Work = Force times Distance = Energy
Work is defined as force times distance. Work is a measure of the energy expended in applying a force to move an object.B.8
Work can also be negative. For example, when uncompressing an ideal spring, the (positive) work done by the spring on its moving end support can be interpreted also as saying that the end support performs negative work on the spring as it allows the spring to uncompress. When negative work is performed, the driving system is always accepting energy from the driven system. This is all simply accounting. Physically, one normally considers the driver as the agent performing the positive work, i.e., the one expending energy to move the driven object. Thus, when allowing a spring to uncompress, we consider the spring as performing (positive) work on whatever is attached to its moving end.
During a sinusoidal mass-spring oscillation, as derived in §B.1.4, each period of the oscillation can be divided into equal sections during which either the mass performs work on the spring, or vice versa.
Gravity, spring forces, and electrostatic forces are examples of conservative forces. Conservative forces have the property that the work required to move an object from point to point , either with or against the force, depends only on the locations of points and in space, not on the path taken from to .
Potential Energy in a Spring
When compressing an ideal spring, work is performed, and this work is stored in the spring in the form of what we call potential energy. Equation (B.6) above gives the quantitative formula for the potential energy stored in an ideal spring after it has been compressed meters from rest.
Kinetic Energy of a Mass
Kinetic energy is energy associated with motion. For example, when a spring uncompresses and accelerates a mass, as in the configuration of Fig.B.2, work is performed on the mass by the spring, and we say that the potential energy of the spring is converted to kinetic energy of the mass.
Suppose in Fig.B.2 we have an initial spring compression by meters at time , and the mass velocity is zero at . Then from the equation of motion Eq.(B.5), we can calculate when the spring returns to rest (). This first happens at the first zero of , which is time . At this time, the velocity, given by the time-derivative of Eq.(B.5),
Mass Kinetic Energy from Virtual Work
From Newton's second law, (introduced in Eq.(B.1)), we can use d'Alembert's idea of virtual work to derive the formula for the kinetic energy of a mass given its speed . Let denote a small (infinitesimal) displacement of the mass in the direction. Then we have, using the calculus of differentials,
Thus, by Newton's second law, a differential of work applied to a mass by force through distance boosts the kinetic energy of the mass by . The kinetic energy of a mass moving at speed is then given by the integral of all such differential boosts from 0 to :
The quantity is classically called the virtual work associated with force , and a virtual displacement .
Energy in the Mass-Spring Oscillator
Summarizing the previous sections, we say that a compressed spring holds a potential energy equal to the work required to compress the spring from rest to its current displacement. If a compressed spring is allowed to expand by pushing a mass, as in the system of Fig.B.2, the potential energy in the spring is converted to kinetic energy in the moving mass.
We can draw some inferences from the oscillatory motion of the mass-spring system written in Eq.(B.5):
- From a global point of view, we see that energy is conserved, since the oscillation never decays.
- At the peaks of the displacement (when is either or ), all energy is in the form of potential energy, i.e., the spring is either maximally compressed or stretched, and the mass is momentarily stopped as it is changing direction.
- At the zero-crossings of , the spring is momentarily relaxed, thereby holding no potential energy; at these instants, all energy is in the form of kinetic energy, stored in the motion of the mass.
- Since total energy is conserved (§B.2.5), the kinetic
energy of the mass at the displacement zero-crossings is exactly the
amount needed to stretch the spring to displacement (or compress
it to ) before the mass stops and changes direction. At all
times, the total energy is equal to the sum of the potential
energy stored in the spring, and the kinetic energy
stored in the mass:
It is a remarkable property of our universe that energy is conserved under all circumstances. There are no known exceptions to the conservation of energy, even when relativistic and quantum effects are considered.B.9
Energy Conservation in the Mass-Spring System
Newton's Laws of Motion