Uniform rectilinear motion

The movement is relative

The movement of a body, seen by an observer, depends on the reference frame in which the observer is situated.

Source: https://th.bing.com/th/id/R.e77fbd76f41c851355c4417b3653c06e?rik=kb076FHHKTn9uQ&riu=http%3a%2f%2fprofademir.no.comunidades.net%2fimagens%2ftraj.gif&ehk=cz0F%2bfhrQcrCGDUQKSOFBZOfiEy%2fpSqtN6v6UbV8%2b7g%3d&risl=&pid=ImgRaw&r=0

An observer B, in a train (which moves on the tracks), and the observer A, stationary on the Earth. For observer A, observer B is in motion, along with the train. However, from the point of view of observer B, observer A is moving in the opposite direction to the movement of the train on Earth.

Uniform rectilinear motion

Distance, speed and time

When a body moves with constant velocity along a rectilinear path, we say that its motion is uniform rectilinear. The word "uniform" indicates that the velocity value remains constant.

We can find the distance traveled by an object by multiplying its speed by the time it takes to travel.

d: distance traveled

v: speed (constant)

t: time taken to cover the distance d

We can write:

This equation applies even if the trajectory is not straight, but don't forget that it is valid only when the velocity remains constant.

Example: if the velocity of a body is v = 30km/h and you want to calculate the distance it traveled during a time t = 3.0h, we already know that:

Note that the unit of time simplifies when performing the multiplication and the result is correctly expressed in km, which is a unit of distance.

Speed x Time Graph

Source: https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEibfDEUChWXuu_jG8HtFyMUqm701J-Mmaekgae5-FYZycF8UpvtE_s9c3AZRvWMADKad0WPv2ws5n4l3KQr74dMqazCH6i5RH9PI9VVLSvyQXdsIUtqUVVBdfAKnIGIvPmmggmZH0WQjca_/s1600/Velocidade+constante.GIF

Assume that the car shown at the top of the figure above is moving in uniform motion and that this speed is maintained for a time t = 5.0h.

We must draw two perpendicular axes to represent these quantities:

• on the horizontal axis represents different values of time t.
• on the vertical axis we represent the values of velocity v corresponding to each time value t.

We thus learn that in uniform motion the v x t graph is a line parallel to the time axis and that the area under this graph gives us the value of the distance traveled.

Distance x Time Graph

In any uniform motion (v = constant) the distance d traveled by an object is directly proportional to the time t elapsed in this journey. This fact is represented mathematically by the relation d ∝, where the symbol ∝ means "is proportional to".

Graph slope

The slope of the d x t graph for uniform motion gives us the value of the velocity of that motion, i.e.

Source: https://pt-static.z-dn.net/files/d0f/dcf0bbffc8937f8fdc7e7b7f1458f3cc.jpg

Negative speed

When a body moves along a trajectory, we usually define one of the directions of movement as being positive; the other sense, then, will be considered negative.

Reference

MAXIMUM, Antonio; ALVARENGA, Beatriz. Physics: High School. Volume 1. 1st Edition. São Paulo: Editora Scipione, 2006.