Grade 9 Physics: Motion Graphs & Speed

Vocabulary terms and definitions covering position-time graphs, motion diagrams, coordinates, slope interpretations, vector velocity, speed, distance, and displacement for Grade 9 Physics.

Position ($x$)

Location relative to an origin; can be positive, zero, or negative. Unit: metre ($\text{m}$).

Time ($t$)

When the position was measured. Unit: second ($\text{s}$).

Instantaneous Position

The object’s position at one particular instant.

Intersection

The point where two graphed lines cross; represents the same time and same position.

Horizontal Axis

The axis on a position–time graph that displays time $t$ in seconds ($\text{s}$).

Vertical Axis

The axis on a position–time graph that displays position $x$ in metres ($\text{m}$), where negative values are allowed.

Motion Diagram

A representation where dots placed at regular time intervals show successive positions of an object.

Data Table

A tabular list of paired values of time $t$ and position $x$.

Position–Time Graph

A graph plotting $(t, x)$ where a straight line represents constant velocity.

Positive Slope (Position–Time Graph)

Indicates positive velocity; represents motion toward the east if east is chosen as $+x$.

Horizontal Line (Position–Time Graph)

Indicates zero velocity; shows that the object is stationary and its position is not changing.

Negative Slope (Position–Time Graph)

Indicates negative velocity; represents motion toward the west if east is chosen as $+x$.

Velocity ($v_x$)

A vector quantity defined by magnitude and direction, calculated using the equation $v_x = \frac{\Delta x}{\Delta t} = \frac{x_2 - x_1}{t_2 - t_1}$.

Distance

The total path travelled by an object; it is always zero or positive.

Displacement

The change in position, calculated as final position minus initial position ($\Delta x = x_2 - x_1$); can be positive, negative, or zero.

Average Speed

Total distance divided by total time ($\frac{\text{total distance}}{\text{total time}}$). Unit: $\text{m/s}$. Has no direction.

Average Velocity

Displacement divided by total time ($\frac{\text{displacement}}{\text{total time}}$). Unit: $\text{m/s}$. Reported with direction or a sign.

Position Magnitude ($|x|$)

The absolute distance of an object from the origin, without regard to its positive or negative sign.