(a) A flatbed truck moving at 28 m/s carries a steel girder that rests on its wooden floor. The girder is not strapped down, in violation with USDOT regulations. If the coefficient of static friction between steel and wood is 0.52, what is the minimum distance over which the truck can come to a stop without the girder sliding toward the cab of the truck? (answer: 77 m) (b) What is the minimum time over which the truck can accelerate forward from 0 m/s to 28 m/s with a constant acceleration without the girder sliding off the back? (answer: 5.5 s)
(a) Given that, Initial speed of the truck () = 28 m/s
Coefficient of static friction () = 0.52
Final speed () = 0m/s (when the truck stops)
Net force on the truck , --------Eqn(1)
Maximum frictional force , -------Eqn(2)
where, N is the normal force and is given by,
substituting in Eqn(2) gives,
-------Eqn(3)
when the truck is going to stop, Net force on the truck is equal to the maximum frictional force. Then, Equating Eqn(1) and Eqn(3),
Gives, Acceleration ------Eqn(4)
where, g - Acceleration due to gravity ( g = 9.81 m/s2)
Substituting the values in Eqn(4)
When the truck is going to stop, then the acceleration is retarding
i.e,
Now, using the kinematic equation
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