Force is applied to a body of mass 3 kg at rest on a frictionless horizontal surface as shown in the force against time (F - t) graph. The speed of the body after 1 s is

The impulse-momentum theorem states that the change in momentum equals the area under the force-time graph: $J = m(v - u)$.

From $t=0$ to $t=1$ s, the graph consists of two rectangles. The first rectangle (0 to 0.5 s) has area $(8)(0.5) = 4$, and the second (0.5 to 1 s) has area $(4)(0.5) = 2$, making the total impulse $J = 6\text{ Ns}$.

With $m = 3\text{ kg}$ and initial velocity $u = 0$, we have $6 = 3v$, so $v = 2\text{ m/s}$.

The speed after 1 second is $2\text{ m/s}$.