WEBVTT

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Here is the problem: A 4 kg ball has 450 J of kinetic energy. How fast is it moving?

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Use the kinetic energy formula, rearranged for the unknown, in SI units.

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Formula: kinetic energy.

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Rearrange for speed v. Take the positive square root (a magnitude).

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Substitute the known values.

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So the answer is speed v equals 15 m/s.

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Kinetic energy grows with the square of speed: doubling v multiplies KE by four. It is never negative, and the speed found from KE is a magnitude.

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Substituted every value back into the formula: both sides agree, which checks out.