Rearranging the relationship between force, change in momentum, and time by dividing both sides by $\Delta t$ tells us that the net force acting on an object is equal to the rate of change of its linear momentum:

In one dimension, on a graph where the vertical axis represents momentum $p$ and the horizontal axis represents time $t$, the term $\frac{\Delta p}{\Delta t}$ corresponds mathematically to the slope of the line. Therefore, the physical quantity of net force is determined by finding the slope of the momentum-time graph.

To calculate the net force during a time interval where the graph forms a straight line, one determines the slope of that linear segment. For a segment defined by an initial point $(t_i, p_i)$ and a final point $(t_f, p_f)$, the net force is calculated as the change in momentum (rise) divided by the change in time (run):

If the graph is linear, the slope is constant, indicating that a constant net force is acting on the object throughout that duration.

...