Why are chemical changes considered unseen?

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In a game of pool, ball a is moving with a velocity v0 of magnitude v0 = 15 ft/s when it strikes balls b and c, which are at rest and aligned as shown. knowing that after the collision the three balls move in the directions indicated and assuming frictionless surfaces and perfectly elastic impact (that is, conservation of energy), determine the magnitudes of the velocities va, vb, and vc.

The first law of thermodynamics states that heat added to a system is neither created nor destroyed but is as it changes into other forms of energy.