In physics, work has a precise meaning: it is energy moved into or out of an object by a force that acts along a distance. Push a cart across a room and you do work on it; the energy you transfer shows up as the cart's kinetic energy.
Key idea
Work = force × distance moved in the direction of the force: $$W = F\,d$$ Force in newtons times distance in meters gives joules: $1\,\mathrm{J} = 1\,\mathrm{N \cdot m}$.
Why it works
Work counts only when the object moves along the force. If nothing moves, $d = 0$ and no energy is transferred, however tiring it feels. If the force is sideways to the motion, like the upward force of your arms while you walk with a box, that force is not pushing the box along, so it does no work on it either.
Worked example: Pushing a cart
You push a cart with a steady 40 N for 5 m. How much work do you do?
- $W = F\,d = 40\,\mathrm{N} \times 5\,\mathrm{m} = 200\,\mathrm{J}$.
Answer: 200 J
Worked example: Lifting
You lift a 10 kg box straight up 1.5 m at a steady speed. How much work do you do?
- To lift it steadily, your force equals its weight: $10 \times 9.8 = 98$ N.
- $W = 98\,\mathrm{N} \times 1.5\,\mathrm{m} = 147\,\mathrm{J}$.
Answer: 147 J
Common mistake
Thinking that holding a heavy box still is work in the physics sense. Your muscles tire, but the box doesn't move, so $W = F \times 0 = 0$ and the box gains no energy. Check: did the object move in the direction of the force?