of special relativity

In special relativity, the dimensionless Lorentz factor appears frequently, and is given bywhere γ is the Lorentz factor and c is the speed of light.

Relative velocity

Relative velocity is a measurement of velocity between two objects as determined in a single coordinate system. Relative velocity is fundamental in both classical and modern physics, since many systems in physics deal with the relative motion of two or more particles.
Consider an object A moving with velocity vector v and an object B with velocity vector w; these absolute velocities are typically expressed in the same inertial reference frame. Then, the velocity of object A object B is defined as the difference of the two velocity vectors:
Similarly, the relative velocity of object B moving with velocity w, relative to object A moving with velocity v is:
Usually, the inertial frame chosen is that in which the latter of the two mentioned objects is in rest.
In Newtonian mechanics, the relative velocity is independent of the chosen inertial reference frame. This is not the case anymore with special relativity in which velocities depend on the choice of reference frame.

Scalar velocities

In the one-dimensional case, the velocities are scalars and the equation is either:
if the two objects are moving in opposite directions, or:
if the two objects are moving in the same direction.

Coordinate systems

Cartesian coordinates

In multi-dimensional Cartesian coordinate systems, velocity is broken up into components that correspond with each dimensional axis of the coordinate system. In a two-dimensional system, where there is an x-axis and a y-axis, corresponding velocity components are defined as
The two-dimensional velocity vector is then defined as. The magnitude of this vector represents speed and is found by the distance formula as
In three-dimensional systems where there is an additional z-axis, the corresponding velocity component is defined as
The three-dimensional velocity vector is defined as with its magnitude also representing speed and being determined by
While some textbooks use subscript notation to define Cartesian components of velocity, others use,, and for the -, -, and -axes respectively.