The rolling resistance for steel on steel is quite low; the coefficient of rolli

Question

The rolling resistance for steel on steel is quite low; the coefficient of rolling friction is typically r=0.002. Suppose a 180,000 kg locomotive is rolling at 15 m/s on level rails.

Part A

If the engineer disengages the engine, how much time will it take the locomotive to coast to a stop?

Express your answer using two significant figures.

770

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Part B

How far will the locomotive move during this time?

Express your answer to two significant figures and include the appropriate units.

770

Explanation / Answer

Friction force F retarding locomotive = 180,000. × 9.81 × 0.002 = 3532 N

Deceleration a = F/m = 3532/180,000. = 19.62 × 10^(-3) m/s².

Time taken to "lose" 15 m/s = t = speed/a = [15/19.62]×10^3 = 770 s

During this time its average speed is 15/2 m/s so it will travel a distance of [15/2]×770
= 5.775 km.

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