(mentor note) post moved here from non-homework thread hence no template. A truck is travelling at a v=180 km/h speed on a straight road. At some point, the driver sees a police car and hits the brakes. If his average deceleration is a=-4m/s^2, how long will it take for its speed to go down to 80 km/h? So, i converted the units to m/s and used the equation u=v-at to find the time, but the result was wrong. What am i missing?