Work is a force that changes the amount of mechanical energy possessed by an object. It can also change the amount of thermal energy or electrical energy within a system. The concept of work is fundamental to how we understand the world around us and it is a key part of physics.
The SI unit for work is the joule (J), defined as the amount of work needed to move an object a given distance in one direction. The more the object moves, the more work is done and the more energy it gains. For example, if a ball is thrown downhill and hits the ground, gravity does positive work on it as it falls. The ball gains kinetic energy from this work, which is what gives it its speed. Likewise, if the ball is lifted uphill against gravity, the lifting force does negative work on it as it rises, and the lifter gains potential energy from this work.
An important principle is that the amount of work a force does is proportional to the amount of displacement the object experiences, with a linear relationship between the two quantities. However, this does not always hold in practice. For example, the lifting force applied to a block of concrete has a much larger effect on its velocity than a similar force applied to a softball. This is due to the greater mass of the concrete, which requires a higher amount of force to produce a given change in velocity.
In addition, some forces are constraining, limiting the movement of an object to a certain range. For instance, the frictional force exerted by a tire on a road prevents a car from moving up or down the slope, even though there is enough force applied by the driving wheel to overcome the gradient of the road. Similarly, the constraints of gravity and air resistance prevent the acceleration of a spacecraft traveling in a circular orbit.
Despite these limitations, it is still possible to do work on a body that has motion in more than one dimension. This is accomplished by dividing the motion into one-way, one-dimensional segments and evaluating the work on each segment. For example, when a briefcase is lifted upward against the pull of gravity on its bottom and against the force of its weight on the top, it does work on itself, adding energy to the case. Alternatively, if a generator causes a briefcase to be pulled downward by its weight, it does work on the briefcase, taking energy away from it.
