Physics · Work, Energy And Power · NEET
Mechanical energy (KE + PE) is conserved only when the net work done by non-conservative forces is zero. In practice this means no friction, no air drag, and no external push or pull removing energy. Gravity and spring force are conservative, so a ball in free fall or a block on a frictionless track keeps E constant. The moment friction or air resistance acts, some mechanical energy turns into heat, so KE + PE goes down.
Total energy is always conserved, but mechanical energy is not. With friction, mechanical energy (KE + PE) decreases because part of it becomes heat and sound. So you must write: initial mechanical energy = final mechanical energy + energy lost to friction. For NEET, if a question mentions a 'rough' surface or 'air resistance', do not set KE + PE constant.
Take a body under only a conservative force F. The work-energy theorem gives W = change in KE. For a conservative force, the same work equals the drop in potential energy: W = -(change in PE). Setting them equal: change in KE = -(change in PE), so change in KE + change in PE = 0, which means KE + PE = constant. That constant total is the conserved mechanical energy.
Conservation of total energy is a universal law: energy is never created or destroyed, it only changes form (heat, light, sound, mechanical). Conservation of mechanical energy is a special, narrower case that holds only when non-conservative forces do no work. Mechanical energy can 'disappear' as heat, but total energy is still conserved.
In free fall the only force is gravity, a conservative force. As the body falls, height h decreases so PE = mgh falls, and speed increases so KE = (1/2)mv^2 rises by exactly the same amount. At every point mgh + (1/2)mv^2 gives the same total, equal to the initial energy at release.
A particle is released from height S from the surface of the Earth. At a certain height its kinetic energy is three times its potential energy. The height from the surface of the Earth and the speed of the particle at that instant are respectively
The sum of the kinetic energy and potential energy of a simple pendulum bob is 0.02 J. The speed of the bob at the equilibrium position is approximately (mass of the bob = 20 g)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
When the only forces doing work on a system are conservative forces (such as gravity or a spring force), the total mechanical energy E = KE + PE of the system remains constant throughout the motion.
Two conditions: (1) only conservative forces do work, and (2) non-conservative forces like friction and air resistance do zero net work. If either fails, mechanical energy is not conserved.
String tension in a pendulum does no work because it is always perpendicular to the bob's velocity. Since it does zero work, it does not change mechanical energy, so KE + PE stays constant during the swing.
No. When friction lowers mechanical energy, that energy becomes heat and sound. Total energy is still conserved; only its mechanical form has decreased.
It is the fastest way to solve problems on free fall, pendulums, vertical circles, inclined planes and springs without using force and acceleration. NEET regularly tests it as a one-line equation: initial (KE + PE) = final (KE + PE).
