Work is a term used in physics to describe energy transfer that occurs when a force displaces an object, typically over a distance. It is a form of energy, similar to heat or electrical energy.
Various examples of work include a ball being thrown and dropped, an airplane flying at a constant speed in the air, a bicycle pedalling along a path, a person running across the street, or a cobbler polishing shoes. The force that displaces the object is called the force of the displacement, and this forces the object to move in some direction.
The work done is called the quotient of the force and the displacement. The quotient is equal to the magnitude of the force times the distance displaced in the direction of the force.
It is important to remember that no work is done unless the force and displacement occur in the same direction. That means a force on a ball that is thrown up is positive, while a force on a ball that is dropped is negative.
This is because the force on the ball is directed toward the ground at a right angle to the motion of the ball. The same is true of a magnetic force.
One way to understand this is to look at the weight of a ball, which is a force multiplied by the distance it is pushed away from the ground. If a ball is dropped and moves down, it is doing positive work because the force is pushing up on the ball in its downwards direction.
Likewise, the same is true of the force on a car that is being driven down a road. If the car is moving down the road at a speed of 5 meters per second, it is doing 20 J of work.
Another way to understand this is to consider a spring. The work on a spring is a function of the force and the distance through which it acts, so the force and distance are both dependent on each other.
In addition to the energy of the force, the spring also has a quantity of work that it does, which is the product of its mass and the distance through which it compresses the air. It is this volume of work that allows it to accelerate a small particle like a molecule, and enables it to lift a heavy object.