力 lì is a pictograph of exertion. The oracle-bone and bronze forms show a single bent shape that most scholars read as a plough pushed through soil, while others see a flexed arm with the muscle and sinew standing out. Either way the image is the same: the body straining against a load. The modern glyph keeps just two strokes, a long downward sweep hooked at the foot and a short stroke pressing across it, the whole thing leaning forward like a limb under strain. 力 is its own radical, the strength radical, and it lends that sense of effort to a wide family of characters such as 加 (to add), 动 (to move), and 努 (to strive).

Three senses branch from the straining body:

1. Physical strength, force. The muscle sense, raw bodily power: 力气 lìqi (physical strength), 体力 tǐlì (stamina), 用力 yònglì (to exert force).
2. Power, capacity, ability. Strength abstracted into what a person or thing can do: 能力 nénglì (ability), 动力 dònglì (driving force), 压力 yālì (pressure).
3. Effort, by force of trying. Strength deliberately poured into a task: 努力 nǔlì (to strive), 尽力 jìnlì (to do one's utmost), 暴力 bàolì (violence, force used wrongly).

The thread running through every word is push: the foot on the plough, the muscle under load, the will set against a task. Wherever 力 appears, something is being moved by force, whether the force is a body lifting a weight, a capacity meeting a demand, or a person bending their whole effort toward an end.

Past the muscle, 力 becomes the measure of what a person or thing can do. 能力 nénglì pairs 能 (be able) with 力 (force) to name ability, the capacity to carry something off. 动力 dònglì joins 动 (to move) with 力 to name the force that drives motion, whether the literal power of an engine or the inner motivation that keeps a person going. And 压力 yālì joins 压 (to press down) with 力 to name pressure, the force bearing down from outside, used both for physical load and for the everyday weight of stress.

Notice how 力 turns a verb into a noun of capacity. 想象 means to imagine, and 想象力 is the power of imagination; 记忆 means to remember, and 记忆力 is the faculty of memory; 影响 means to influence, and 影响力 is influence as a measurable force. In each, the 力 suffix says "the strength to do this," converting an action into the standing ability behind it.

See the two strokes as a flexed arm: the long hooked sweep is the limb, the short stroke pressing across it is the swollen muscle straining under a load. That arm under strain is the whole character. Everything 力 means is the body, or something like a body, pushing against weight.

Keep the muscle and you get raw strength (力气, 体力). Abstract the muscle into capacity and you get ability and the forces around it (能力, 动力, 压力). Aim the muscle at a task and you get effort (努力, 尽力). Whenever you meet 力, look for the load it is pushing against, and the sense will follow.