Force
Definition
A push or a pull on an object. Forces can start motion, stop motion, change direction, or squeeze and stretch things.
Symbol: · Unit: newton (N)
Technical definition
An interaction that changes an object's momentum over time. Newton's second law quantifies it: the net force on an object equals its mass times its acceleration, F = ma. Force is a vector — direction matters as much as size.
Example
When you kick a soccer ball, your foot applies a force of several hundred newtons for about a tenth of a second, and the ball shoots off at 25 m/s. The same kick on a bowling ball barely moves it — more mass needs more force for the same acceleration.
Force is not energy, and a force does not guarantee motion. You can push against a brick wall with 400 N of force all day and do zero joules of work, because the wall never moves. Force causes acceleration; work requires displacement. Mixing them up makes energy problems unsolvable.