The crowbar offers a very simple demonstration of the concept of a fulcrum.
The fulcrum is possibly one of the earliest tools, enabling the applification of a much larger force than by sheer strength alone.
To understand the working of this 'machine' simply needs the rules governing moments of force about a fulcrum. A 'moment of force' is just given by the product of the force (F or f) and its perpendicular distance from the fulcrum (b or a), e.g. on the diagram the moment of applied force is F × b. In the case of the crowbar these two moments must be equal and thus the relation f = F × ( b ÷ a ) can be derived. Thus the force generated at the working end of the crowbar is greater than the force applied (since b is greater than a).
Of course, this may appear to be giving you something for nothing, i.e. more force for no extra effort, but the amount of energy transfered is the same. Energy is proportional to the force times the distance moved. There is only a small applied force but you move the handle through a large distance compared to the working end, and the energy out does equal the energy in, i.e. ( F.c ) = ( f.d )
John Bland, html-ized by Nick
Return to the crowbar appreciation page
This page was last reviewed on Tuesday March 24th 1998
Nicholas Clark
<Nicholas.Clark@Liverpool.ac.uk>