work
When you push a stalled car and it rolls forward, or lift a box onto a shelf, you feel like you are doing work — and in physics that everyday word becomes a precise idea. Work is what a force does when it acts on an object while the object moves. Crucially, only motion along the direction of the force counts. Holding a heavy suitcase perfectly still may exhaust your muscles, but because nothing moves, the physicist says you have done zero work on the suitcase.
The precise rule: the work done by a constant force is W = F d cos(theta), where F is the size of the force, d is the distance the object moves, and theta is the angle between the force and the direction of motion. When the force points the same way as the motion (theta = 0, cos = 1) you get the most work, W = F d; when the force is sideways to the motion (theta = 90 degrees, cos = 0) it does no work at all — which is why the string of a whirling ball does no work on it. Work is measured in joules (J): one joule is one newton pushing through one metre, 1 J = 1 N x 1 m.
Work can be negative. If the force opposes the motion (theta between 90 and 180 degrees), cos(theta) is negative, so the work is negative — the force is taking energy away rather than adding it. Friction sliding against a moving block, or gravity acting on a ball you throw upward, both do negative work. This signed bookkeeping is the whole point of work: it is the exact amount of energy a force pumps into, or drains out of, a moving object.
You push a box 3 m across the floor with a steady 20 N force aimed along the floor. The work you do is W = F d = 20 N x 3 m = 60 J. If instead you push at 60 degrees below horizontal, only the horizontal part counts: W = 20 x 3 x cos(60 degrees) = 20 x 3 x 0.5 = 30 J.
Same push, same distance — but a slanted force does less work, because only the along-the-motion part counts.
In physics, 'work' is not effort or tiredness. Holding a weight still, or carrying a suitcase horizontally at steady speed, does zero work on it even though it feels hard — because the force has no component along the object's motion.