Velocity of a particle moving along positive x-direction is v=(40-10 t). Here, t is in seconds At time t=0, the x-coordinate of particle is zero. Find the time when the particle is at a point   (-60 m,0) from origin.

Comparing with v=u+at, we have u=40 m/s and a=-10 m/s2Distance of 60 m from origin may be at x=+60 m and x=-60 m.From the figure, we can see that at these two points particle is at three times, t1, t2 and t3.For X=+60 m ort1 and t2s=ut+12at2 ⇒  60=(+40)t+12(-10)t2Solving this equation, we gett1=2 s and t2=6 sFor X=-60 mor t1s=ui+12at2 -60=(+40)t+12(-10)t2Solving this equation, we get positive value of t is t3=2(2+7)s