Mechanical Reasoning - Finding Force at a Dashboard with a Pry Bar
During an extrication drill, a training manikin is pinned behind the dashboard of a junked car. The crew has a 60-inch pry bar, a 1-ton come-along, a 3:1 rope pulley system, and wooden cribbing. The training officer estimates the dash needs about 600 pounds of force to move. You place the pry bar's fulcrum 8 inches from the dash; two firefighters put their weight on the long end and together apply about 55 pounds. The dash does not move. A firefighter then slides a 4-foot steel pipe over the come-along handle, saying, 'This will double our pull.' The come-along's cable is rated at 2,000 pounds and its handle is designed for about 80 pounds of input. The bar is rated for 1,500 pounds of applied load at the load end, and the crew is working against a 10-minute drill clock with the rope system not yet rigged.
1. What mechanical advantage does the bar give as currently set, what force actually reaches the dash, and by how much does it fall short of 600 pounds?
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Mechanical advantage of a first-class lever is the ratio of the arms, not the length of the bar:
- Effort arm = 60 − 8 = 52 in
- Load arm = 8 in
- MA = 52 ÷ 8 = 6.5
Force at the dash: 55 lb × 6.5 = 357.5 lb, roughly 358 lb.
Shortfall: 600 − 357.5 = 242.5 lb, so the current setup delivers about 60% of what the officer estimates is needed.
Sanity check on the estimate itself: the crew is measuring their effort by 'putting their weight on it,' which is a guess, and the 600 lb figure is also an estimate. Real extrication forces are not that tidy, and a dash that 'does not move' stiffens as it deforms. The point of stating the shortfall is not precision - it is that the crew is not marginally short, they are short by a factor of about 1.7, which is why leaning harder will not solve it.
2. Keeping the same bar, the same 55 pounds of effort, and the same overall length, where should the fulcrum go to produce at least 600 pounds? What tradeoff do you warn the crew about?
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Work backward from the target.
- Required MA = 600 ÷ 55 = 10.9
- Load arm = 60 ÷ 10.9 ≈ 5.5 in from the dash
- Effort arm = 60 − 5.5 = 54.5 in
So the fulcrum moves from 8 inches to about 5.5 inches from the load - roughly 2.5 inches closer. Check: 54.5 ÷ 5.5 = 9.9; 55 × 9.9 = 545 lb. Round a little tighter, to a 5-inch load arm, and you get 55 × (55 ÷ 5) = 605 lb. Aim for 5 inches rather than 5.5 to leave margin.
Tradeoffs to state out loud:
- Travel, not free force. The effort end now moves about 10 inches for every inch the dash moves. Leverage buys force with distance; the crew will pump the bar a lot and see the dash creep.
- Geometry risk. With the fulcrum 5 inches from the load, the bar tip and the firefighters' hands crowd the work area, and the bar is easier to knock off its fulcrum. A slipped fulcrum dumps 600 pounds of stored load into whatever is under it - use cribbing to block the fulcrum and take up the gap as the dash moves.
- Structural load. 600 lb is well under the bar's 1,500 lb load-end rating, so the bar is fine - but the dash, the A-pillar, and the manikin's surroundings may not be, and a sudden release is the injury waiting to happen. Crib, then pry, then crib again.
3. Evaluate the 'cheater bar' idea on the come-along: what will it actually do, and what is the real risk?
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What it does mechanically: sliding pipe over a handle lengthens the lever arm about the handle's pivot, so the rotational effort for a given hand force rises roughly in proportion to the added length. If the pipe roughly doubles the effective handle length, a comfortable 55 lb hand pull produces something like 110 lb of effective input at the handle - which is exactly the problem.
Why it is the wrong answer here:
- The handle is designed for about 80 lb of input. Doubling the input puts it at roughly 138% of design, well past the 80 lb figure before anyone even leans on it.
- The cable is rated at 2,000 lb (the 1-ton rating). A come-along pulled near its rating with an over-length handle can exceed that, and a cable that parts under tension is a whip, not a snap.
- The pipe can slip off the handle or bend it, releasing the stored energy the ratchet was holding.
The deeper reasoning point: a come-along's pull is limited by its rating, not by how hard you can push the handle. A cheater bar raises input without raising capacity. If the crew needs more than the tool is rated to give, the correct moves are to add a properly rated pulley system (the 3:1 already on scene), use a larger tool, or reposition and crib so the required force drops. The drill clock is not a reason to overload hardware - a failed come-along at a real extrication costs far more than the ten minutes.