Physics · Work, Energy And Power · NEET
No, and this is the most common mix-up. The law of conservation of energy is the big rule: TOTAL energy (all forms) is always constant. Conservation of MECHANICAL energy (KE + PE = constant) is a smaller special case that only holds when just conservative forces (like gravity or spring force) do work. If friction or air drag acts, mechanical energy drops, but the missing part turns into heat, so TOTAL energy is still conserved.
It does not disappear. Non-conservative forces like friction and air drag convert mechanical energy (KE + PE) into heat and sound. In the 2017 NEET raindrop question, gravity gives 10 J but the drop lands with only 1.25 J of KE. The 'missing' 8.75 J was turned into heat by air resistance. Total energy stays the same, so the law of conservation of energy still holds.
No. The kinetic energy is not destroyed, it is transformed. Friction between the ball and floor changes the KE into thermal energy (the floor and ball warm up very slightly) and a little sound. Add up all forms and the total is unchanged. Energy is never destroyed, only converted.
In an inelastic collision, kinetic energy is not conserved because some KE turns into heat, sound, or deformation. But total energy IS conserved. When a book says 'energy is lost', it means mechanical energy is lost from the moving objects, not that energy vanished from the universe.
Only when non-conservative forces do no work, for example a smooth (frictionless) slide or a free fall in vacuum. Then all PE becomes KE, so mgh = (1/2)mv squared, giving v = sqrt(2gh). If friction or air drag is present, you must add a work-against-friction term, or use energy conservation with a heat loss term.
Consider a drop of rain water having mass 1 g falling from a height of 1 km. It hits the ground with a speed of 50 m/s. Take g constant with a value 10 m/s squared. The work done by the (i) gravitational force and the (ii) resistive force of air is:
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:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Energy can neither be created nor destroyed; it can only change from one form to another. The total energy of an isolated system stays constant over time.
Yes. For NEET it holds in every situation. Even when mechanical energy seems to be 'lost', it has just converted into heat, sound, or another form, so the total is always the same. At a deeper level, mass itself is a form of energy (E = mc squared).
Conservation of energy applies to total energy in all cases. Conservation of mechanical energy (KE + PE = constant) is a special case that holds only when non-conservative forces such as friction do no work.
Yes, total energy is still conserved. Mechanical energy decreases, but the lost amount appears as heat and sound. Add up all forms and the total does not change.
You equate the total energy at two points, usually KE + PE at start equals KE + PE at end. If a non-conservative force acts, include its work: change in KE = work by all forces, as in the raindrop PYQ.
