Because labor is a measure of energy, it may seem strange to imagine that work may be negative—but it can! Work is the amount of energy used by a force over a certain distance. If you try to stop the net from moving, you will apply a force in the other direction. This new force is called work. Since energy cannot be created or destroyed, if you apply enough force in one direction, all the energy stored in the object being pulled will be converted into other forms of energy, primarily heat. This is why engines use friction to create heat - friction is an irreversible process so it cannot produce energy itself, but it can convert energy from another source. For example, if I rub two sticks together, we generate some heat and light some fireflies! The energy from our hands rubbing together was first stored as kinetic energy in our arms and then transformed into potential energy as we lifted our arms high into the air. Finally, this energy was transformed back into kinetic energy as we dropped them again.
Work can be positive or negative. A clock's hand is pushing down on the hour hand and lifting up on the minute hand; this is positive work. When a book is opened, its cover is lying flat and its pages are standing up; this is also positive work. However, when a book is closed, its cover is standing up and its pages are lying down; this is negative work. When ice melts, water molecules move from the solid state into the liquid state and increase their entropy.
When an item travels in the opposite direction to the direction of force application, it performs negative work. Taking a pail of water from a well, for example. The force is applied to the rope downward, yet the movement of the bucket is upward. As a result, the negative work. Done by the rope is equal to mgh, where m is the mass of the bucket and g is acceleration due to gravity.
Negative work can also be called work done by inertia. If object A has mass m and velocity v before a force acts on it, then after the act it will have mass m-v and velocity 0. So its work value will be (m-v)xg = -mvg or -w where w is w o rk. This means that A has done work on B (the force acting on it). In other words, A has accelerated B.
Work can only be done on physical objects. It cannot be created or destroyed. So if you have a box with some stuff in it that weighs 10 pounds and you lift it up into the air so it's out of reach, that thing still did positive work on you because muscle power is positive work too. Yet it didn't weigh anything less when you let go so it can only have increased your energy level somehow... probably through oxygen consumption.
Work can be positive or negative; if the force has a component in the same direction as the object's displacement, it is producing positive work. If the force contains a component in the opposite direction as the displacement, it causes negative work. For example, if you were to lift a weight off the ground and hold it above your head, you would have done negative work on the weight.
Negative work is done on an item by a force in the setting of classical mechanics, as you explain, when the motion of the object is in the opposite direction to the force. Such negative work shows that the force is tending to slow the item down, i.e., reduce its kinetic energy. Reducing the kinetic energy of an object lowers its potential energy, and objects have a maximum potential energy per unit mass. Thus, negative work amounts to using up available energy.
In physics textbooks, this topic is often covered under the heading of "irreversible processes". Physical laws are not violated by irreversible processes such as heat flow or chemical reactions. Rather, they provide the framework for understanding these processes. Irreversibility arises as a consequence of our inability to reverse the process of entropy increase (or decay for radioactive elements).
An example of negative work in daily life occurs when a person lowers himself using a rope into a well. The effort required to do this work is used up, instead of being stored as elastic strain energy in the rope.
Another example is when someone pushes against a door to open it. This action uses up some of the pusher's mechanical energy and reduces the door's potential energy.
Yet another example is when someone pulls on a leash to move his dog toward him. Once again, some of the person's mechanical energy is used up and the dog's potential energy is reduced.
When a force works in the opposite direction of displacement, the work done by a body might be negative. Work done by a force is positive if the applied force has a component in the displacement direction. Work done by a force is zero if there is no displacement.
For example, consider a force pulling up on a weight. The weight will fall back down again, but because it is being pulled up, work is done lifting the weight high into the air. This work is negative because it goes against the natural tendency for things to fall down. Lifting a weight up uses energy from your muscles and nerves; this action can be good or bad depending on what type of work you are doing. Lifting weights is useful for building muscle mass and strength. Lifting heavy objects is also used by athletes when they want to increase their power output during exercises such as jumping or throwing.
Work can also be negative if a force tries to pull something apart. For example, if you were to try to pull two strands of hair apart, then you would need to apply a force against the force trying to pull them together, which would require negative work. This could happen, for example, if one strand was tied up with string and you tried to pull it free. The work done by the force holding the hairs together would be negative because it goes against the natural state of things to stick together.