Hello Here we must use the equation of motion v^2 = u^2 + 2as; where v is final velocity, u is initial velocity, a is the acceleratoin and is the distance travelled. We select this one because the time of collision is unknown to us. We know the truck stopped so its final velocity is 0; thus v = 0. Converting the initial velocity to SI units, we get 3.89 m/s. The distance traveled, s, is 0.062 meters. Inserting all of these values into the equation, 0 = (3.89)^2 + 2(a)(0.062) and solving for a, we get a to be -122.0 ms^(-2) The negative sign indicates the acceleration is in the opposite direction to the initial motion, which means the truck decelerated. This is consistent with the given condition.
