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Physics is the natural science that studies matter, motion, energy, and force. For competitive exams like SSC CGL, Railway NTPC, UPSC, NDA, and Defence exams, important Physics topics include: Newton's Laws, gravitation, pressure, work-energy-power, light (reflection, refraction), sound, electricity and magnetism, nuclear physics, and everyday physics phenomena. NCERT Physics Class 9 and 10 are the standard preparation books.

The Law of Conservation of Energy states that energy cannot be created or destroyed, only converted from one form to another. Total energy of an isolated system remains constant. Examples: A ball falling converts potential energy to kinetic energy. In a hydroelectric dam, potential energy of water converts to kinetic energy then to electrical energy. This fundamental law is asked in all science sections of competitive exams.

The Doppler Effect is the change in frequency (or wavelength) of a wave as observed by someone moving relative to the wave source. When source moves toward observer, frequency increases (higher pitch/blue shift). When source moves away, frequency decreases (lower pitch/red shift). Applications: Police radar guns, Doppler weather radar, medical ultrasound, and astronomical observation (redshift proves universe is expanding).

Laws of Reflection: 1) Angle of incidence = Angle of reflection (measured from normal), 2) Incident ray, reflected ray, and normal are in the same plane. Laws of Refraction (Snell's Law): n1 sin(i) = n2 sin(r) where n is refractive index. When light enters denser medium it bends toward normal. Total Internal Reflection is used in optical fibers and allows light to travel long distances.

Nuclear Fission is the splitting of a heavy nucleus (like Uranium-235) into smaller nuclei, releasing enormous energy. Used in nuclear power plants and atomic bombs. Nuclear Fusion is the combining of two light nuclei (like Hydrogen/Deuterium) to form a heavier nucleus, releasing even more energy. Powers the Sun and hydrogen bombs. Both are important topics for SSC, UPSC, and NDA exams.