Newton’s third law of motion:Consider two bodies engaged in a mutual interaction. Label the bodies 1and 2 respectively. Let be the force on body 1 due to the interactionwith body 2, and be the force on body 2 due to the interaction withbody 1.F1,2 F2,1If the acceleration of a body is the result of an outside force, thensomewhere in the universe there must be an equal and opposite forceacting on another body. The interaction may be a complicated one, but aslong as the forces are equal and opposite, Newton’s laws are satisfied.All real Forces arise due to interaction!1 212 12 12 21 21 212 12 12 21 2Gravitational force: ˆ ˆ ˆCoulomb force: ˆm m F G r r rrq q F k r F Fr      Newton’s 3rd law emphasizes Conservation of Momentum Validity of Newton’s laws• Validity of the first two laws– The first law is always valid (add a pseudo force).– The second law F = 𝒑ሶ holds but F and phave different expressions in therelativistic limit.• The 3rd law is not valid in the relativisticlimit. Why????


gravitational and frictional influences.
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