A truck tractor pulls to trailers, one behind the other, at a constant speed of 100km/h.?

Question

It takes .635s for the big rig to completely pass onto a bridge 450m long. For what duration of time is all or part of the truck-trailer combination on the bridge?

Answer 1

to understand the trick behind >> analyse this first

-------------------------

a car moving with 60 km/h overtakes another car moving with 42 km/h. if both are 5 meter long, what is the time of overtaking.

> relative speed of fast car wrt another is = 60 -42 = 18 km/h

> meaning that fast car moves with 18 km/h and another car is at rest.

........................... |... L1 ... |

......................................... | ......... | overtake starts

.............. | .. L2 .. | overtake ends

distance moved in overtake = L1 + L2 = 5+5 =10 m

time taken = 10/[18*5/18] = 10/5 = 2 sec

===================================

truck-trailor

given that big-rig takes [t=0.635 s] to completely pass ON the bridge OR MEANS TAKES 0.635 s TO COMPLETELY COME OUT OF THE BRIDGE

L2 = 0.635 * [100*5/18] = 17.64 meter

.......................... |... 450 ... |

......................................... | ......... | overtake starts

.............. | .. L2 .. | overtake ends

distance moved in overtaking bridge by all parts = 450 + L2 = 450+ 17.64 =467.64 m

duration time is all or part of the truck-trailer combination on the bridge= 467.64/[100*5/18] = 16.84 sec