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# When timing the 100-m run, officials at the finish line are instructed to start their stopwatches at the sight of smoke from the starter’s pistol and not at the sound of its firing. Explain. What would happen to the times for the runners if the timing started when sound was heard?

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Since speed of light equals $c = 3 \cdot 10^{8}\, \mathrm{m/s}$ and speed of sound equals $343\, \mathrm{m/s}$, speed of light is greater than speed of sound, therefore, they would see the smoke before they hear the sound and if they first hear the sound their time will be longer.

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