Uniform motion

In uniform motion, velocity is constant. This means that both the magnitude of velocity (i.e., the speed) and the direction of motion remain the same throughout the motion. Because of this, average and instantaneous velocity are the same throughout the motion, and we have one and only one equation.

\[\vec{v}=\dfrac{\Delta\vec{d}}{\Delta t}\]
Important Note

There is a total of 3 parameters: \(\vec{v}\), \(\Delta\vec{d}\) and \(\Delta t\). We need to know 2 parameters in order to calculate the third.

In the displacement-time graph, the relation is always linear. The slope (i.e., velocity) is zero if and only if the object is stationary.

D-T graph - Stationary, positive position
Figure 1. Object is stationary on the positive side (east in this case) of the reference point.
D-T graph - Stationary, negative position
Figure 2. Object is stationary on the negative side (west in this case) of the reference point.

In the displacement-time graph, the slope is positive if and only if the object is moving in the positive direction, and is negative if and only if the object is moving in the negative direction.

D-T graph - Positive velocity (1)
Figure 3. Object starting from the reference point and moving in the positive direction (east in this case).
D-T graph - Negative velocity (1)
Figure 4. Object starting from the reference point and moving in the negative direction (west in this case).

The \(\boldsymbol{y}\)-intercept is the point where the object starts moving. The origin of coordinates is the reference point.

D-T graph - Positive velocity (2)
Figure 5. Object starting from the negative side (west in this case) of the reference point and moving in the positive direction (east in this case).
D-T graph - Negative velocity (2)
Figure 6. Object starting from the positive side (east in this case) of the reference point and moving in the negative direction (west in this case).

In the velocity-time graph, the relation is always a horizontal line.

V-T graph - Stationary
Figure 7. The blue graph represents an object moving at constant velocity in the positive direction, while the red graph represents an object moving at constant velocity in the negative direction.