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Q Would you please explain the state
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ment made in the March 21 issue of Commercial Motor in the article "Make Fast!" The last sentence of one paragraph reads: "When the case shoots forward it hits the front wall with an energy of impact of 13,440 ft.lb. equivalent to the blow delivered when a weight of 7 ton falls through 6ft". What force in lb or tons would the headboard have to withstand in this instance?
AIn the example quoted, the author obviously assumes a rate of retardation of 19 (100 per cent braking efficiency) and that there is no friction between the load and the floor. Thus, as stated, the kinetic energy of the load of 1 ton 12.240Ib) when it strikes the headboard, is equal to the kinetic energy it would acquire after falling 6ft =2,240 x 6 = 13,440 ft.lb. The force in lb or tons on the headboard created by the impact of a sliding load entirely depends on the -give" in the headboard and/or load. No moving body can be brought to rest instantaneously and the give in this case represents the distance moved by the centre of gravity of the load after initial contact with the headboard has been made and the load has come to rest.
If a cushion of some sort were placed to the rear of the headboard to resist the impact, and the distance in which the load was brought
13,400 ft. lb' 13.4401b lft
16 tonsl. If the distance were reduced to 1in. (ft), the force would be increased to 72 tons, while a reduction to kin. would increase the force to 720 tons.
The example given in the article is, of course. of academic interest in that it represents an extremely unlikely combination of conditions, namely a rate of retardation of g, complete lack of friction between the load and the floor and absence of any means of restraint. However, it serves to illustrate the magnitude of the impact force on the headboard if the load slides during an emergency stop. If the load were located immediately behind the headboard and the coefficient of friction between the load and vehicle deck were 0.5. the headboard would only have to resist a force of ton. If the coefficient of friction were 1.0, the force on the headboard would be nil.