Definition and Measurement of Motion, Speed, Velocity and Friction

What is Motion? What are the differences between speed and velocity? What is friction and how can it be reduced? These are questions relating to fluids and mechanics and form part of the very basics in physic and engineering.

Motion is defined as the change of the position of a body with time

Types of Motion

1) Random Motion:
This is a type of motion in which a body moves irregularly with no preferred direction.

2) Translational Motion:
The translational motion is a type of motion in which every point in a moving body remain fixed in relation to other points.

3) Rotational Motion:
This is a type of motion where every point in a body moves in circles.

4) Oscillatory or Vibratory Motion:
This is a type of motion in which a body moves to and fro, reversing it’s direction, and returning regularly to it’s position.


Dynamics is about the motion of objects and the forces acting on them. It deals with the forces that keeps an airplane in flight through air. The founder of mechanics is Sir Isaac Newton.


The term ‘ Speed ‘ describes the rate at which a body covers a distance.
Speed is the distance travelled in unit time.

Speed / time = distance.

If you run round the corner of a street to see a friend and Travel a distance of 24m in 20s, then your average speed is:

24m / 20s or 1.2m/s

Similarly if a car takes 2h to travel along a winding road from a town to another town 96km away, the average speed of the car is:

distance / time.

96km / 2h = 48km/h

The S.I unit of distance is metre.
The S.I unit of speed is metre per second….. (ms^-1)

Uniform Speed:

Uniform Speed or constant speed happens when a body
covers equal distances in equal time intervals.


Velocity graph

Velocity is defined as the rate of change of displacement.

Velocity = displacement / time.

V = s / t.

Uniform Velocity:

When a body moves with equal displacements in equal time intervals, it is said to be in uniform velocity.


A car travels at an average speed of 100kmh^-1, What distance does it cover in 5min.

100km per hour = 100 * km per min / 60.

Speed. = distance / time.

distance = speed * time

———– * 5

= 8.3km

Distance time graph.

Distance graph

Distance time graph provides useful information about the motion of an object.
If the graph is a straight line , the speed ( or velocity ) is uniform since the body covers equal distances in equal times.

Instantaneous Velocity:
Instantaneous Velocity is the velocity at any instant of time.
Example of an instantaneous velocity: The speedometers of moving vehicles.

Angular Velocity:
Angular Velocity is the angle turned through, divided by the elapsed time.

Acceleration is the rate of change of velocity with time. It’s unit is metre – per – second square. (ms ^-2 )

Acceleration (a) = velocity / time taken.

A car traveling at 36km/h accelerates uniformly at 2m/s^2. Find its velocity in km/h in 5s time.
NB: The velocity of 36km/h is therefore changed to metre per second.

Since 36km/h = 10m/s
initial velocity (u) = 10m/s
Now a = 2m/s^2 , t = 5s
Final Velocity (v) = u + at = 10 + (2*5) =10+10 = 20m/s
v = 72km/h

When the velocity increases with time, the rate of the change of velocity with time us termed ‘ acceleration ‘ but when the velocity decreases with time, it us called ‘ retardation or “deceleration” ‘ . Retardation or Deceleration is also known as ” negative acceleration”.

Uniform Acceleration:
Uniform Acceleration is defined as the motion of an object whose velocity increases by equal amounts in equal time intervals.

Retardation or Deceleration:
Retardation or Deceleration means that the velocity decreases by equal amounts in equal times.

Deceleration = velocity / time

Example of Deceleration:

Deceleration. = ( 98 – 48 ) km/h / 10s

=5km/h per s


FORCE is a change in the state of rest or of uniform motion in a straight line of a body

Types of Force:

There are different types of forces. Frictional force, contact force, gravitational force, Tensional force, Air resistance force, Normal force, Electrical force and Magnetic force. Read more on these forces


Friction is the force which acts at the separation between two objects in contact and tends to oppose the motion of one over the other.

Types of Friction

1) Static or Limiting Friction:
This is the maximum force that must be overcome before a body can move over another.

2) Kinetic or Dynamic Friction:
This is the for e that must be overcome so that a body can move with uniform speed over another body.

Advantages of Friction

1) Without friction, it would be impossible to walk or stop walking after getting started.
2) Friction enables the automobile tyres to make a firm grip with the roadway, thus aiding their movements.
3) Friction enables screws and nails to remain in place after being screwed into a position.
4) Friction enables the brakes to stop the car.
5) Friction is used in grinding stone to sharpen knives and chisels.

Disadvantages of Friction:

1) It leads to much wear and tear on the moving parts of a machine.
2)It causes loss if energy in a machine.
3)It causes the heating of engines.

Methods of reducing friction:

1) Application of lubricants
2)The use of ball it roller bearings
3) Streamlining of body shapes of moving objects.

Ideas of Circular Motion:

1)The earth moving around the sun
2)The planets moving around the sun
3)The moon circling the earth…..etc

What is a Centripetal Force?:

Centripetal Force is the inward force required to keep an object moving with a constant speed in a circular path.

What is a Centrifugal Force?:

A centrifugal Force acts in opposite direction to the centripetal force.