## Motion & position vector

The definition of motion

We say that a body is in motion when the position of a body changes with respect to an origin.

For example, a car is going from city A to city B through a highway, or the Moon orbiting Earth.

The simplest type of motion is the motion along a straight line (one-dimensional motion).

Position vector

Position vector is a vector starting at origin, where the position x is equal to zero, ending at the current body’s position. As an example:

In the above vector,l is the unit vector and x is the body’s position.

Points:

a.It shows the instantaneous position of a moving body, and do not give any information about the body movement.

b. Its magnitude shows the length of a straight line connecting the position of the body to the origin at any instant.

c.Shifting origin changes the position vector.

d. If a body is placed in negative positions, the position vector direction is negative, and if a body is placed in positive positions, the position vector direction is positive.

e.The motion of a body is always considered relative to other bodies. It means that a O x=4 km movement leads to stagnation if the movement origin starts moving with the body.

Displacement vector

Displacement is a vector which connects the start of a movement to its end.

Points:

a.If the origin changes, the position vector changes too, but the displacement vector remains unchanged.

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