Category: WORK / ENERGY vis a vis WORK / FORCE - DISPLACEMENT
Work is defined as force through displacement. Given that detailed information the work of an evenct can be calculated. On the other hand for an event, work is one of a few terms in an energy equation and given those terms are know, work can be determined. Examples of this category show this truth.
WORK to Raise the Castle Bridge
Our system is the simple span of a drawbridge. To "select the bridge as system," is to isolate it. To do this, let your mind trace the entirety of the boundary of bridge. The event for the bridge will be that the "drawing ropes" pulling it from its horizontal to its vertical position.
What least work is required to raise the span?
A tank contains a shallow body of water. Its bottom, left wall, and ends are fixed. The right wall can be pushed to the left and will remain vertical should that happen. Seals prevent all leakage of water. Figure 1. shows the wall in its initial and final positions.
Calculate the work required to push the movable wall a short distance to the left. Note that the depth of the tank (into the page) is L.
The sketch, (1) shows a buoy floating in the sea. A cable lowered from a dock is attached to the buoy and the buoy is hauled to state (2). Take the system to be the buoy, a short length of the cable and part of the sea. The cable moves the buoy for condition (1) to condition (2) as shown.
Calculate the least work required of the cable.