Physics · Work, Energy And Power · NEET
In NCERT the full derivation sits in the Nuclei chapter, but Work, Energy and Power introduces it as the widest form of the law of conservation of energy. Before Einstein, mass and energy were thought to be conserved separately. Einstein showed they are the same thing, so the energy budget must include mass. For NEET, remember the formula E = m c^2 and the idea that mass is a store of energy. You do not need special relativity derivation for the mechanics chapter.
c is the speed of light in vacuum, c = 3 x 10^8 m/s. It is squared because that is what Einstein's special relativity gives. The squaring is the key: c is already a huge number, and squaring makes it about 9 x 10^16. So even 1 kg of mass corresponds to 9 x 10^16 J. That is why nuclear energy is millions of times larger than chemical energy. Memory: 'the square is what makes it scary.'
Put mass in kilograms into E = m c^2. Step 1: convert grams to kg (1 g = 10^-3 kg). Step 2: use c^2 = (3 x 10^8)^2 = 9 x 10^16. Step 3: multiply. Example, 1 g: E = 10^-3 x 9 x 10^16 = 9 x 10^13 J. This is the exact NCERT worked example, so learn these numbers.
The correct NEET statement is that total energy is conserved, where mass is counted as energy through E = m c^2. Rest mass alone is NOT conserved because some mass converts to energy (this lost mass is called mass defect). So say 'mass-energy is conserved,' not 'mass is conserved.' This is the exact trap examiners set.
It is the biggest version of that law. Conservation of energy says energy only changes form, never vanishes. Einstein widened this by adding mass as one of the forms energy can take. So in the full accounting, kinetic energy plus potential energy plus mass-energy (m c^2) stays constant. This is why the next topic, the law of conservation of energy, builds directly on this idea.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
E = m c^2, where E is energy in joules, m is mass in kilograms, and c = 3 x 10^8 m/s is the speed of light in vacuum. This is Einstein's mass-energy relation from special relativity.
E = m c^2 = (10^-3 kg)(3 x 10^8)^2 = 10^-3 x 9 x 10^16 = 9 x 10^13 J. This is the standard NCERT example and shows the enormous energy hidden in a small mass.
Because nuclear reactions convert small amounts of mass into energy through E = m c^2, and c^2 is about 9 x 10^16. NCERT notes nuclear process energies are about a million times larger than chemical process energies.
No. Rest mass is not conserved; part of it becomes energy (mass defect). Total mass-energy is conserved. This distinction is a common NEET trap, so use the phrase 'mass-energy is conserved.'
The joule (J). If mass is in kg and c is in m/s, then m c^2 has units kg x (m/s)^2 = kg m^2 s^-2 = joule.
