Calculating Time for Truck-Trailer Combination to Pass Bridge

[zdickz]

Homework Statement

A truck tractor pulls two trailers, one behind the other, at a constant speed of 100 km/h. It takes 0.610 s for the big rig to completely pass onto a bridge 390 m long. For what duration of time is all or part of the truck-trailer combination on the bridge?

d = 390 m
v of combo = 100km/h = 27.78 m/s

Homework Equations

Total time equals the time to cover the distance of 390 m plus the time for the truck to pass onto and off of the bridge. So...

The Attempt at a Solution

390 m / 27.78 m.s = 14.04 s + 2(0.610) = 15.3 s

I got credit for the problem from webassign, but they have an answer of 14.7 s as the true answer. What can I do to improve my accuracy?

Thank you in advance.

 

[p21bass]

Homework Statement

390 m / 27.78 m.s = 14.04 s + 2(0.610) = 15.3 s

This, technically, is incorrect. Once the truck is fully on the bridge (0.610s), it has already covered part of the 390m bridge. Thus, it only needs to go another 390m to be fully off the bridge. It doesn't go 390m and then take another 0.610s to get off.

So, the equation should have been (390m)/(27.78m/s) = 14.04s + 0.610s = 14.65s.

Rounded to 14.7s.

 

[zdickz]

Once the truck is fully on the bridge (0.610s), it has already covered part of the 390m bridge. Thus, it only needs to go another 390m to be fully off the bridge.

Thank you!