Work is the term used to describe the transference of energy. Work is given the math variable "W." That's a capital "W." Because it is used to describe the flow of energy it has the same units as energy, Joules.
Work is the "dot product" of two vectors. The dot product gives an answer that is a scalar. This means that work is component of force parallel to the displacement times the displacement. Mathematically this is written as,
The easy application of this formulas when the force and displacement point in the same direction as show in the animation below.
But what if the force and the displacement are not parallel? In that case you draw a right triangle using the force as the hypotenuse and with one of the triangle's sides parallel to the displacement. The side of the force triangle that is parallel to the displacement is the force that is used when calculating work.That is why instead of "F" for force, the parallel lines are used as a subscript on the letter "F" to indicate parallel
The parallel lines mean to use trigonometry to find the piece of the force that is parallel to the displacement.
Example
If the force and displacement are parallel to each other, then the angle is zero degrees. The work formula reduces to
Practice 
A wagon is pulled by a 100 N force as shown below. The wagon is pulled for 10 m at
a constant velocity of 5 m/s. How much work is done by the pulling force? 


To describe if energy is being added to or removed from a body, physicists use the prepositions "by" and "on." When the force and displacement point in the same general direction, then work is done by source of the force. It is energy being added to the body.
If the force and displacement point in opposite directions, the work is done on the body that is the source of the force. It is being removed, or subtracted, from the body.
When the tractor's front wheels were lifting up, it was applying a force. But the tractor and the sled where not moving. There was no work done yet because there wasn't any displacement. Once the tractor and sled began to move work was being done and energy was transferred. 
Practice 
Identify if work is done "on " or "by" the force.
Engine's force: A car is moving forward along a road at a constant velocity. 
Identify if work is done "on " or "by" the force.
Engine's force: A car is moving forward along a road at a constant velocity.
The engine's force and displacement point in the same direction. So the work is by the engine's force. 


Practice 
Identify if work is done "on " or "by" the force.
Rocket engine's force: A rocket is launched to the sky. 
Identify if work is done "on " or "by" the force.
Rocket engine's force: A rocket is launched to the sky.
The rocket engine's force points up and displacement points up too.They point in the same direction. So the work is by the rocket engine's force. 


Practice 
Identify if work is done "on " or "by" the force.
Water: A diver enters the water below & slows down. 
Identify if work is done "on " or "by" the force.
Water: A diver enters the water below & slows down.
The water is slowing down the diver as the diver moves down. Work is done ON the water. 


Practice 
Identify if work is done "on " or "by" the force.
(Parachute: A skydiver drops out of a plane and drops downwards with an open parachute. 
Identify if work is done "on " or "by" the force.
(Parachute: A skydiver drops out of a plane and drops downwards with an open parachute.
The parachute is slowing down the skydiver as the skydiver moves down. Work is done ON the parachute. 


Practice 
Identify if work is done "on " or "by" the force.
Gravity: A skydiver drops out of a plane and moves downwards with an open parachute. 
Identify if work is done "on " or "by" the force.
Gravity: A skydiver drops out of a plane and moves downwards with an open parachute.
The force of gravity is pulling the skydiver down in the direction of the displacement. Work is done BY gravity. 


Practice 
Identify if work is done "on " or "by" the force.
Engine's force: A car is moving forward along a road at a while accelerating. 
Identify if work is done "on " or "by" the force. Engine's force: A car is moving forward along a road at a while accelerating.
The engine's force and displacement point in the same direction. So the work is BY the engine. The speed or acceleration of the car does not affect the outcome. 


Practice 
Identify if work is done "on " or "by" the force.
Pushing force: A box is pushed along a floor to the right by a force that is pointing towards the right too and downwards at a 30° angle with the floor. 
Identify if work is done "on " or "by" the force. Pushing force: A box is pushed along a floor to the right by a force that is pointing towards the right too and downwards at a 30° angle with the floor.
The force is genrally pointing in the same direction as the displacement. So work is done BY the force. 


