For a $2 \ kg$ mass,the $v-t$ graph is given. Find the force experienced by the mass in $OA$,$AB$,and $BC$.

(N/A) The force experienced by a mass is given by Newton's second law of motion,$F = ma$,where $a$ is the acceleration calculated from the slope of the $v-t$ graph.
$1$. For portion $OA$:
Acceleration $a = \frac{v_f - v_i}{t_f - t_i} = \frac{20 - 0}{10 - 0} = 2 \ m \ s^{-2}$.
Force $F = ma = 2 \ kg \times 2 \ m \ s^{-2} = 4 \ N$.
$2$. For portion $AB$:
The velocity is constant,so acceleration $a = 0 \ m \ s^{-2}$.
Force $F = ma = 2 \ kg \times 0 \ m \ s^{-2} = 0 \ N$.
$3$. For portion $BC$:
Acceleration $a = \frac{v_f - v_i}{t_f - t_i} = \frac{0 - 20}{40 - 30} = -2 \ m \ s^{-2}$.
Force $F = ma = 2 \ kg \times (-2) \ m \ s^{-2} = -4 \ N$.
The negative sign indicates a retarding force.