Describing linear motion (One Dimension)


Two scientist that contributed greatly to the field of Mechanics were Galileo Galilei (1564-1642) and Isaac Newton (1642-1727).

What do you think is the meaning of mechanics? Well mechanics is the study of motion /movements and the related elements of force and energy. You need to know that mechanics is divided into two section; one section describe how objects move. This section is called Kinematics, so this is where you find the use of graphs such as velocity vs. time graphs and position/distance vs. time graphs. We will look at those later. The other section of mechanics focus on why objects move the way they do and also the force acting on the object. This section is called Dynamics. So the two section of mechanics are Kinematics and Dynamics.

We will be looking at Translational motion and also how to calculate it in term of average speed, velocity, displacement and acceleration. Translational motion means in a straight line. This is telling you there is no twisting taking place. Hope you understand that, if not read it over and think about it. You need to know that a reference frame is key when dealing with measurement of position, distance or speed. Example, you are on a boat travelling to Europe and you are running across the deck of the boat. The boat speed will be the reference frame for your speed. It’s also important to specific direction of motion as well.

In science we often use a set of coordinate’s axis to signify a reference frame. I bet you know about the x and y-axis that are always perpendicular to each other. Look at the diagram below that was design by SciencezoneJA.


Average Speed

You should know that speed is the rate of change of distance, if you did not now you do. Average speed is the total distance travelled divided by the total time taken:


(clip_image004= the bar above x is used to state average values.

What is the difference between speed and velocity? Well speed is just a number with unit (it only indicate rate of change and give no direction) and velocity tell you how fast an object is moving and also in which direction (velocity is a vector quantity, because it has magnitude and direction). The unit for average speed is ms-1 or m/s.

Instantaneous speed is the speed at any instant (time) during the rate of change of distance. Look at this example, we travel with an average speed of 10m/s. But that does not mean we travel 10m/s for all our journey. So what was our speed when we were not traveling at 10m/s. That’s what instantaneous speed deals with; your speed at any given moment/instant during your travel.

Average Velocity

Before we deal with average velocity, you need to know what velocity is. Velocity is the rate of change of displacement and displacement is change in position (you were at A and move to B; but you did not return back to A). Displacement occur only when an object does not return to its initial position. See diagram below to get a better understanding:


Average velocity is total Displacement divided by Total time taken:


The unit for velocity is ms-1 or m/s. Just like for speed.

Please note that the average speed and average velocity have the same magnitude when motions/movement occurs all in one direction. But a difference in value for average speed and velocity, can occur when dealing with average values.

What do you think instantaneous velocity? Look at instantaneous speed and think about it.

When an object move to the right average velocity is positive and when an object move to the left its negative. The direction in which the displacement occur, is the same direction the average velocity occur.

Bonus Information

The following  symbol represent;

s- displacement (measure in metre (m))

t- Time (measure in second (s))

v- Speed (measure in metre per second (m/s))

v- Velocity (measure in metre per second (m/s))

Displacement is calculated using the formula;


u= initial position (in metre (m)), v= final position (in metre (m)) and t= time (in second).

If the data is requested by the examiner in kilometre and hour, or whatever unit(s). You should state the data in the requested unit. So you need to learn how to convert between units.

We will look at instantaneous velocity, acceleration, deceleration and describing linear motion with equations in the next post.


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