Reference frame and distance

Road in the distance

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Refrence frame

A frame

Frame of reference is a system for specifying the precise location of objects in space and time.

A simple expirement can show the need of a refrence frame :

We can choose a student in our classroom . Let’s say student E, as you can see in the given classroom figure. We want to answer to a simple question: «where is E sitting ?».

English reference frame_1

A classroom figure.

We want to answer the question but we can’t measure without a starting point. We can choose point A but we can also choose points B or C or even D.The choosen point (A or B or C or even D) will be our refrence frame . We can not conduct any measure without a refrence frame .

We can measure the distance AE or BE or CE or even DE. We can see that when we choose another point as a refrence frame, the distance of E is changing.


If we want to measure changes in position, we must chose a frame of refrence. The selected refrence frame must be fixed throught our measures.

Physics is a science that studies changes, a body that stands still throughout the course of our study has no interest.

Kinematics is the branch of physics that describes motion.

One-dimensional motion

We often choose the x axis as the line along which the motion takes place. Then the position of an object at any moment is given by its x coordinate.

We can also choose the y axis when a body is falling from a roof of a building.


Stopping Distance in a car

Distance is the length of the path taken to move from one position to another, regardless of direction.


A cute mouse is moving in a tube from – 2 m to + 5 m. Determine the total distance that mouse traveled.


The mouse traveled d = 2 + 5 = + 7 m .

Note that – 2 is +2 bacause distance is the lengh a positive number.

Lenght units

The lenght unit is 1 meter (m), a base unit.


We also use:

1 kilometer (km) = 10m.

1 centimeter (cm) = 10-2 m.

1 millimeter (mm) = 10-3 m.

1 micrometer (μm) = 10-6 m.

1 nanometer (nm) = 10-9 m.

English and Americans use

1 inch (in) = 2.54 cm , where 2.54 is a conversion factor.

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