POST 2

question 1

A lorry haning a mass of 1.5t is travel-ling along a level road at 72 km/h. when the brakes are applied, the speed decreases to 18 km/h. determine how much the kinetic enegy of the lorry is reduced.

Initial velocity of lorry, v1                  = 72 km/h

                                       km                    m            1h

                            =  72   −−−   x  1000     −−−    x    −−−

                                        H                     km         3600s

                                    72

                               =  −−−   = 20 m/s

                                   3.6

Final velocity of lorry, v2                =  18

                                                        −−−  =   5 m/s and

                                                         3.6

Mass of lorry , m                             = 1.5t = 1500 kg

Initial kinetic energy of the lorry        1                    1

                                                  = −−− mv ²   =   −−−  (1500)(20) ²

                                                      2          ¹        2                   

                                              

             = 300 kj

Final kinetic energy of the lorry         1                  1

                                                 =  −−  mv²  =   −− ( 1500)(5) ²

                                                      2     ²           2

                                                    

 = 18.75 kj

Hence, the change in kinetic energy    = 300 – 18.75

                            

                                                        = 281.25 kj