Frnd here the initial velocity(u)is 40km/hr as the body at firts was moving with that speed where as the final velocity will be(v)will be 0 as the body tends to stop after the breakes are applied .Acceteration is negative which also results only when the final velocity is less than the initial velocity .here u can evaluate v-u/t= -0.6m/2s or -0.3m/s .but first change 40km/hr to m/s i.e100m/9s.get time and use the formula s=ut+1/2at sqaure. Or simply u can use the formula v square-u square=2as frm here u can get the value of s(distance) which will be the easiest
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