Why is it so easy for you to use a screw driver to loosen/tighten screws, but nearly impossible/very hard to do it with just your hand?
In: Physics
To tighten a screw by hand, you’re limited to gripping just the head. This would require you to pinch it between two finger tips, which has a maximum grip strength of maybe 20 pounds. Friction between skin and metal is iffy, so we’ll say that you only get 10 pounds of grip. That is applied at a distance of, at best, a quarter inch from the center. This gives you a torque (rotating force) on the screw of 2.5 inch-pounds.
With a screwdriver, you could instead grip with 60 pounds at least since you can use your whole hand. On top of that, you’re twisting farther from the center – maybe 3/4 of an inch instead of 1/4. That allows for 22.5 inch-pounds, assuming the same friction from earlier. With a good rubber handle, friction may also be noticeably higher.
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