Reaction time and stopping distance

Question

A pedestrian was stuck and seriously injured by a truck which left skid marks 28.5m long. Prior to skidding, the truck was travelling at 72km/hr^-1.

 

1. Determine the speed of the truck in metres per second (m/s^-1) prior to skidding.

 

2. If the truck driver took 0.75s to react, estimate the stopping distance in metres of the truck.

Answer 1

1.

Use conversion factors to convert speed from km/hr to m/s

1km = 1000m
1 hour = 60 minutes = 3600 seconds

Speed of the truck prior to skidding = 72 km / hr

                                                                = 72 (1000 m) / (3600 s)

                                                                = 72000 m / 3600 s

                                                                = 20 m/s

Speed of the truck im meters per second prior to skidding is 20 m/s

 

Answer 2

2.

stopping distance = skidding distance + reacting distance

                          = 28.5 m+ speed of the truck prior to skidding x reaction time

                          = 28.5 m + 20m/s x 0.75s

stopping distance = 28.5m + 20 m = 43.5m

Comments

Stopping distance = skidding distance + reacting distance

                           = 28.5 m+ speed of the truck prior to skidding x reaction time

                           = 28.5 m + (20m/s x 0.75s)

Stopping distance = 28.5m + 15m = 43.5m.